Eos Energy Enterprises, Inc. is distributing at no charge to the holders of our common stock, par value $0.0001 per share, and holders of the Company’s warrants issued on April 14, 2023, May 17, 2023, December 19, 2023 and November 21, 2025 (the “Participating Warrants”) as of 5:00 p.m. (New York City time) on July 1, 2026 (the “Record Date” and the holders collectively, the “Rights Offering Participants”) subscription rights (“Rights”) entitling the holder to purchase up to 27,367,171 Units (“Units”) at the Subscription Price (as defined below) with an aggregate offering value of up to approximately $150 million (the “Rights Offering”). The subscription price per Unit shall be equal to $5.481 (the “Subscription Price”).

Each Rights Offering Participant will receive one Right entitling the holder to purchase 0.071193 of a Unit at the Subscription Price, for each share of our common stock, or each share of our common stock underlying a Participating Warrant, as applicable, owned by such holder on the Record Date.

Each Unit shall consist of one share of our common stock (“New Shares”) and 0.4388 of a warrant to purchase common stock (“Warrants”), whose issuance is subject to certain conditions as discussed herein (the “Closing Conditions”). See “Description of the Rights Offering—Conditions to the Rights Offering.” Each whole Warrant entitles the holder to purchase one share of our common stock at an exercise price of $5.481 per whole share, subject to adjustment and exercised solely through cashless exercise. The Warrants will be exercisable immediately upon completion of this Rights Offering and will expire on the tenth anniversary of the completion of this Rights Offering, unless earlier exercised or redeemed. The Warrants will be subject to redemption by us, at our option, in whole but not in part, at any time on or after the five year anniversary of the issue date while they are exercisable and prior to their expiration, for $0.01 per Warrant, on not less than thirty (30) days’ written notice, if the last reported sale price of our common stock has been at least $10.962 per share, subject to adjustment, on each of twenty (20) trading days within the thirty (30) trading-day period ending on the business day prior to the date on which such notice of the redemption is given. Subject to the satisfaction or waiver of the Closing Conditions, the New Shares and the Warrants comprising the Units will separate upon the closing of this Rights Offering and will be issued separately; however, they may only be purchased as a Unit and the Units will not trade as a separate security. For a more detailed discussion, see “Description of the Rights Offering―Subscription Rights―Basic Subscription Rights.” Following the closing of the Rights Offering, there is no assurance that the price will remain at the current trading price, and the price may decline to the Subscription Price, or to a price lower than the Subscription Price. For a more detailed discussion, see “Description of the Rights Offering.”

The purpose of this Rights Offering is to raise equity capital in a cost-effective manner that provides all of our existing stockholders the opportunity to participate. We anticipate using the net proceeds of the Rights Offering, if any, to fund our contribution to Frontier Power USA Parent, LLC, a Delaware limited liability company (“Frontier”) in exchange for a number of Frontier’s Class B Units, at a price of $1.00 per Class B Unit. For a more detailed discussion, see “Summary―Recent Developments” and “Use of Proceeds.”

The Rights will be distributed and exercisable beginning on July 2, 2026. The Rights will expire and will have no value if they are not exercised prior to the expiration date of this Rights Offering, which is currently expected to be 5:00 p.m. (New York City time) on July 21, 2026 (the “Expiration Date”), unless we, in our sole discretion, extend the period for exercising the Rights or terminate it earlier. We will extend the duration of the Rights Offering as required by applicable law, and may choose to extend the Rights Offering if we decide that the degree of participation in this Rights Offering by holders of our common stock and Participating Warrants is less than the level we desire. You should carefully consider whether or not to exercise your Rights before the Expiration Date. We reserve the right to modify or cancel the Rights Offering at any time before the expiration of the Rights Offering, for any reason.

There is no minimum number of Units that we must sell in order to complete the Rights Offering. If you exercise your Rights in full, you may also exercise an over-subscription privilege to purchase additional Units up to the number of Units underlying the basic subscription right (or 200% combined) that remain unsubscribed at the expiration of the Rights Offering, subject to the availability and allocation of Units among persons exercising this over-subscription privilege and certain other limitations as further described elsewhere in this prospectus supplement and the accompanying prospectus. See “Description of the Rights Offering―Subscription Rights—Over-Subscription Privilege.” Stockholders who do not participate in the Rights Offering will continue to own the same number of shares, but will own a smaller percentage of the total shares outstanding after the Rights Offering to the extent that other stockholders participate in the Rights Offering. Rights that are not exercised by the Expiration Date will expire and have no value.

We are distributing the Rights and offering the underlying securities directly to you. We have not employed any brokers, dealers or underwriters in connection with the solicitation or exercise of Rights in the Rights Offering and no commissions, fees or discounts will be paid in connection with the Rights Offering. Broadridge Corporate Issuer Solutions, LLC is acting as the subscription agent. Sodali & Co. is acting as the information agent for the Rights Offering. Continental Stock Transfer & Trust Company is acting as the warrant agent for the Warrants. While certain of our directors, officers and other employees may solicit responses from you, those directors, officers and other employees will not receive any commissions or compensation for their services other than their normal compensation.

We have applied to have the Rights admitted to trading on Nasdaq Capital Market (“Nasdaq”), where we expect them to begin trading under the symbol “EOSER” on July 6, 2026 and to continue trading through July 21, 2026. We have applied to have the Warrants admitted to trading on Nasdaq under the symbol “EOSEW.” However, no assurance can be given that such listing application will be approved. If our Warrants listing application is not approved, the Warrants may not be traded on Nasdaq when issued, or at all. The subscription period for the Rights will begin on July 2, 2026 and will close at 5:00 p.m. (New York City time) on July 21, 2026. We do not intend to apply for listing of the Units offered hereby.

Our common stock trades on Nasdaq under the symbol “EOSE.” The subscription price for each Unit in this offering represents an approximately 0.3% discount to the last reported sale price of our common stock on Nasdaq on July 1, 2026, which was $5.55 per share.

Investing in our securities involves a high degree of risk. Before making an investment decision, you should carefully consider the risks described under “Risk Factors” beginning on page S-18 of this prospectus supplement and under Item 1A in our Annual Report on Form 10-K for the year ended December 31, 2025 which has been filed with the Securities and Exchange Commission and is incorporated by reference in this prospectus supplement and the accompanying prospectus.

Our board of directors (the “Board of Directors” or the “Board”), reserves the right to terminate the Rights Offering for any reason at any time before the completion of the Rights Offering. If we terminate the Rights Offering, all subscription payments received will be returned as soon as practicable, without interest or penalty.

The Board is making no recommendations regarding your exercise of any Rights. You should carefully consider whether to exercise your Rights before the Expiration Date. You may not revoke or revise any exercises of Rights once made, unless we terminate the Rights Offering.

This prospectus supplement and the accompanying prospectus do not constitute an offer to sell or the solicitation of an offer to subscribe for or purchase securities to any person to whom or in any jurisdiction in which such offer or solicitation is unlawful.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined if this prospectus supplement or the accompanying prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

This document has two parts. The first part consists of this prospectus supplement, which describes the specific terms of this offering and the distribution of rights to purchase units offered hereby. The second part is the accompanying prospectus, which provides more general information, some of which may not apply to this offering. If the description of this offering varies between this prospectus supplement and the accompanying prospectus, you should rely on the information in this prospectus supplement.

Before purchasing any units you should carefully read both this prospectus supplement and the accompanying prospectus, together with the additional information in the documents we have listed under the heading “Where You Can Find More Information.”

We have not authorized anyone to provide any information other than that contained or incorporated by reference in this prospectus supplement and the accompanying prospectus or in any free writing prospectus prepared by or on behalf of us or to which we have referred you. We take no responsibility for, and can provide no assurance as to the reliability of, any other information that others may give you. We are not making an offer of these securities in any jurisdiction where the offer is not permitted. You should not assume that the information contained in or incorporated by reference in this prospectus supplement or the accompanying prospectus is accurate as of any date other than their respective dates.

This prospectus supplement, the accompanying prospectus, and the information incorporated herein and therein by reference includes trademarks, service marks and trade names owned by us or other companies. All trademarks, service marks and trade names included or incorporated by reference into this prospectus supplement or the accompanying prospectus are the property of their respective owners.

All references in this prospectus supplement and the accompanying prospectus to “Eos,” the “Company,” “we,” “us,” “our,” or similar references refer to Eos Energy Enterprises, Inc., a Delaware corporation, and its subsidiaries taken as a whole, except where the context otherwise requires or as otherwise indicated.

The SEC allows us to incorporate by reference information in this document. This means that we can disclose important information to you by referring you to another document filed separately with the SEC. The information incorporated by reference is considered to be part of this document, except for any information that is superseded by information that is included directly in this document. Any statement contained in this prospectus supplement, or in a document incorporated or deemed to be incorporated by reference herein, shall be deemed to be modified or superseded to the extent that a statement contained herein, or in any subsequently filed document that also is incorporated or deemed to be incorporated by reference herein, modifies or supersedes such statement.

We are incorporating by reference the filings listed below and any additional documents that we may file with the SEC pursuant to Section 13(a), 13(c), 14 or 15(d) of the Exchange Act on or after the date hereof and prior to the termination of any offering (other than documents or information deemed to have been furnished and not filed in accordance with SEC rules):

The SEC maintains a website at www.sec.gov, from which you can inspect these documents and other information we have filed electronically with the SEC. You may also request copies of these documents, at no cost to you, from our website (https://www.eose.com), or by writing or telephoning us at the following address:

This prospectus supplement, including the documents incorporated by reference in this prospectus supplement and accompanying prospectus, contains forward-looking statements. The words “anticipate,” “believe,” “continue,” “could,” “estimate,” “expect,” “intends,” “may,” “might,” “plan,” “possible,” “potential,” “predict,” “project,” “should,” “would” and similar expressions, as they relate to us, are intended to identify forward-looking statements. These statements appear in a number of places in this prospectus supplement and the documents incorporated by reference herein and include statements regarding our intent, belief or current expectations. Forward-looking statements are based on our management’s beliefs, as well as assumptions made by, and information currently available to, them. Because such statements are based on expectations as to future financial and operating results and are not statements of fact, actual results may differ materially from those projected. Factors which may cause actual results to differ materially from current expectations include, but are not limited to:

Should one or more of these risks or uncertainties materialize, or should any of our assumptions prove incorrect, actual results may vary in material respects from those projected in these forward-looking statements. Forward-looking statements speak only as of the date they are made. Readers are cautioned not to put undue reliance on forward-looking statements and, except as required by law, we assume no obligation and do not intend to update or revise these forward-looking statements, whether as a result of new information, future events, or otherwise. See also Part I, Item 1A, “Risk Factors” disclosures contained in our Annual Report on Form 10-K for the year ended December 31, 2025 for additional discussion of the risks and uncertainties that could cause the our actual results to differ materially from those expressed or implied in its forward-looking statements.

The following are examples of what we anticipate will be common questions about the Rights Offering. The answers are based on selected information included elsewhere in this prospectus supplement. The following questions and answers do not contain all of the information that may be important to you and may not address all of the questions that you may have about the Rights Offering. This prospectus supplement and the documents incorporated by reference contain more detailed descriptions of the terms and conditions of the Rights Offering and provide additional information about us and about our business, including potential risks related to the Rights Offering, our common stock, and our business.

Exercising the Rights and investing in our securities involve a high degree of risk. We urge you to carefully read the section titled “Risk Factors” beginning on page S-18 of this prospectus supplement and all other information included in, or incorporated by reference into, this prospectus supplement in its entirety before you decide whether to exercise your Rights.

An investment in our securities involves a high degree of risk. Before deciding whether to invest in our securities, you should consider carefully the risks described below, together with the other information in this prospectus supplement, the accompanying prospectus, the information and documents incorporated herein and therein by reference, and in any free writing prospectus that we have authorized for use in connection with this offering. You should also consider the risks, uncertainties and assumptions discussed under the heading “Risk Factors” included in our most recent Annual Report on Form 10-K, which is on file with the Securities and Exchange Commission, and is incorporated herein by reference. If any of these risks actually occurs, our business, financial condition, results of operations or cash flow could be seriously harmed. This could cause the trading price of our common stock to decline, resulting in a loss of all or part of your investment. The risks and uncertainties described below are not the only ones facing us. Additional risks and uncertainties not presently known to us, or that we currently see as immaterial, may also harm our business. Please also read carefully the section above titled “Special Note Regarding Forward-Looking Statements.”

The Rights do not have an established trading market. Although we expect that the Rights will be admitted to trading on Nasdaq, as discussed elsewhere in this prospectus supplement, there is no assurance that an active trading market in the Rights will develop during the applicable trading periods or that there will be sufficient liquidity for the Rights during such periods. If an active trading market does not develop or is not substantial, the liquidity and trading price of the Rights could be adversely affected. Because the trading price of the Rights depends on a variety of factors, including the trading price of our common stock, the trading price of the Rights may be volatile and subject to significantly greater price fluctuations than our common stock.

Any Rights that are not exercised by a holder during the subscription period will expire and become null and void, and holders of the Rights will not receive any compensation for them. The rights offering is designed to enable us to raise capital in a manner that gives its eligible holders the opportunity to subscribe for the New Shares pro rata to their holding prior to the rights offering, subject to applicable securities laws. To the extent that holders do not exercise their Rights, their proportionate ownership and voting interest in us will decline. Even if holders elect to sell their Rights, the consideration they receive, if any, may not be sufficient to fully compensate them for the dilution of their percentage ownership of common stock that may be caused as a result of the offering.

Under the laws and regulations of jurisdictions outside the United States, certain holders of Rights may not be able to exercise or sell their Rights unless we take action to register or otherwise qualify the offering under the laws of such jurisdictions, which we do not intend to do. In addition, neither of us or the Rights Agent will mail or otherwise seek to deliver Rights certificates to holders of our common stock and Participating Warrants in such jurisdictions, unless they deliver an opinion of reputable counsel stating that they are permitted to receive such materials and to participate in the rights offering, as described elsewhere in this prospectus supplement.

In such circumstances, we have made arrangements with the Rights Agent (acting through its broker-dealer) to use commercially reasonable efforts to mail this prospectus supplement and the accompanying prospectus to holders of common stock in such jurisdictions, in order to advise them of their entitlements and, upon instructions from such holders, to sell the related Rights and send the proceeds of such sales, net of brokers’ and other fees and expenses, to the entitled holders. If holders in such jurisdictions fail to appropriately instruct the Rights Agent or are otherwise unable to exercise or sell their Rights, their Rights will expire and become worthless.

Certain holders of our common stock hold their securities in certificated form. Because of the logistical difficulties that are inherent to distributing physical subscription forms for the Rights to such holders, certain holders may be unable to exercise their rights on a timely basis or at all. If such holders are unable to exercise their Rights, or if the Rights subscription form required for the exercise of their Rights is not received until after the completion of the applicable subscription period, their Rights will expire and become worthless.

The closing of the Rights Offering is subject to the satisfaction or waiver of certain closing conditions that are outside of our control, including: (i) the absence of prohibitions or enjoinments by any court of competent jurisdiction and (ii) the substantially concurrent closing of the JV Transaction. As these conditions are outside of our control, we cannot assure you that the conditions to the completion of the Rights Offering will be satisfied in a timely manner or at all.

In addition, we do not intend, but have the right, to terminate the Rights Offering at any time prior to the Expiration Date.

In determining the subscription price for the Rights Offering, our board of directors, with the advice and input of management and advisors, considered a number of factors, including: the likely cost of capital from other sources and the general conditions of the securities markets, the price at which our stockholders might be willing to participate in the Rights Offering, historical and current trading prices for our common stock, our need for liquidity and capital, the value of the Warrants being issued as a component of the Unit and the desire to provide an opportunity to our stockholders to participate in the Rights Offering on a pro rata basis. In conjunction with its review of these factors, our board of directors also reviewed a range of subscription prices in various prior rights offerings of public companies. The subscription price is not necessarily related to our book value, net worth or any other established criteria of value and may or may not be considered the fair value of the Units to be offered in the Rights Offering. The market price of our common stock may decline during or after the Rights Offering, including below the subscription price for the Units. After the date of this prospectus supplement, our common stock may trade at prices above or below the subscription price.

The market price of our common stock may fluctuate and decline below the subscription price of the Rights. Moreover, given that the market price of the Rights depends to a significant degree on the price of our common stock, a decline in the trading price of the common stock would also be expected to negatively affect the trading price of the Rights. The market price of our common stock may fluctuate due to a variety of factors, including:

Subscriptions for our common stock pursuant to the exercise of the Rights, as applicable, will be binding and irrevocable, and the subscription price may be materially different from the market price of the Company’s common stock. We cannot assure holders of Rights that the market price of our common stock will not decline below the subscription price after such holders elect to exercise their Rights. If that occurs, such holders will have committed to buying common stock at a price above the prevailing market price, and such holders will suffer an immediate unrealized loss as a result. In addition, we cannot assure holders of Rights that, following the exercise of the Rights, they will be able to sell their common stock, as applicable, at a price equal to or greater than the subscription price.

Securities research analysts publish their own annual and/or quarterly projections regarding our operating results. These projections may vary widely from one another and may not accurately predict the results we actually achieve. Our share price may decline if we fail to meet securities research analysts’ projections. Similarly, if one or

more of the analysts who covers us downgrades our shares or publishes inaccurate or unfavorable research about our business, our share price could decline. If one or more of these analysts ceases coverage of us or fails to publish reports on us regularly, our share price or trading volume could decline. There can be no assurance we will be covered by security research analysts. However, if we are covered by securities research analysts, if securities or industry analysts cease coverage of us at any time, the trading price of our shares and the trading volume could decline.

If you exercise Rights, you may not be able to resell the common stock that you receive in the Rights Offering until you, or your custodian bank, broker, dealer or other nominee, if applicable, have received those shares. Moreover, you will have no rights as a stockholder of the shares you received in the Rights Offering until we issue the shares to you. Although we will endeavor to issue the shares as soon as practicable after completion of the Rights Offering, and after all necessary calculations have been completed, there may be a delay between the Expiration Date of the Rights Offering and the time that the shares are issued. In addition, we cannot assure you that, following the exercise of your Rights, you will be able to sell your common stock at a price equal to or greater than the subscription price.

No minimum subscription is required for consummation of the Rights Offering. We do not have formal commitments from our other stockholders for the amount we seek to raise pursuant to the Rights Offering. It is possible that no rights will be exercised in connection with the Rights Offering. As a result, we cannot assure you of the amount of proceeds that we will receive in the Rights Offering. Therefore, if you exercise all or any portion of your Rights, but other stockholders do not, we may not raise the desired amount of capital in the Rights Offering, the market price of our common stock could be adversely impacted and we will find it necessary to pursue alternative means of financing, which may be dilutive to your investment.

We do not intend, but have the right, to terminate the Rights Offering at any time prior to the Expiration Date. If we determine to terminate the Rights Offering, we will not have any obligation with respect to the Rights except to return any money received from subscribing stockholders as soon as practicable, without interest or deduction. Because our Board of Directors reserves the right to terminate the Rights Offering at any time, we concluded a floor price for each Unit was neither necessary nor in the best interests of the Company.

Stockholders who desire to purchase Units in the Rights Offering must act promptly to ensure that all required forms and payments are actually received by the subscription agent prior to the Expiration Date, which is currently set to be 5:00 p.m. on July 21, 2026 unless we, in our sole discretion, extend the period for exercising the Rights or terminate it earlier. If you are a beneficial owner of shares, you must act promptly to ensure that your custodian bank, broker, dealer or other nominee acts for you and that all required forms and payments are actually received by the subscription agent prior to the Expiration Date. Your nominee may establish a deadline prior to the Expiration Date by which you must provide it with your instructions to exercise your Rights and payment for your Units. We will not be responsible if your custodian bank, broker, dealer or nominee fails to ensure that all required forms and payments are actually received by the subscription agent prior to the Expiration Date. If you fail to complete and sign the required subscription forms, send an incorrect payment amount, or otherwise fail to follow the subscription procedures that apply to your desired transaction the subscription agent may, depending on the circumstances, reject your subscription or accept it to the extent of the payment received. Neither we nor our subscription agent will undertake to contact you concerning, or attempt to correct, an incomplete or incorrect subscription form or payment. We have the sole discretion to determine whether a subscription exercise properly follows the subscription procedures.

By signing the rights certificate and exercising their rights, each stockholder agrees, solely with respect to such stockholder’s exercise of rights in the Rights Offering, that we have the right to void and cancel (and treat as if never exercised) any exercise of rights, and securities issued pursuant to an exercise of rights, if any of the agreements, representations or warranties of a subscriber in the subscription documents are false.

Any uncertified personal check used to pay the subscription price in the Rights Offering must clear prior to the Expiration Date, and the clearing process may require at least five (5) business days. As a result, if you choose to use an uncertified personal check to pay the subscription price, it may not clear prior to the Expiration Date, in which event you would not be eligible to exercise your Rights. You may eliminate this risk by paying the subscription price by a certified check, bank draft, cashier’s check, U.S. postal money order, or wire transfer of funds to ensure that the subscription agent receives your funds prior to the expiration of the Rights Offering.

Exercise of the over-subscription privilege will only be honored if and to the extent that the basic subscription rights have not been exercised in full. If sufficient Units are available, we will seek to honor your over-subscription request in full, up to the number of Units available under your basic subscription right (or 200% combined). If, however, over-subscription requests exceed the number of Units available to be purchased pursuant to the over-subscription privilege, we will allocate the available Units proportionately among stockholders who exercised their over-subscription privileges based on the number of Units each stockholder subscribed for under such stockholder’s basic subscription rights. As a result, you may not receive any or all of the Units for which you exercise your over-subscription privilege.

As soon as practicable after the Expiration Date, the subscription agent will determine the number of Units that you may purchase pursuant to the over-subscription privilege. If you have properly exercised your over-subscription privilege, we will issue the shares of common stock and Warrants included in the Units purchased in the Rights Offering to you in book-entry, or uncertificated, form as soon as practicable after the Expiration Date and after all allocations and adjustments have been effected. If you request and pay for more Units than are allocated to you, we will refund the overpayment, without interest or deduction. In connection with the exercise of the over-subscription privilege, custodian banks, brokers, dealers and other nominee holders of Rights who act on behalf of beneficial owners will be required to certify to us and to the subscription agent as to the aggregate number of Rights exercised, and the number of Units requested through the over-subscription privilege, by each beneficial owner on whose behalf the nominee holder is acting.

We believe, and intend to take the position, that the distribution of the Rights should be treated as a non-taxable distribution with respect to existing shares of our common stock for U.S. federal income tax purposes. However, the authorities governing transactions such as the Rights distribution are complex and unclear in certain respects. Our position regarding the tax-free treatment of the Rights distribution is not binding on the Internal Revenue Service (the “IRS”) or the courts. If this position is determined by the IRS or a court to be incorrect, the fair market value of the Rights would be treated as a dividend to the extent of your pro rata share of our current and accumulated earnings and profits, if any, with any excess being treated as a return of capital to the extent thereof and then as capital gain. If you are a Non-U.S. Holder, any such amount treated as a dividend generally would be subject to U.S. federal withholding tax at a 30% rate, unless an applicable income tax treaty provides otherwise. Although no assurance can be given, the Company anticipates that it will not have current or accumulated earnings and profits through the end of 2026. For a more detailed discussion, see “Material U.S. Federal Income Tax Consequences.” You should consult your tax advisor as to the particular considerations applicable to you of the Rights distribution.

As described under “Description of the Rights Offering—Warrants,” the terms of each Warrant provide for an adjustment to the number of shares of our common stock for which the Warrant may be exercised or to the exercise price of the Warrant on the occurrence of certain events. An adjustment which has the effect of preventing dilution

generally is not a taxable event. Holders of the Warrants would, however, be treated as receiving a constructive distribution from us if, for example, the adjustment to the number of such shares or to such exercise price increases the Warrant holders’ proportionate interest in our assets or earnings and profits (e.g., through an increase in the number of shares of our common stock that would be obtained upon exercise or through a decrease in the exercise price of the Warrants), including as a result of a distribution of cash or other property to the holders of shares of our common stock which is taxable to such holders. Any constructive distribution deemed received by you would be subject to tax (including U.S. federal withholding tax if you are a non-U.S. holder) in the same manner as if you received a cash distribution from us equal to the fair market value of the increased interest resulting from the adjustment. It is possible that the U.S. federal withholding tax on the constructive dividend deemed received by a non-U.S. holder would be withheld from or set off against other payments to which you would otherwise be entitled, such as, in some circumstances, any payments on our common stock, or sales proceeds payable to you or your other funds or assets. For a more detailed discussion, see “Material U.S. Federal Income Tax Consequences.” You should consult your tax advisor as to the particular considerations applicable to you with respect to any such adjustments.

The Warrants do not have an established trading market. Although we applied for the Warrants to be admitted to trading on Nasdaq, as discussed elsewhere in this prospectus supplement, there is no assurance that the Warrants will be admitted for trading, an active trading market in the Warrants will develop or that there will be sufficient liquidity for the Warrants. If an active trading market does not develop or is not substantial, the liquidity and trading price of the Warrants could be adversely affected. Because the trading price of the Warrants depends on a variety of factors, including the trading price of our common stock, the trading price of the Warrants may be volatile and subject to significantly greater price fluctuations than our common stock.

The Warrants being issued in connection with this Rights Offering become exercisable upon issuance and will expire ten years after issuance, unless earlier exercised or redeemed. We cannot provide you any assurance that the market price of our common stock will ever exceed the exercise price per whole share prior to their date of expiration. Any Warrants not exercised by their date of expiration will expire for no value and we will be under no further obligation to the Warrant holder.

We may call the Warrants for redemption, at our option, in whole but not in part, at any time on or after the five year anniversary of the issue date, for $0.01 per Warrant, on not less than thirty (30) days’ written notice, if the last reported sale price of our common stock has been at least $10.962 per share, subject to adjustment, on each of twenty (20) trading days within the thirty (30) trading-day period ending on the business day prior to the date on which such notice of the redemption is given. If we give a notice of redemption, you will be forced to sell or exercise your Warrants or accept the redemption price. The notice of redemption could come at a time when it is not advisable or possible for you to exercise the Warrants. As a result, you would be unable to benefit from owning the Warrants being redeemed. A notice of redemption may also result in our common stock becoming more volatile and being subject to greater selling pressure which could result in declines in the market price for our common stock.

If you choose not to exercise your Rights or if you exercise fewer than all of your Rights and other stockholders fully exercise their Rights or exercise a greater proportion of their Rights than you exercise, the percentage of our common stock owned by these other stockholders will increase relative to your ownership percentage, and your voting and other rights in us will likewise be diluted. However, even if all stockholders choose to exercise their Rights in full, the percentage of our common stock owned by each such stockholder on a fully diluted basis will still be diluted, and each such stockholders’ voting and other rights in us may still be diluted, due to the issuance of 20,017,772 CCM Warrants to CCM and 10,008,886 HBC Warrants to HBC, each exercisable for one share of our common stock at an exercise price of $5.481 per whole share, subject to adjustment.

The closing of the JV Transaction is subject to the satisfaction or waiver of certain closing conditions that are outside of our control, including: (i) the completion of the Rights Offering; (ii) the Department of Energy’s consent to the JV Transaction; and (iii) the execution of certain commercial framework guidelines by the parties to the JV Transaction.

As these conditions are outside of our control, we cannot assure you that the conditions to the completion of the JV Transaction will be satisfied in a timely manner or at all, which may affect when and whether the JV Transaction will close. If the JV Transaction is not completed, the price of our common stock could fall to the extent that their current price reflects an assumption that JV Transaction will be completed. Furthermore, if JV Transaction is not completed and/or the JV Agreement with CCM, HBC and Frontier is terminated, we may suffer other consequences that could adversely affect our business, results of operations and share price, including the following:

Furthermore, any prolonged delay until the completion of the JV Transaction could adversely affect our business if we do not have access to the benefits we expect to receive from the JV Transaction in a timely manner. Although the Company expects that the JV Transaction will result in various benefits, there can be no assurance regarding when or the extent to which the Company will be able to realize such benefits. Achieving the anticipated benefits, including expected project pipeline, is subject to a number of uncertainties. Costs associated with the transaction could exceed current estimates, negatively impacting the economics of the transaction or impacting the Company’s results of operations or financial condition.

Although we cannot determine what the actual net proceeds from the sale of the Units in the Rights Offering will be until the Rights Offering is completed, assuming all Rights are exercised, we estimate that the aggregate net proceeds from the sale of the Units, before deducting estimated offering expenses, will be approximately $150 million.

We anticipate using the net proceeds of the Rights Offering, if any, to fund our contribution to Frontier in exchange for a number of Frontier’s Class B Units, at a price of $1.00 per Unit. “Summary―Recent Developments.”

Our net tangible book value as of March 31, 2026, was approximately $(874.1) million, or $(2.57) per share of our common stock (based on 339,459,021 shares outstanding as of March 31, 2026). Net tangible book value per share is equal to our total net tangible book value, which is our total tangible assets less our total liabilities, divided by the number of shares of our outstanding common stock. Dilution per share equals the difference between the amount per share paid by purchasers of shares of common stock that are included in the Units offered in the Rights Offering and the net tangible book value per share of our common stock immediately after the Rights Offering.

After giving effect to the assumed sale of 27,367,171 Units in the Rights Offering (consisting of 27,367,171 shares of our common stock and warrants to purchase an aggregate of 12,008,714 shares of common stock upon exercise), based on the assumed subscription price of $5.481 per Unit (the Subscription Price), and after deducting estimated offering expenses payable by us, our pro forma net tangible book value as of March 31, 2026 would have been approximately $(724.1) million, or $(1.97) per share. This represents an immediate increase in pro forma net tangible book value to existing stockholders of $0.60 per share and an immediate dilution to purchasers in the Rights Offering of $7.45 per share.

The following table illustrates this per-share dilution (assuming a fully subscribed for Rights Offering of 27,367,171 Units at the assumed subscription price of $5.481 per Unit) (the Subscription Price), but excluding the issuance of shares of common stock upon exercise of warrants.

The information above is illustrative only and will adjust based on the actual subscription price and the actual number of Units sold in the Rights Offering. The number of shares of our common stock to be outstanding after this offering is based on 339,459,021 shares of common stock outstanding as of March 31, 2026, and excludes:

To the extent that any options are exercised, new options are issued under our equity incentive plans or we otherwise issue additional shares of common stock in the future at a price less than the public offering price, there will be further dilution to new investors.

We have never declared or paid cash dividends on our common stock. We currently do not anticipate paying any cash dividends in the foreseeable future. Any future determination to declare cash dividends will be made at the discretion of our board of directors, subject to applicable laws, and will depend on our financial condition, results of operations, and capital requirements, general business conditions and other factors that our board of directors may deem relevant.

We will distribute to each Right Offering Participant on the Record Date, which is July 1, 2026, at no charge, one Right for each share of common stock owned. Each Right will entitle the rights holder to purchase 0.071193 of a Unit at the Subscription Price, upon timely delivery of the required documents and payment of the subscription price. Each Unit consists of one share of our common stock at the Subscription Price and 0.4388 of a Warrant, each whole warrant entitling the holder to purchase one share of our common stock at an exercise price of $5.481. If rights holders wish to exercise their Rights, they must do so prior to 5:00 p.m. (New York City time) on July 21, 2026, the Expiration Date for the Rights Offering, unless we, in our sole discretion, extend the period for exercising the Rights or terminate it earlier. After the Expiration Date, the Rights will expire and will have no value. See below “—Expiration of the Rights Offering and Extensions, Amendments and Termination.” You are not required to exercise all of your Rights. If you transfer a portion of your Rights, you may exercise your over-subscription privilege if you exercise all of the remaining rights represented by the rights certificate you receive back from the subscription agent following the transfer. We will issue to the record holders who purchase Units in the Rights Offering the shares and Warrants in book-entry, or uncertificated, form as soon as practicable after the Rights Offering has expired.

Subject to the allocation described below, each Right also grants the holder an over-subscription privilege to purchase additional Units that are not purchased by other rights holders pursuant to their basic subscription rights up to the number of Units underlying the basic subscription right (or 200% combined). You are entitled to exercise your over-subscription privilege only if you exercise your basic subscription right in full.

If you wish to exercise your over-subscription privilege, you should indicate the number of additional Units that you would like to purchase in the space provided on your rights certificate, as well as the number of shares that you beneficially own without giving effect to any Units to be purchased in this Rights Offering. When you send in your rights certificate, you must also send the full purchase price, as provided herein, for the number of additional Units that you have requested to purchase (in addition to the payment, as provided herein, due for Units purchased through your basic subscription right). If the number of Units remaining after the exercise of all basic subscription rights is not sufficient to satisfy all requests for Units pursuant to over-subscription privileges, you will be allocated additional Units in the proportion which the number of Units you purchased through the basic subscription right bears to the total number of Units that all over-subscribing stockholders purchased through the basic subscription right. The subscription agent will return any excess payments in the form in which made.

As soon as practicable after the Expiration Date, the subscription agent will determine the number of Units that you may purchase pursuant to the over-subscription privilege. If you request and pay for more Units than are allocated to you, we will refund the overpayment in the form in which made. You will not receive a refund or other compensation for any unissued fractional Units. In connection with the exercise of the over-subscription privilege, custodian banks, brokers, dealers and other nominee holders of Rights who act on behalf of beneficial owners will be required to certify to us and to the subscription agent as to the aggregate number of Rights exercised, and the number of Units requested through the over-subscription privilege, by each beneficial owner on whose behalf the nominee holder is acting.

The subscription price per Unit will be $5.481. Subscribers must fund their subscriptions pursuant to both the basic subscription right and over-subscription privilege at the Subscription Price.

In determining the subscription price, our board of directors, with the advice and input of management and advisors, considered a number of factors, including: the likely cost of capital from other sources and general conditions of the securities markets, the price at which our stockholders might be willing to participate in the Rights Offering, historical and current trading prices for our common stock, our need for liquidity and capital, the value of the Warrants being issued as a component of the Unit, and the desire to provide an opportunity to our stockholders to participate in the Rights Offering on a pro rata basis. In conjunction with its review of these factors, our board of

directors also reviewed a range of subscription prices in various prior rights offerings of public companies. The subscription price is not necessarily related to our book value, net worth or any other established criteria of value and may or may not be considered the fair value of our Units to be offered in the Rights Offering. You should not consider the subscription price as an indication of value of us or our common stock. The market price of our common stock may decline during or after the Rights Offering, including below the subscription price for the Units. You should obtain a current quote for our common stock before exercising your Rights and make your own assessment of our business and financial condition, our prospects for the future, and the terms of the Rights Offering.

Each whole Warrant entitles the holder to purchase one share of our common stock at an exercise price of $5.481 per whole share, subject to adjustment and exercised solely through cashless exercise. Each Warrant will be exercisable immediately upon completion of this Rights Offering and will expire on the tenth anniversary of the completion of this Rights Offering, unless earlier exercised or redeemed. Upon a cashless exercise of any Warrants, the exercising holder will be entitled to receive a number of shares of our common stock equal to the quotient obtained by dividing (x) the product of the number of shares of our common stock underlying the relevant Warrants, multiplied by the excess of the fair market value of our common stock over the exercise price, by (y) such fair market value. The “fair market value” of our common stock will be the last reported sale price of our common stock on the date of exercise; provided that the “fair market value” in respect of any exercise following our notice of redemption of the Warrants will be the average of the last reported sale prices of our common stock for the ten (10) trading days ending on the third trading day prior to the date on which the notice of redemption is sent to the holders of the Warrants.

A holder of a Warrant may notify us in writing in the event it elects to be subject to a requirement that such holder will not have the right to exercise such Warrant to the extent that, after giving effect to such exercise, such person (together with such person’s affiliates), to the warrant agent’s actual knowledge, would beneficially own in excess of 4.9% or 9.8% (or such other amount as a holder may specify) of the shares of our common stock outstanding immediately after giving effect to such exercise.

The Warrants will be subject to redemption by us, at our option, in whole but not in part, at any time on or after the five year anniversary of the issue date while they are exercisable and prior to their expiration, for $0.01 per Warrant, on not less than thirty (30) days’ written notice, if the last reported sale price of our common stock has been at least $10.962 per share, subject to adjustment, on each of twenty (20) trading days within the thirty (30) trading-day period ending on the business day prior to the date on which such notice of the redemption is given.

If the number of outstanding shares of our common stock is increased by a stock dividend payable in shares of our common stock, or by a split-up of shares of our common stock or other similar event, then, on the effective date of such stock dividend, split-up or similar event, the number of shares of our common stock issuable on exercise of each Warrant will be increased in proportion to such increase in the outstanding shares of our common stock. A rights offering to holders of our common stock entitling holders to purchase shares of our common stock at a price less than the fair market value will be deemed a stock dividend of a number of shares of our common stock equal to the product of (i) the number of shares of our common stock actually sold in such rights offering (or issuable under any other equity securities sold in such rights offering that are convertible into or exercisable for our common stock) and (ii) one (1) minus the quotient of (x) the price per share of our common stock paid in such rights offering divided by (y) the fair market value. For these purposes (i) if the rights offering is for securities convertible into or exercisable for our common stock, in determining the price payable for our common stock, there will be taken into account any consideration received for such rights, as well as any additional amount payable upon exercise or conversion and (ii) fair market value means the volume weighted average price of our common stock as reported during the ten (10) trading day period ending on the trading day prior to the first date on which the shares of our common stock trade on the applicable exchange or in the applicable market, regular way, without the right to receive such rights.

In addition, if, at any time while the Warrants are outstanding and unexpired, we pay a dividend or make a distribution in cash, securities or other assets to the holders of our common stock on account of such shares of our common stock (or other shares of our capital stock into which the Warrants are convertible), other than as described in the immediately preceding paragraph, then the exercise price will be decreased, effective immediately after the effective date of such event, by the amount of cash and/or the fair market value (as determined by our board of directors, in good faith) of any securities or other assets paid on each share of our common stock in respect of such event.

If the number of outstanding shares of our common stock is decreased by a consolidation, combination, reverse stock split or reclassification of shares of our common stock or other similar event, then, on the effective date of such consolidation, combination, reverse stock split, reclassification or similar event, the number of shares of our common stock issuable on exercise of each Warrant will be decreased in proportion to such decrease in outstanding shares of our common stock.

Whenever the number of shares of our common stock purchasable upon the exercise of the Warrants is adjusted, as described above, the exercise price will be adjusted by multiplying the exercise price immediately prior to such adjustment by a fraction (x) the numerator of which will be the number of shares of our common stock purchasable upon the exercise of the Warrants immediately prior to such adjustment, and (y) the denominator of which will be the number of shares of our common stock so purchasable immediately thereafter.

In case of any reclassification or reorganization of the outstanding shares of our common stock (other than those described above in this “—Adjustments” section or that solely affects the par value of such shares of our common stock), or in the case of any merger or consolidation of us with or into another entity or conversion of us as another entity (other than a consolidation or merger in which we are the continuing corporation and that does not result in any reclassification or reorganization of our outstanding shares of common stock), or in the case of any sale or conveyance to another entity of the assets or other property of us as an entirety or substantially as an entirety in connection with which we are dissolved, the holders of the Warrants will thereafter have the right to purchase and receive, upon the basis and upon the terms and conditions specified in the Warrants and in lieu of the shares of our common stock immediately theretofore purchasable and receivable upon the exercise of the rights represented thereby, the kind and amount of shares of stock or other securities or property (including cash) receivable upon such reclassification, reorganization, merger or consolidation, or upon a dissolution following any such sale or transfer, that the holder of the Warrants would have received if such holder had exercised their Warrants immediately prior to such event.

Continental Stock Transfer & Trust Company is acting as the warrant agent for the Warrants.

No fractional shares will be issued upon exercise of the Warrants. If, upon exercise of the Warrants, a holder would be entitled to receive a fractional interest in a share of our common stock, we will, upon exercise, round down to the nearest whole number of shares of common stock to be issued to the Warrant holder.

The Warrants will be issued in registered form under a warrant agreement between Continental Stock Transfer & Trust Company, as warrant agent, and us. You should review a copy of the warrant agreement, which is filed as Exhibit 4.2 to the Current Report on Form 8-K filed by the Company on July 2, 2026 for a complete description of the terms and conditions applicable to the Warrants.

You may exercise your Rights at any time prior to 5:00 p.m. (New York City time) on July 21, 2026, the Expiration Date for the Rights Offering unless we, in our sole discretion, extend the period for exercising the Rights or terminate it earlier. If you do not exercise your Rights before the Expiration Date, your Rights will expire and will have no value. We will not be required to sell Units to you if the subscription agent receives your rights certificate or payment, after the Expiration Date, regardless of when you sent the rights certificate and payment.

We may, in our sole discretion, extend the time for exercising the Rights. We may extend the Expiration Date at any time after the Record Date. If the commencement of the Rights Offering is delayed for a period of time, the Expiration Date may be similarly extended. We will extend the duration of the Rights Offering as required by applicable law, and may choose to extend the duration of the Rights Offering for any reason. We may extend the Expiration Date by giving oral or written notice to the subscription agent on or before the scheduled Expiration

Date. If we elect to extend the Expiration Date, we will issue a press release announcing such extension no later than 9:00 a.m. (New York City time) on the next business day after the most recently announced Expiration Date.

We reserve the right, in our sole discretion, to amend or modify the terms of the Rights Offering. We also reserve the right to terminate the Rights Offering at any time prior to the Expiration Date for any reason, in which event all funds received in connection with the Rights Offering will be returned without interest or deduction to those persons who exercised their Rights as soon as practicable.

If you do not indicate the number of Rights being exercised, or do not forward full payment of the total subscription price payment for the number of Rights that you indicate are being exercised, then you will be deemed to have exercised your Rights with respect to the maximum number of whole Units that may be exercised with the aggregate subscription price payment you delivered to the subscription agent. If your aggregate subscription price payment is greater than the amount you owe for exercise of your basic subscription right in full, you will be deemed to have exercised your over-subscription privilege to purchase the maximum number of Units with your over-payment.

If an insufficient number of Units is available to fully satisfy all over-subscription privilege requests, the available Units will be distributed proportionately among rights holders who exercise their over-subscription privilege based on the number of Units each rights holder subscribed for under the basic subscription right. The proration process will be repeated until all Units have been allocated or all over-subscription exercises have been fulfilled, whichever occurs earlier. Any excess subscription payments received by the subscription agent caused by proration will be returned by the subscription agent to you by mail, without interest or penalty, as soon as practicable after the Expiration Date of the Rights Offering. The subscription agent will return any excess payments in the form in which it was made. You will not receive a refund or other compensation for any unissued fractional Warrants.

Rights holders will only be entitled to purchase a whole number of Units representing a whole number of shares of common stock, rounded down to the nearest whole number of Units a holder would otherwise be entitled to purchase. As we will not sell fractional Units, we will not issue fractional shares of common stock or Warrants in the Rights Offering. The number of shares of common stock and Warrants issued to you shall correspond to the number of Units issued to you (and, for the avoidance of doubt, the number of Warrants issued to you will be rounded down to the nearest whole number).

The excess amount for any fractional Units will be returned to you as soon as practicable, in the form in which made. You will not receive interest or a deduction on any payments refunded to you under the Rights Offering. No subscriber will receive a refund or other compensation for any unissued fractional Units.

The common stock and Warrants comprising the Units will separate upon the closing of this Rights Offering and will be issued separately; however, they may only be purchased as a Unit and the Unit will not trade as a separate security. Holders may not recombine shares of common stock and Warrants to receive a Unit.

The closing of the Rights Offering is subject to the satisfaction or waiver of certain closing conditions that are outside of our control, including: (i) the absence of prohibitions or enjoinments by any court of competent jurisdiction and (ii) the substantially concurrent closing of the JV Transaction. We may waive any of these conditions and choose to proceed with the Rights Offering even if one or more of these events occur. In addition, we do not intend, but have the right, to terminate the Rights Offering at any time prior to the Expiration Date. If we terminate the Rights Offering, in whole or in part, all affected Rights will expire without value and all subscription payments in the form in which received by the subscription agent will be returned in the form in which paid, without interest or deduction, as soon as practicable. See also “—Expiration of the Rights Offering and Extensions, Amendments and Termination.”

The exercise of Rights is irrevocable and may not be cancelled or modified. Your Rights will not be considered exercised unless the subscription agent receives from you, your custodian bank, broker, dealer or nominee, as the

case may be, all of the required documents properly completed and executed and your full subscription price payment in cash, as provided herein, prior to the Expiration Date, which is currently set to be 5:00 p.m. (New York City time) on July 21, 2026 unless we, in our sole discretion, extend the period for exercising the Rights or terminate it earlier. Rights holders may exercise their rights as follows:

Rights holders who are registered holders of our common stock or Participating Warrants as of the Record Date may exercise their Right by properly completing and executing the rights certificate together with any required signature guarantees and forwarding it, together with payment in full, as provided herein, of the subscription price for each Unit for which they subscribe, to the subscription agent at the address set forth under the subsection titled “—Delivery of Subscription Materials and Payment,” on or prior to the Expiration Date.

Banks, trust companies, securities dealers and brokers (each, a “Nominee”) that hold shares of our common stock on the Record Date as nominee for more than one beneficial owner may, upon proper showing to the subscription agent, exercise such beneficial owner’s Right through DTC on the same basis as if the beneficial owners were stockholders on the Record Date. Such Nominee may exercise the Right on behalf of the exercising beneficial owner through DTC’s PSOP Function on the “agents subscription over PTS” procedure by (1) providing a certification as to the aggregate number of Rights exercised by the beneficial owner on whose behalf such Nominee is acting, and (2) instructing DTC to charge the Nominee’s applicable DTC account for the subscription payment for the new Units to facilitate the delivery of the full subscription payment to the subscription agent. DTC must receive the subscription instructions and payment for the new Units no later than the Expiration Date.

Rights holders who are beneficial owners of shares of our common stock as of the Record Date and whose shares are registered in the name of a custodian bank, broker, dealer or other nominee, or would prefer to have an institution conduct the transaction relating to the rights on their behalf, should instruct their custodian bank, broker, dealer or other nominee or institution to exercise their rights and deliver all documents and payment, on their behalf, prior to the Expiration Date. A rights holder’s Rights will not be considered exercised unless the subscription agent receives from such rights holder or the rights holder’s custodian bank, broker, dealer, or other nominee or institution, as the case may be, all of the required documents and such holder’s full subscription price payment.

If you subscribe for fewer than all of the Units that you are eligible to purchase pursuant to the basic subscription privilege represented by your rights certificate, you may, under certain circumstances, request from the subscription agent a new rights certificate representing the unused Rights and then attempt to sell your unused Rights. See “—Method of Transferring and Selling Rights” below. Alternatively, you may transfer a portion of your Rights and request from the subscription agent a new rights certificate representing the Rights you did not transfer. If you exercise less than all of your Rights represented by a single rights certificate, you may not exercise the over-subscription privilege.

You must timely pay the full subscription price, in U.S. currency, for the full number of Units at the Subscription Price you wish to acquire pursuant to the exercise of rights (including any exercise of the over-subscription privileges, if available) by delivering:

Rights certificates received after the Expiration Date of the Rights Offering will not be honored, and we will return your payment to you in the form received as soon as practicable, without interest or deduction.

The subscription agent will be deemed to receive payment upon:

If you are paying by uncertified personal check, please note that payment will not be deemed to have been received by the subscription agent until the check has cleared, which could take at least five (5) business days. Any personal check used to pay for Units must clear the appropriate financial institutions prior to 5:00 p.m. (New York City time) on July 21, 2026, the expected Expiration Date, unless we, in our sole discretion, extend the period for exercising the Rights or terminate it earlier. Accordingly, holders that wish to pay the subscription price by means of an uncertified personal check are urged to make payment sufficiently in advance of the expiration of the Rights Offering to ensure such payment is received and clears by such date. If you elect to exercise your Rights, we urge you to consider using a certified check, cashier’s check, bank draft, U.S. Postal money order, or wire transfer of funds to ensure that the subscription agent receives your funds prior to the expiration of the Rights Offering.

You should read the instruction letter accompanying the rights certificate carefully and strictly follow it. DO NOT SEND RIGHTS CERTIFICATES OR PAYMENTS TO THE COMPANY. We will not consider your subscription received until the subscription agent has received delivery of a properly completed and duly executed rights certificate and payment of the full subscription amount. The risk of delivery of all documents and payments is on you or your nominee, not us or the subscription agent.

The method of delivery of rights certificates and payment of the subscription amount to the subscription agent will be at the risk of the holders of rights, but, if sent by mail, we recommend that you send those certificates and payments by overnight courier or by registered mail, properly insured, with return receipt requested, and that a sufficient number of days be allowed to ensure delivery to the subscription agent and clearance of payment before the expiration of the subscription period.

Unless a rights certificate provides that the shares of common stock and Warrants are to be delivered to the record holder of such rights or such certificate is submitted for the account of a bank or a broker, signatures on such rights certificate must be guaranteed by an “Eligible Guarantor Institution,” as such term is defined in Rule 17Ad-15 of the Securities Exchange Act of 1934 (an “Eligible Institution”), subject to any standards and procedures adopted by the subscription agent. See “—Medallion Guarantee May Be Required.”

If you completed any part of the Rights certificate to provide that the common stock and Warrants that comprise the Units purchased pursuant to your exercise of Rights were to be (x) issued in a name other than that of the registered holder, or (y) issued to an address other than that shown on the front of the Rights certificate, your signature on each Rights certificate must be guaranteed by an Eligible Institution, such as a member firm of a registered national securities exchange or a member of the Financial Industry Regulatory Authority, Inc., or a commercial bank or trust company having an office or correspondent in the United States, or by a member of a Stock Transfer Association approved medallion program such as STAMP, SEMP or MSP subject to standards and procedures adopted by the subscription agent.

The subscription agent for this Rights Offering is Broadridge Corporate Issuer Solutions, LLC. We will pay all fees and expenses of Broadridge Corporate Issuer Solutions, LLC related to the Rights Offering and have also

agreed to indemnify Broadridge Corporate Issuer Solutions, LLC from certain liabilities that it may incur in connection with the Rights Offering. Broadridge Corporate Issuer Solutions, LLC can be contacted at the following address and telephone number:

The information agent for this Rights Offering is Sodali & Co. We will pay all fees and expenses of Sodali & Co. related to the Rights Offering and have also agreed to indemnify Sodali & Co. from certain liabilities that it may incur in connection with the Rights Offering. Sodali & Co. can be contacted at the following address, telephone number and e-mail address:

We expect to list the Rights on Nasdaq on July 6, 2026 under the symbol “EOSER.” We expect that Rights may be purchased or sold through usual investment channels until the close of business on July 21, 2026. However, there will not be a public market for the Rights prior to the rights distribution date. There can be no assurance that an active trading market will develop or be sustained for the Rights following the rights distribution. We also cannot assure you of the prices at which the Rights will trade, if at all. If you do not exercise or sell your Rights you will lose any value inherent in the Rights, respectively. See Risk Factors.”

You may transfer Rights in whole by endorsing the rights certificate for transfer. Please follow the instructions for transfer included in the information sent to you with your rights certificate. If you wish to transfer only a portion of the rights, you should deliver your properly endorsed rights certificate to the subscription agent. With your rights certificate, you should include instructions to register such portion of the rights evidenced thereby in the name of the transferee (and to issue a new rights certificate to the transferee evidencing such transferred rights). You may only transfer whole rights and not fractions of a right. If there is sufficient time before the expiration of the Rights Offering, the subscription agent will send you a new rights certificate evidencing the balance of the rights issued to you but not transferred to the transferee. You may also instruct the subscription agent to send the rights certificate to one or more additional transferees. If you wish to sell your remaining rights, you may request that the subscription agent send you certificates representing your remaining (whole) rights so that you may sell them through your broker or dealer. You may also request that the subscription agent sell your rights for you, as described below.

If you wish to transfer all or a portion of your rights, you must notify the subscription agent on or before 11:00 a.m. New York City time on the fifth business day before the Expiration Date for the subscription agent to:

If you wish to transfer your rights to any person other than a bank or broker, the signatures on your rights certificate must be guaranteed by an eligible institution.

If you choose not to sell your rights through your broker or dealer, you may seek to sell your rights through the subscription agent. If you wish to have the subscription agent seek to sell your rights, you must deliver your properly executed rights certificate, with appropriate instructions, and a properly completed and executed Internal Revenue

Service Form W-8 or W-9, as applicable, to the subscription agent. If you want the subscription agent to seek to sell only a portion of your rights, you must send the subscription agent instructions setting forth what you would like done with the rights, along with your rights certificate and a properly completed and executed Internal Revenue Service Form W-8 or W-9, as applicable.

If the subscription agent sells rights for you, it will send you a check for the net proceeds from the sale of any of your rights, reduced by any applicable tax withholding (including backup withholding), as soon as practicable after the Expiration Date. If your rights can be sold, the sale will be deemed to have been made at the weighted average net sale price of all rights sold by the subscription agent. The aggregate fees charged by the subscription agent for selling rights will be deducted from the aggregate sale price for all such rights in determining the weighted average net sale price of all such rights. We cannot assure you, however, that a market will develop for the Rights, or that the subscription agent will be able to sell your Rights.

The subscription agent must have received your order to sell your rights on or before 11:00 a.m., New York City time, on the fifth business day before the Expiration Date. If less than all sales orders received by the subscription agent are filled, it will prorate the sales proceeds among you and the other holders of rights based on the number of rights that each holder has instructed the subscription agent to sell during that period, irrespective of when during the period the instructions are received by it. The subscription agent is required to sell your rights only if it is able to find buyers. If the subscription agent cannot sell your Rights by 5:00 p.m., New York City time, on the fourth business day before the Expiration Date, the subscription agent will return your rights certificate to you by overnight delivery.

If you sell your rights through your broker or dealer, you will likely receive a different amount of proceeds than if you sell the same amount of rights through the subscription agent. If you sell your rights through your broker or dealer instead of the subscription agent, your sales proceeds will be the actual sales price of your rights rather than the weighted average sales price described above.

The amount of time needed by your transferee to exercise or sell its rights depends upon the method by which the transferor delivers the rights certificates, the method of payment made by the transferee and the number of transactions which the holder instructs the subscription agent to effect. You should also allow up to ten business days for your transferee to exercise or sell the rights transferred to it. Neither we nor the subscription agent will be liable to a transferee or transferor of rights if rights certificates or any other required documents are not received in time for exercise or sale prior to 5:00 p.m. (New York City time) on the Expiration Date.

You will receive a new rights certificate upon a partial exercise, transfer or sale of rights only if the subscription agent receives your properly endorsed rights certificate no later than 11:00 a.m., New York City time, on the fifth business day before the Expiration Date. If your instructions and rights certificate are received by the subscription agent after that time and date, you will not receive a new rights certificate and therefore will not be able to sell or exercise your remaining rights.

You are responsible for all commissions, fees and other expenses (including brokerage commissions and transfer taxes) incurred in connection with the purchase, sale or exercise of your rights, except that we will pay any fees of the subscription agent associated with the exercise of rights. Any amounts you owe will be deducted from your account.

If you do not exercise or sell your Rights before 5:00 p.m. (New York City time) on the Expiration Date, your Rights will expire and will no longer be exercisable; and thus, you will lose any value inherent in the Rights.

You should deliver your Rights certificate and payment of the subscription price, as provided herein, or, if applicable, nominee holder certifications, to the subscription agent by one of the methods described below:

Your delivery to an address or by any method other than as set forth above will not constitute valid delivery and we may not honor the exercise of your Rights.

You should direct any questions or requests for assistance concerning the method of subscribing for Units or for additional copies of this prospectus to the information agent.

Broadridge Corporate Issuer Solutions, LLC, the subscription agent, will hold funds received in payment for Units in a segregated account pending completion of the Rights Offering. The subscription agent will hold this money until the Rights Offering is completed or is withdrawn or terminated. If the Rights Offering is canceled for any reason, all subscription payments received by the subscription agent will be returned to subscribers, without interest or penalty, as soon as practicable.

There is no guaranteed delivery period in connection with this Rights Offering, so you must ensure that you properly complete all required steps prior to 5:00 p.m. (New York City time) on July 21, 2026, unless we, in our sole discretion, extend the period for exercising the Rights or terminate it earlier.

If you are a broker, a trustee or a depositary for securities who holds shares of our common stock for the account of others as of the Record Date, you should notify the respective beneficial owners of such shares of the Rights Offering as soon as possible to find out their intentions with respect to exercising their Rights. You should obtain instructions from the beneficial owners with respect to their Rights, as set forth in the instructions we have provided to you for your distribution to beneficial owners. If a beneficial owner so instructs, you should complete the appropriate Rights certificates and submit them to the subscription agent with the proper payment. If you hold shares of our common stock for the account(s) of more than one beneficial owner, you may exercise the number of Rights to which all such beneficial owners in the aggregate otherwise would have been entitled had they been direct record holders of our common stock or Participating Warrants on the Record Date, provided that you, as a nominee record holder, make a proper showing to the subscription agent by submitting the form entitled “Nominee Holder Certification” substantially in the form accompanying this prospectus supplement. If you did not receive this form, you should contact the subscription agent to request a copy.

If you are a beneficial owner of shares of our common stock or will receive Rights through a custodian bank, broker, dealer or other nominee, we will ask your custodian bank, broker, dealer or other nominee to notify you of the Rights Offering. If you wish to exercise your Rights, you will need to have your custodian bank, broker, dealer or other nominee act for you. If you hold shares of our common stock directly under your name in stock certificate(s) or in book-entry, or uncertificated, form, but would prefer to have your custodian bank, broker, dealer or other nominee act for you, you should contact your nominee and request it to effect the transactions for you. Your nominee may establish a deadline prior to the Expiration Date by which you must provide it with your instructions to exercise your Rights and payment for your shares.

To indicate your decision with respect to your Rights, you should complete and return to your custodian bank, broker, dealer or other nominee the form entitled “Beneficial Owners Election Form” substantially in the form accompanying this prospectus supplement. You should receive the “Beneficial Owners Election Form” from your custodian bank, broker, dealer or other nominee with the other Rights Offering materials. If you wish to obtain a separate Rights certificate, you should contact the nominee as soon as possible and request that a separate Rights certificate be issued to you. You should contact your custodian bank, broker, dealer or other nominee if you do not receive this form but you believe you are entitled to participate in the Rights Offering. We are not responsible if you do not receive this form from your custodian bank, broker, dealer or nominee or if you receive it without sufficient time to respond.

We will decide all questions concerning the timeliness, validity, form and eligibility of the exercise of your Rights and any such determinations by us will be final and binding. We, in our sole discretion, may waive, in any particular instance, any defect or irregularity, or permit, in any particular instance, a defect or irregularity to be

corrected within such time as we may determine. We will not be required to make uniform determinations in all cases. We may reject the exercise of any of your Rights because of any defect or irregularity. We will not accept any exercise of Rights until all irregularities have been waived by us or cured by you within such time as we decide, in our sole discretion. Our interpretations of the terms and conditions of the Rights Offering will be final and binding. Neither we, nor the subscription agent, will be under any duty to notify you of any defect or irregularity in connection with your submission of Rights certificates and we will not be liable for failure to notify you of any defect or irregularity. We reserve the right to reject your exercise of Rights if your exercise is not in accordance with the terms of the Rights Offering or in proper form. We will also not accept the exercise of your Rights if our sale of Units to you could be deemed unlawful under applicable law.

Once you submit the form of rights certificate to exercise any Rights, you may not revoke or change your exercise or request a refund of monies paid. All exercises of rights are irrevocable, even if you subsequently learn information about us that you consider to be unfavorable. You should not exercise your rights unless you are certain that you wish to purchase additional Units at the Subscription Price.

All shares of our common stock and Warrants that you purchase in the Rights Offering as part of the Units will be issued in book-entry, or uncertificated, form, meaning that you will receive a direct registration (DRS) account statement from our transfer agent reflecting ownership of these securities if you are a holder of record of shares or Warrants. If you hold your shares of common stock in the name of a custodian bank, broker, dealer, or other nominee, DTC will credit your account with your nominee with the securities you purchased in the Rights Offering. Subject to state securities laws and regulations, we have the discretion to delay distribution of any securities you may have elected to purchase by exercise of your rights in order to comply with state securities laws.

We will resolve all questions regarding the validity and form of the exercise of your Rights, including time of receipt and eligibility to participate in the Rights Offering. Our determination will be final and binding. Once made, subscriptions and directions are irrevocable, and we will not accept any alternative, conditional or contingent subscriptions or directions. We reserve the absolute right to reject any subscriptions or directions not properly submitted or the acceptance of which would be unlawful. You must resolve any irregularities in connection with your subscriptions before the subscription period expires, unless waived by us in our sole discretion. Neither the subscription agent nor we shall be under any duty to notify you or your representative of defects in your subscriptions. A subscription will be considered accepted, subject to our right to modify or cancel the Rights Offering, only when a properly completed and duly executed Rights certificate and any other required documents and payment of the full subscription amount have been received by the subscription agent (and any payment by uncertified personal check has cleared) and any defects or irregularities therein waived by us. Our interpretations of the terms and conditions of the Rights Offering will be final and binding.

You will have no rights as a holder of the shares of our common stock you purchase in the Rights Offering until shares are issued in book-entry form or your account at your broker, dealer, bank or other nominee is credited with the shares of our common stock purchased in the Rights Offering. Holders of Warrants issued in connection with the Rights Offering will not have rights as holders of our common stock until such Warrants are exercised and the shares of common stock underlying the Warrants are issued to the holder. You will have no right to revoke your subscriptions after you deliver your completed rights certificate, subscription payment, as provided herein, and any other required documents to the subscription agent.

The subscription agent will not mail rights certificates to you if you are a stockholder whose address is outside the United States or if you have an Army Post Office or a Fleet Post Office address. Instead, we will have the subscription agent hold the Rights certificates for your account. To exercise your rights, you must notify the

subscription agent prior to 11:00 a.m. (New York City time) at least five (5) business days prior to the Expiration Date, and establish to the satisfaction of the subscription agent that it is permitted to exercise your Rights under applicable law. If you do not follow these procedures by such time, your rights will expire and will have no value.

An investment in Units (and the common stock and respective Warrants that comprise such Units) must be made according to your evaluation of your own best interests and after considering all of the information herein, including the section titled “Risk Factors.” Neither we nor our board of directors are making any recommendation regarding whether you should exercise your Rights.

The number of shares of our common stock that will be outstanding after the Rights Offering will depend on the number of Units that are purchased in the Rights Offering. Assuming no additional shares of common stock are issued by us prior to consummation of the Rights Offering and assuming all offered Units are sold in the Rights Offering at the Subscription Price, we will issue 27,367,171 shares of common stock. In that case, we will have approximately 381,642,806 shares of common stock outstanding after the Rights Offering. This would represent an increase of approximately 7.7% in the number of outstanding shares of common stock. We would also issue Warrants to purchase an aggregate of an additional 12,008,714 shares of our common stock.

Neither we, nor the subscription agent, will charge a brokerage commission or a fee to Rights holders for exercising their rights. However, if you exercise your Rights through a custodian bank, broker, dealer or nominee, you will be responsible for any fees charged by your custodian bank, broker, dealer or nominee.

If you have any questions or require assistance regarding the method of exercising your Rights or requests for additional copies of this document or any document mentioned herein, you should contact the subscription agent at the address and telephone number set forth above under “—Delivery of Subscription Materials and Payment.”

The Rights Offering is not a transaction or series of transactions which has either a reasonable likelihood or a purpose of producing a “going private effect” as specified in Rule 13e-3 of the Exchange Act. Given the structure of the Rights Offering, as described in this prospectus supplement, Eos will continue to be registered pursuant to Section 12 of the Exchange Act and intends to remain listed on Nasdaq following completion of the Rights Offering.

Eos is not making the Rights Offering in any state or other jurisdiction in which it is unlawful to do so, nor is Eos distributing or accepting any offers to purchase any of our securities from Rights holders who are residents of those states or of other jurisdictions or who are otherwise prohibited by federal or state laws or regulations to accept or exercise the Rights. Eos may delay the commencement of the Rights Offering in those states or other jurisdictions, or change the terms of the Rights Offering, in whole or in part, in order to comply with the securities law or other legal requirements of those states or other jurisdictions. Subject to state securities laws and regulations, Eos also has the discretion to delay allocation and distribution of any securities you may elect to purchase by exercise of your Rights in order to comply with state securities laws. Eos may decline to make modifications to the terms of the Rights Offering requested by those states or other jurisdictions, in which case, if you are a resident in one of those states or jurisdictions or if you are otherwise prohibited by federal or state laws or regulations from accepting or exercising the Rights you will not be eligible to participate in the Rights Offering.

The following discussion is a summary of the material U.S. federal income tax consequences of the receipt, exercise (or expiration) and disposition of Rights received in the Rights distribution, the ownership and disposition of Warrants received in respect of such Rights, and the ownership and disposition of New Shares received in respect of such Rights and shares of our common stock received upon the exercise of such Warrants (the “Warrant Shares” and, the Warrant Shares together with such New Shares, the “Offering Shares”). This discussion does not purport to be a complete analysis of all potential tax effects. The effects of other U.S. federal tax laws, such as estate and gift tax laws, and any applicable state, local or non-U.S. tax laws are not discussed. This discussion is based on the U.S. Internal Revenue Code of 1986, as amended (the “Code”), Treasury Regulations promulgated thereunder, judicial decisions, and published rulings and administrative pronouncements of the IRS, in each case in effect as of the date hereof. These authorities may change or be subject to differing interpretations. Any such change or differing interpretation may be applied retroactively in a manner that could adversely affect a holder of Rights, Warrants or Offering Shares. We have not sought and will not seek any rulings from the IRS regarding the matters discussed below. There can be no assurance the IRS or a court will not take a contrary position to that discussed below regarding the tax consequences of the receipt of Rights by persons holding shares of our common stock entitled to receive Rights in the Rights distribution, the exercise (or expiration) of such Rights or the Warrants, the disposition of such Rights or Warrants, and the ownership and disposition of Offering Shares acquired upon exercise of such Rights or Warrants.

This discussion is limited to Rights, Warrants and Offering Shares, in each case, that are held as a “capital asset” within the meaning of Section 1221 of the Code (generally, property held for investment). This discussion does not address all U.S. federal income tax consequences relevant to a holder’s particular circumstances, including the impact of the alternative minimum tax or the Medicare contribution tax on net investment income. In addition, it does not address consequences relevant to holders subject to special rules, including, without limitation:

If an entity treated as a partnership for U.S. federal income tax purposes receives Rights in the Rights distribution, the tax treatment of a partner in the partnership will depend on the status of the partner, the activities of the partnership and certain determinations made at the partner level. Accordingly, partnerships and the partners in such partnerships should consult their tax advisors regarding the U.S. federal income tax consequences to them.

This discussion is for information purposes only and is not tax advice. Investors should consult their tax advisors with respect to the application of U.S. federal income tax laws to their particular situations as well as any tax consequences of the receipt, ownership, exercise and disposition of Rights, Warrants or Offering Shares arising under the U.S. federal estate or gift tax laws or under the laws of any state, local or non-U.S. taxing jurisdiction or under any applicable income tax treaty.

The initial exercise price of the Warrants may reflect a discount to the trading price of our common stock as of the date the Warrants are issued. Although not entirely free from doubt, we believe (and intend to take the position) that the Warrants should be treated for U.S. federal income tax purposes as rights to acquire our common stock, rather than being deemed exercised at issuance or considered a separate class of our stock, and the remainder of this discussion assumes that treatment is proper. If the Warrants were treated as having been exercised at issuance or as a separate class of our stock, the U.S. federal income tax consequences of the ownership, exercise and disposition of the Warrants may differ from those described below. You should consult your tax advisor with respect to the proper treatment of the Warrants for U.S. federal income tax purposes.

For purposes of this discussion, a “U.S. Holder” is any beneficial owner of Rights, Warrants or Offering Shares, as the case may be, that, for U.S. federal income tax purposes, is or is treated as any of the following:

Although the authorities governing transactions such as this offering are complex and unclear in certain respects (including with respect to the effects of the over-subscription privilege), we believe and intend to take the position that a U.S. Holder’s receipt of Rights pursuant to this offering should not be treated as a taxable distribution with respect to such holder’s existing shares of our common stock for U.S. federal income tax purposes. Section 305(a) of the Code generally provides that the receipt by a stockholder of a right to acquire stock or warrants is not included in the taxable income of the stockholder; however, the general non-recognition rule in Section 305(a) of the Code is subject to exceptions described in Section 305(b) of the Code, which include “disproportionate distributions.” A disproportionate distribution is generally a distribution or a series of distributions, including deemed distributions, that has the effect of the receipt of cash or other property by some stockholders (including holders of rights to acquire stock and holders of debt instruments convertible into stock) and an increase in the proportionate interest of other stockholders (including holders of rights to acquire stock and holders of debt instruments convertible into stock) in a corporation’s assets or earnings and profits.

We have not made any distributions of cash or property (other than stock or rights to acquire stock) with respect to our outstanding stock or warrants and, although there can be no assurance in this regard, we currently do not intend to make any distributions of cash or property (other than stock or rights to acquire stock) with respect to our outstanding stock or warrants in the foreseeable future. However, we have made, and expect to continue to make, regular payments of interest with respect to our outstanding convertible notes, which are treated as distributions of cash or other property to stockholders for this purpose. Although the matter is not free from doubt, we believe, and intend to take the position that, anti-dilution adjustments required to be made in respect of our convertible notes, shares of preferred stock and certain of our outstanding warrants should prevent the Rights distribution from constituting a disproportionate distribution to our holders of common stock for purposes of Section 305(b) of the Code.

This position regarding the non-taxable treatment of this offering is not binding on the IRS or the courts. If this position is finally determined by the IRS or a court to be incorrect, whether because, contrary to our expectations, distributions of cash or property (other than stock or rights to acquire stock) are made with respect to our stock or warrants, because the issuance of the Rights is a “disproportionate distribution” or for any other reason, the fair market value of the Rights would be taxable to U.S. Holders of our Common Stock generally in the manner as described below under “—Distributions on Common Stock,” and a U.S. Holder’s initial tax basis in a Right would be equal to the Right’s fair market value on the distribution date. Although we currently do not expect to have current or accumulated earnings and profits through the end of 2026, there can be no assurance in that regard.

The remainder of this discussion is based upon the treatment of the Rights distribution as a non-taxable distribution with respect to a U.S. Holder’s existing shares of our common stock for U.S. federal income tax purposes.

If, on the date a U.S. Holder receives Rights, the fair market value of the Rights is less than 15% of the fair market value of the U.S. Holder’s existing shares of our common stock with respect to which the Rights were distributed, Section 307(b) of the Code provides that the Rights will be allocated a zero tax basis for U.S. federal income tax purposes, unless the U.S. Holder elects to allocate the tax basis in the holder’s existing shares of our common stock between the existing shares of our common stock and the Rights in proportion to the relative fair market values of the existing shares of our common stock and the Rights, determined on the date of receipt of the Rights. If a U.S. Holder chooses to allocate tax basis between existing common shares and the Rights, the U.S. Holder must make this election on a statement included with its timely filed U.S. federal income tax return (including extensions) for the taxable year in which the U.S. Holder receives the Rights. Such an election is irrevocable.

However, if on the date a U.S. Holder receives Rights, the fair market value of the Rights is 15% or more of the fair market value of the U.S. Holder’s existing shares of our common stock, then the U.S. Holder must allocate tax basis in the existing shares of our common stock between those shares and the Rights the U.S. Holder receives in proportion to their fair market values, determined on the date the U.S. Holder receives the Rights. Please refer to the discussion below regarding the U.S. tax treatment of a U.S. Holder that, at the time of the receipt of a Right, no longer holds the shares of our common stock with respect to which the Right was distributed.

We have not obtained, and do not intend to obtain, an appraisal of the fair market value of the Rights on that Rights distribution date. However, as discussed above, it is expected that the Rights will be traded on Nasdaq on that date.

A U.S. Holder’s holding period in the Rights it receives will include the holding period of the U.S. Holder’s existing shares of our common stock with respect to which the Rights are distributed.

A U.S. Holder will not recognize gain or loss upon the exercise of a Right received in this offering. A U.S. Holder’s adjusted tax basis, if any, in the Right plus the subscription price will establish the U.S. Holder’s initial tax basis in the Units received upon exercise of such U.S. Holder’s Right. The U.S. Holder generally will be required to allocate its initial tax basis in those Units among the New Shares and Warrants comprising the Units, based on their relative fair market values on the exercise date for the Right. We have not obtained, and do not intend to obtain an appraisal of the fair market value of the Warrants. It is unclear whether a U.S. Holder’s holding period for a New Share or a Warrant acquired upon exercise of a Right will begin on the date of exercise or the day following the date of exercise.

If, at the time of the receipt or exercise of a Right, the U.S. Holder no longer holds the shares of our common stock with respect to which the right was distributed, then certain aspects of the tax treatment of the receipt and exercise of the Right are unclear, including (i) the allocation of the tax basis between the shares of our common stock previously sold and the Right, (ii) the impact of such allocation on the amount and timing of gain or loss recognized with respect to the shares of our common stock previously sold, and (iii) the impact of such allocation on the tax basis of the New Shares and Warrants acquired upon exercise of the Right. Furthermore, if a U.S. Holder exercises the Rights and sells other shares of our common stock within the 61-day period beginning 30 days before the exercise date and ending 30 days after the exercise date, the “wash sale” rules may disallow the recognition of any loss upon the sale of our common stock. If a U.S. Holder exercises a Right after disposing of shares of our common stock with respect to which the Right is received, the U.S. Holder should consult its tax advisor.

If a U.S. Holder allows Rights received in the Rights distribution to expire, the U.S. Holder should not recognize any gain or loss for U.S. federal income tax purposes, and the U.S. Holder should re-allocate any portion of the tax basis in its existing common shares previously allocated to the Rights that have expired to such U.S. Holder’s existing common shares.

Upon a sale, exchange, or other taxable disposition of Rights received in the Rights distribution, a U.S. Holder generally will recognize capital gain or loss equal to the difference between the amount realized and the U.S. Holder’s adjusted tax basis, if any, in such Rights. Such capital gain or loss will be long-term capital gain or loss if the U.S. Holder’s holding period of the Rights exceeded one year at the time of disposition. Long-term capital gains recognized by certain non-corporate taxpayers, including individuals, generally are subject to reduced rates of taxation. The deductibility of capital losses is subject to limitations.

We do not anticipate declaring or paying cash distributions to holders of our common stock in the foreseeable future. However, if we do make distributions of cash or property on our common stock, such distributions will constitute dividends to the extent paid out of our current or accumulated earnings and profits, as determined for U.S. federal income tax purposes. Dividends received by a corporate U.S. Holder may be eligible for a dividends-received deduction, subject to applicable limitations. Dividends received by certain non-corporate U.S. Holders, including individuals, are generally taxed at the lower applicable capital gains rate, provided that certain holding period and other requirements are satisfied. Distributions in excess of our current and accumulated earnings and profits will constitute a return of capital and first be applied against and reduce a U.S. Holder’s adjusted tax basis in its common stock, as the case may be, but not below zero. Any excess will be treated as capital gain and will be treated as described below under “—Sale, Exchange or Other Disposition of Common Stock or Warrants.”

As described above under “Description of the Rights Offering—Warrants,” the terms of each Warrant provide for an adjustment to the number of shares of our common stock for which the Warrant may be exercised or to the exercise price of the Warrant on the occurrence of certain events. An adjustment which has the effect of preventing dilution generally is not a taxable event. U.S. Holders of the Warrants would, however, be treated as receiving a constructive distribution from us if, for example, the adjustment to the number of such shares or to such exercise price increases the Warrant holders’ proportionate interest in our assets or earnings and profits (e.g., through an increase in the number of shares of our common stock that would be obtained upon exercise or through a decrease in the exercise price of the Warrants), including as a result of a distribution of cash or other property to the holders of shares of our common stock which is taxable to such holders. Any constructive distribution deemed received by a U.S. Holder would be subject to tax in the same manner as if the U.S. Holder received a cash distribution from us equal to the fair market value of the increased interest resulting from the adjustment. Generally, a U.S. Holder’s adjusted tax basis in its Warrants would be increased to the extent any such constructive distribution is treated as a dividend.

Upon a sale, exchange, or other taxable disposition of our Offering Shares or Warrants (which, in general, would include a redemption of our warrants that is treated as a taxable exchange of such warrants as described below under “—Exercise, Lapse or Redemption of a Warrant”), a U.S. Holder generally will recognize capital gain or loss equal to the difference between the amount realized (not including any amount attributable to declared and unpaid dividends, which will be taxable to U.S. Holders who have not previously included such dividends in income as described above under “—Distributions on Common Stock”) and the U.S. Holder’s adjusted tax basis in the Offering Shares or Warrants, as the case may be. Such capital gain or loss will generally be long-term capital gain or loss if the U.S. Holder’s holding period for the Offering Shares or the Warrants, as the case may be, exceeded one year at the time of disposition. Long-term capital gains recognized by certain non-corporate U.S. Holders, including individuals, generally are subject to reduced rates of taxation. The deductibility of capital losses is subject to limitations.

The tax consequences of an exercise of a Warrant (which is permitted only on a cashless basis) are not clear under current tax law. An exercise may be tax-free, either because the exercise is not a realization event or because the exercise is treated as a recapitalization for U.S. federal income tax purposes. In either tax-free situation, a U.S. Holder’s tax basis in the Offering Shares received would equal such U.S. Holder’s basis in the Warrants. If the exercise were treated as not being a realization event, it is unclear whether a U.S. Holder’s holding period in the Offering Shares received would be treated as commencing on the date following the date of exercise or on the date of exercise of the Warrant. If the exercise were treated as a recapitalization, the holding period of the Offering Shares received would include the holding period of the Warrants exercised therefor.

It is also possible that the exercise of a Warrant could be treated in part as a taxable exchange in which gain or loss would be recognized. In such event, a portion of the Warrants to be exercised could, for U.S. federal income tax purposes, be deemed to have been surrendered in consideration for the exercise price of the remaining Warrants, which would be deemed to be exercised. For this purpose, a U.S. Holder would be deemed to have surrendered a number of Warrants having an aggregate value equal to the exercise price for the number of Warrants deemed exercised. A U.S. Holder would recognize capital gain or loss in an amount equal to the difference between the exercise price of the Warrants deemed exercised and such U.S. Holder’s tax basis in the Warrants deemed surrendered. Such gain or loss would be long-term or short-term depending on the U.S. Holder’s holding period in the Warrants deemed surrendered. In this case, a U.S. Holder’s tax basis in the Offering Shares received would equal the sum of such U.S. Holder’s initial investment in the Warrants deemed exercised and the exercise price of such Warrants. It is unclear whether a U.S. Holder’s holding period for the Offering Shares received would commence on the date following the date of exercise or on the date of exercise of the Warrant; in either case, the holding period would not include the period during which the U.S. Holder held the Warrant.

Due to the absence of authority on the U.S. federal income tax treatment of the exercise of a Warrant on a cashless basis, including when a U.S. Holder’s holding period would commence with respect to the Offering Shares received, there can be no assurance as to which, if any, of the alternative tax consequences and holding periods described above would be adopted by the IRS or a court of law. Accordingly, U.S. Holders should consult their own tax advisors regarding the tax consequences of an exercise of their Warrants.

If a Warrant lapses unexercised, a U.S. Holder generally will recognize a capital loss equal to such holder’s tax basis in the Warrant.

If we redeem Warrants for cash or if we purchase Warrants in an open market transaction, such redemption or purchase generally will be treated as a taxable disposition to U.S. Holders, taxed as described above under “—Sale, Exchange or Other Disposition of Common Stock or Warrants.”

If we give notice of an intention to redeem Warrants and a U.S. Holder exercises its Warrants, receiving an amount of Offering Shares in respect thereof, we intend to treat such exercise as a redemption of Warrants for the Offering Shares for U.S. federal income tax purposes. Such redemption should be treated as a “recapitalization” within the meaning of Section 368(a)(1)(E) of the Code. Accordingly, a U.S. Holder should not recognize any gain or loss on the redemption of Warrants for shares of our common stock. A U.S. Holder’s aggregate tax basis in the Offering Shares received in the redemption generally should equal its aggregate tax basis in the Warrants redeemed and the holding period for the Offering Shares received should include its holding period for the surrendered Warrants. However, there is some uncertainty regarding this tax treatment and it is possible such a redemption could be treated differently, including as, in part, a taxable exchange in which gain or loss would be recognized in a manner similar to that discussed above for an exercise of Warrants in the absence of a notice of our intention to redeem Warrants. Accordingly, U.S. Holders are urged to consult their own tax advisors regarding the tax consequences of a redemption of Warrants for shares of our common stock.

A U.S. Holder may be subject to information reporting and backup withholding with respect to the receipt of dividend payments (including constructive dividends) or proceeds from the sale or other taxable disposition of Rights, Warrants or Offering Shares. Certain U.S. Holders are exempt from backup withholding, including corporations and certain tax-exempt organizations. A U.S. Holder will be subject to backup withholding unless such holder provides the holder’s taxpayer identification number and otherwise complies with applicable requirements of the backup withholding rules or the holder provides proof of an applicable exemption.

Backup withholding is not an additional tax. Any amounts withheld under the backup withholding rules may be allowed as a refund or a credit against a U.S. Holder’s U.S. federal income tax liability, provided the required information is timely furnished to the IRS. U.S. Holders should consult their tax advisors regarding their qualification for an exemption from backup withholding and the procedures for obtaining such an exemption.

For purposes of this discussion, a “Non-U.S. Holder” is any beneficial owner of Rights, Warrants or Offering Shares, as the case may be, that is neither a U.S. Holder nor an entity treated as a partnership for U.S. federal income tax purposes.

The discussion assumes that the receipt of the Rights will be treated as a nontaxable distribution. See “—Tax Considerations Applicable to U.S. Holders—Receipt of the Rights” above. In such case, Non-U.S. Holders will not be subject to U.S. federal income tax (or any withholding thereof) on the receipt, exercise or expiration of the Rights. Otherwise, the fair market value of the Rights would be treated as a potentially taxable distribution on our existing common stock, generally in the manner as described below under “—Distributions on Common Stock.” Although we currently do not expect to have current or accumulated earnings and profits through the end of 2026, there can be no assurance in that regard.

The U.S. federal income tax treatment of the exercise of a Warrant by a Non-U.S. Holder, the lapse of a Warrant held by a Non-U.S. Holder or the redemption of a Warrant held by a Non-U.S. Holder generally will correspond to the U.S. federal income tax treatment of the exercise, lapse or redemption of a Warrant held by a U.S. Holder, as described under “Tax Considerations Applicable to U.S. Holders—Exercise, Lapse or Redemption of a Warrant” above, although to the extent the exercise or redemption of a Warrant results in a taxable exchange, the consequences would be similar to those described under “Tax Considerations Applicable to Non-U.S. Holders—Sale or Other Disposition of Rights, Warrants or Common Stock” below.

We do not anticipate declaring or paying cash distributions to holders of our common stock in the foreseeable future. However, if we do make distributions of cash or property on our common stock, such distributions will constitute dividends for U.S. federal income tax purposes to the extent paid from our current or accumulated earnings and profits, as determined under U.S. federal income tax principles. Amounts not treated as dividends for U.S. federal income tax purposes will constitute a return of capital and first be applied against and reduce a Non-U.S. Holder’s adjusted tax basis in its Offering Shares, but not below zero. Any excess will be treated as capital gain and will be treated as described below under “—Sale, Exchange or Other Disposition of Common Stock or Warrants.”

Subject to the discussion below on effectively connected income, dividends paid to a Non-U.S. Holder (including deemed dividends as described above under “Tax Considerations Applicable to U.S. Holders—Possible Constructive Distributions”) will be subject to U.S. federal withholding tax at a rate of 30% of the gross amount of the dividends (or such lower rate specified by an applicable income tax treaty), provided that the Non-U.S. Holder furnishes a valid IRS Form W-8BEN or W-8BEN-E (or other applicable documentation) claiming such benefits. In the case of any constructive dividend, it is possible that the U.S. federal withholding tax on the constructive dividend would be withheld from or set off against other payments to which you would otherwise be entitled, such as, in some circumstances, any payments on our common stock, or sales proceeds payable to you or your other funds or assets.

A Non-U.S. Holder that does not timely furnish the required documentation, but that qualifies for a reduced treaty rate, may obtain a refund of any excess amounts withheld by timely filing an appropriate claim for refund with the IRS. Non-U.S. Holders should consult their tax advisors regarding their entitlement to benefits under any applicable income tax treaty.

If dividends paid to a Non-U.S. Holder are effectively connected with the Non-U.S. Holder’s conduct of a trade or business within the United States (and, if required by an applicable income tax treaty, the Non-U.S. Holder maintains a permanent establishment in the United States to which such dividends are attributable), the Non-U.S.

Holder will be exempt from the U.S. federal withholding tax described above. To claim the exemption, the Non-U.S. Holder must furnish to the applicable withholding agent a valid IRS Form W-8ECI, certifying that the dividends are effectively connected with the Non-U.S. Holder’s conduct of a trade or business within the United States.

Any such effectively connected dividends will be subject to U.S. federal income tax on a net income basis at the regular graduated U.S. federal income tax rates. A Non-U.S. Holder that is a corporation also may be subject to a branch profits tax at a rate of 30% (or such lower rate specified by an applicable income tax treaty) on such effectively connected dividends, as adjusted for certain items. Non-U.S. Holders should consult their tax advisors regarding any applicable tax treaties that may provide for different rules.

A Non-U.S. Holder will not be subject to U.S. federal income tax on any gain realized upon the sale or other taxable disposition of Rights, Warrants or Offering Shares unless:

Gain described in the first bullet point above generally will be subject to U.S. federal income tax on a net income basis at the regular rates. A Non-U.S. Holder that is a corporation also may be subject to a branch profits tax at a rate of 30% (or such lower rate specified by an applicable income tax treaty) on such effectively connected gain, as adjusted for certain items.

A Non-U.S. Holder described in the second bullet point above will be subject to U.S. federal income tax at a rate of 30% (or such lower rate specified by an applicable income tax treaty) on gain realized upon the sale or other taxable disposition of Rights, Warrants or Offering Shares, which may be offset by U.S. source capital losses of the Non-U.S. Holder (even though the individual is not considered a resident of the United States), provided that the Non-U.S. Holder has timely filed U.S. federal income tax returns with respect to such losses.

With respect to the third bullet point above, we believe we currently are not, and do not anticipate becoming, a USRPHC. Because the determination of whether we are a USRPHC depends, however, on the fair market value of our USRPIs relative to the fair market value of our non-U.S. real property interests and our assets used or held for use in a trade or business, there can be no assurance we currently are not a USRPHC or will not become one in the future. Even if we are or were to become a USRPHC, gain arising from the sale or other taxable disposition of Rights, Warrants or Offering Shares by a Non-U.S. Holder will not be subject to U.S. federal income tax:

Whether or not our Rights are treated as “regularly traded” on an established securities market under the applicable Treasury Regulations depends in part on facts and circumstances at the time of and after the Rights Offering. There can be no guarantee that our common stock or our Rights will be treated as “regularly traded” for these purposes. Non-U.S. Holders should consult their tax advisors regarding potentially applicable income tax treaties that may provide for different rules.

Payments of dividends on our common stock will not be subject to backup withholding, provided that the applicable withholding agent does not have actual knowledge or reason to know that the beneficial owner is a United States person and the beneficial owner either certifies its non-U.S. status, such as by furnishing a valid IRS Form W-8BEN, W-8BEN-E, or W-8ECI, or otherwise establishes an exemption. However, information returns are required to be filed with the IRS in connection with any distributions (including deemed distributions) on Warrants or Offering Shares paid (or deemed paid) to a Non-U.S. Holder, regardless of whether such distributions constitute dividends or whether any tax was actually withheld. In addition, proceeds of the sale or other taxable disposition of Rights, Warrants or Offering Shares within the United States or conducted through certain U.S.-related brokers generally will not be subject to backup withholding or information reporting if the applicable withholding agent receives the certification described above and does not have actual knowledge or reason to know that such holder is a United States person or the holder otherwise establishes an exemption. Proceeds of a disposition of Rights, Warrants or Offering Shares conducted through a non-U.S. office of a non-U.S. broker generally will not be subject to backup withholding or information reporting.

Copies of information returns that are filed with the IRS may also be made available under the provisions of an applicable treaty or agreement to the tax authorities of the country in which the Non-U.S. Holder resides or is established.

Backup withholding is not an additional tax. Any amounts withheld under the backup withholding rules may be allowed as a refund or a credit against a Non-U.S. Holder’s U.S. federal income tax liability, provided the required information is timely furnished to the IRS.

Provisions of the Code commonly referred to as “FATCA” require withholding of 30% on payments of dividends on Warrants or Offering Shares, as well as of gross proceeds of dispositions of Warrants or Offering Shares, to “foreign financial institutions” (which is broadly defined for this purpose and in general includes investment vehicles) and certain other non-U.S. entities unless various U.S. information reporting and due diligence requirements (generally relating to ownership by U.S. persons of interests in or accounts with those entities) have been satisfied, or an exemption applies. An intergovernmental agreement between the United States and an applicable foreign country may modify these requirements. Under proposed regulations, the preamble to which states that taxpayers may rely on the proposed regulations until final regulations are issued, this withholding tax will not apply to the gross proceeds from the sale, exchange, redemption or other taxable disposition of our common stock. If FATCA withholding is imposed, a beneficial owner that is not a foreign financial institution generally may obtain a refund of any amounts withheld by filing a U.S. federal income tax return (which may entail significant administrative burden).

Investors should consult their tax advisors regarding the potential application of withholding under FATCA to their receipt, ownership, and exercise and disposition of Rights, Warrants and Offering Shares.

We are distributing rights certificates and copies of this prospectus supplement to those persons who were holders of our common stock or Participating Warrants on 5:00 p.m. (New York City time) on July 1, 2026, the Record Date, following the effective date of the registration statement of which this prospectus supplement forms a part. We expect to list our Rights on Nasdaq under the symbol “EOSER.” We have applied to have the Warrants admitted to trading on Nasdaq under the symbol “EOSEW.” However, no assurance can be given that such listing application will be approved. If our Warrants listing application is not approved, the Warrants may not be traded on Nasdaq when issued, or at all. Except as described herein, we have not employed any brokers, dealers or underwriters in connection with the solicitation or exercise of Rights in the Rights Offering and no commissions, fees or discounts will be paid in connection with the Rights Offering. While certain of our directors, officers and other employees may solicit responses from you, those directors, officers and other employees will not receive any commissions or compensation for their services other than their normal compensation, and will not register with the SEC as brokers in reliance on certain safe harbor provisions contained in Rule 3a4-1 under the Exchange Act.

As soon as practicable after the Record Date, we will distribute the Rights, rights certificates and copies of this prospectus supplement to individuals who owned shares of common stock on the Record Date. If your shares are held in the name of a custodian bank, broker, dealer or other nominee, then you should send your subscription documents and subscription payment to that record holder. If you are the record holder, then you should send your subscription documents, rights certificate, and subscription payment to the subscription agent, Broadridge Corporate Issuer Solutions, LLC, at the following address. If sent by mail, we recommend that you send documents and payments by registered mail, properly insured, with return receipt requested, and that a sufficient number of days be allowed to ensure delivery to the subscription agent. Do not send or deliver these materials to the Company.

In the event that the Rights Offering is not fully subscribed, holders of Rights who exercise all of their Rights pursuant to their basic subscription right will have the opportunity to subscribe for additional Units pursuant to the over-subscription privilege. See further the section titled “Description of the Rights Offering―Subscription Rights―Over-subscription privilege.”

We have not agreed to enter into any standby or other arrangement to purchase or sell any Rights or any of our securities.

We have not entered into any agreements regarding stabilization activities with respect to our securities. If you have any questions, you should contact the information agent at Sodali & Co., by telephone at (203) 658-9400 (banks and brokers), (833) 225-0490 (individuals call toll-free) or by email at EOSE.info@investor.sodali.com. We have agreed to pay Sodali & Co. a fee plus certain expenses, which we estimate will total approximately $17,500. We estimate that our total expenses in connection with the Rights Offering will be approximately $2,000,000.

Other than as described herein, we do not know of any existing agreements between any stockholder, broker, dealer, underwriter or agent relating to the sale or distribution of Units or the shares of common stock and warrants comprising the Units.

The validity of the securities offered hereby and certain matters of U.S. federal and New York State law will be passed upon for us by Davis Polk & Wardwell LLP, New York, New York.

The financial statements of Eos Energy Enterprises, Inc. as of December 31, 2025 and 2024, and for each of the two years in the period ended December 31, 2025, incorporated by reference in this prospectus supplement, have been audited by Deloitte & Touche LLP, an independent registered public accounting firm, as stated in their report. Such financial statements are incorporated by reference in reliance upon the report of such firm given their authority as experts in accounting and auditing.

Eos Energy Enterprises, Inc. may from time to time offer and sell common stock, preferred stock, debt securities, warrants, units or rights issuable to our stockholders to purchase shares of our common stock or preferred stock, to purchase warrants exercisable for shares of our common stock or preferred stock, or to purchase units consisting of two or more of the foregoing. Specific terms of the preferred stock, debt securities, warrants, units or rights will be provided in supplements to this prospectus.

We may sell the securities covered by this prospectus in a number of different ways and at varying prices. The securities may be sold directly to you, through agents, or through underwriters and dealers. If agents, underwriters or dealers are used to sell the securities, we will name them and describe their compensation in a prospectus supplement.

Our common stock is listed on the Nasdaq Capital Market (“Nasdaq”) under the symbol “EOSE.” The closing price of our common stock on May 12, 2026 was $8.10 per share.

Investing in these securities involves certain risks. You should review carefully the risks and uncertainties described under the heading “Risk Factors” beginning on page 12 of our annual report on Form 10-K for the year ended December 31, 2025, which is incorporated herein by reference, as amended or supplemented from time to time by any risk factors we include in subsequent annual or quarterly reports on Form 10-K or 10-Q, respectively, and incorporated herein by reference.

Neither the Securities and Exchange Commission (the “SEC”) nor any state securities commission has approved or disapproved these securities, or determined if this prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

This prospectus is part of a registration statement that we filed with the SEC utilizing a “shelf” registration process. Under this shelf process, we may sell any combination of the securities described in this prospectus in one or more offerings. This prospectus provides you with a general description of the securities we may offer. Each time we sell securities, we will provide a prospectus supplement that will contain specific information about the terms of that offering. The prospectus supplement may also add, update or change information contained in this prospectus. You should read both this prospectus and any prospectus supplement together with additional information described under the heading “Where You Can Find More Information.”

In this prospectus, unless the context otherwise requires, the terms “Company,” “we,” “us,” “our,” and “Eos” refer to Eos Energy Enterprises, Inc., a Delaware corporation. Prior to the consummation of the Business Combination (as defined below), the Company was known as B. Riley Principal Merger Corp. II, or “BMRG”.

The SEC allows us to incorporate by reference information in this document. This means that we can disclose important information to you by referring you to another document filed separately with the SEC. The information incorporated by reference is considered to be part of this document, except for any information that is superseded by information that is included directly in this document. Any statement contained in this prospectus, or in a document incorporated or deemed to be incorporated by reference herein, shall be deemed to be modified or superseded to the extent that a statement contained herein, or in any subsequently filed document that also is incorporated or deemed to be incorporated by reference herein, modifies or supersedes such statement.

We are incorporating by reference the filings listed below and any additional documents that we may file with the SEC pursuant to Section 13(a), 13(c), 14 or 15(d) of the Exchange Act on or after the date hereof and prior to the termination of any offering (other than documents or information deemed to have been furnished and not filed in accordance with SEC rules):

The SEC maintains a website at www.sec.gov, from which you can inspect these documents and other information we have filed electronically with the SEC. You may also request copies of these documents, at no cost to you, from our website (https://www.eose.com), or by writing or telephoning us at the following address:

This prospectus, including the documents incorporated by reference in this prospectus, contains forward-looking statements. The words “anticipate,” “believe,” “continue,” “could,” “estimate,” “expect,” “intends,” “may,” “might,” “plan,” “possible,” “potential,” “predict,” “project,” “should,” “would” and similar expressions, as they relate to us, are intended to identify forward-looking statements. These statements appear in a number of places in this prospectus and the documents incorporated by reference herein and include statements regarding our intent, belief or current expectations. Forward-looking statements are based on our management’s beliefs, as well as assumptions made by, and information currently available to, them. Because such statements are based on expectations as to future financial and operating results and are not statements of fact, actual results may differ materially from those projected. Factors which may cause actual results to differ materially from current expectations include, but are not limited to:

Should one or more of these risks or uncertainties materialize, or should any of our assumptions prove incorrect, actual results may vary in material respects from those projected in these forward-looking statements. Forward-looking statements speak only as of the date they are made. Readers are cautioned not to put undue reliance on forward-looking statements and, except as required by law, we assume no obligation and do not intend to update or revise these forward-looking statements, whether as a result of new information, future events, or otherwise. See also Part I, Item 1A, “Risk Factors” disclosures contained in our Annual Report on Form 10-K for the year ended December 31, 2025 for additional discussion of the risks and uncertainties that could cause the our actual results to differ materially from those expressed or implied in its forward-looking statements.

Before making an investment decision, you should carefully consider the risks described under “Risk Factors” in the applicable prospectus supplement and in our then most recent annual report on Form 10-K, and in any updates to those risk factors in our quarterly reports on Form 10-Q, together with all of the other information appearing or incorporated by reference in this prospectus and any applicable prospectus supplement, in light of your particular investment objectives and financial circumstances.

We will receive proceeds from the issuance and sale of our common stock, preferred stock, debt securities, warrants or units and from the exercise of any warrants or rights. Unless otherwise indicated in a prospectus supplement, the net proceeds from our sale of securities will be used for general corporate purposes, including working capital, acquisitions, retirement of debt and other business opportunities.

The following is a description of the material terms of, and is qualified in its entirety by, our certificate of incorporation and amended and restated bylaws. These documents are filed as exhibits and incorporated by reference into the registration statement of which this prospectus forms a part.

Our certificate of incorporation authorizes the issuance of 601,000,000 shares of capital stock, consisting of (x) 600,000,000 authorized shares of Common Stock and (y) 1,000,000 authorized shares of preferred stock, par value $0.0001 per share. As of March 31, 2026, there were 339,459,021 shares of Common Stock outstanding. As of March 31, 2026, 59 shares of preferred stock were designated as Series A-1 Preferred Stock, none of which are outstanding; 7 shares of preferred stock were designated as Series A-2 Preferred Stock, none of which are outstanding; 31.940063 shares of preferred stock were designated as Series B-1 Preferred Stock, all of which are outstanding; 28.806463 shares of preferred stock were designated as Series B-2 Preferred Stock, all of which are outstanding; 38.259864 shares of preferred stock were designated as Series B-3 Preferred Stock, all of which are outstanding and 16.150528 shares of preferred stock were designated as Series B-4 Preferred Stock, all of which are outstanding. There is no cumulative voting with respect to the election of directors.

Except as otherwise required by law or as otherwise provided in any certificate of designation for any series of preferred stock, the holders of Common Stock possess all voting power for the election of our directors and all other matters requiring stockholder action and will at all times vote together as one class on all matters submitted to a vote of the stockholders. Holders of Common Stock are entitled to one vote per share on matters to be voted on by stockholders.

Holders of Common Stock are entitled to receive such dividends and other distributions, if any, as may be declared from time to time by our board of directors (the “Board”) in its discretion out of funds legally available therefor and shall share equally on a per share basis in such dividends and distributions.

In the event of our voluntary or involuntary liquidation, dissolution, distribution of assets or winding-up, the holders of Common Stock will be entitled to receive an equal amount per share of all of our assets of whatever kind available for distribution to stockholders, after the rights of the holders of the preferred stock have been satisfied and after payment or provision for payment of our debts and other liabilities.

Our stockholders have no preemptive or other subscription rights and there is no sinking fund or redemption provisions applicable to Common Stock.

The Board is divided into three (3) classes with only one class of directors being elected in each year and each class serving a three (3) year term. The directors hold their office for a term of three (3) years or until their respective successors are elected and qualified, subject to such director’s earlier death, resignation, disqualification or removal.

The transfer agent and registrar for our Common Stock is Continental Stock Transfer & Trust Company.

Our certificate of incorporation provides that shares of preferred stock may be issued from time to time in one or more series. Our Board is authorized to fix the voting rights, if any, designations, powers, preferences, the relative, participating, optional or other special rights and any qualifications, limitations and restrictions thereof,

applicable to the shares of each series. Our Board may, without stockholder approval, issue preferred stock with voting and other rights that could adversely affect the voting power and other rights of the holders of the Common Stock and could have anti-takeover effects. The ability of our Board to issue preferred stock without stockholder approval could have the effect of delaying, deferring or preventing a change of control of the Company or the removal of existing management.

On June 21, 2024, we filed with the Secretary of State of the State of Delaware the Certificate of Designation of Series A-1 Non-Voting Non-Convertible Preferred Stock (the “Series A-1 Certificate of Designation”) and issued 59 shares of Series A-1 Preferred Stock to satisfy the terms of the Credit Agreement. Under the terms of the Series A-1 Certificate of Designation, each share of Series A-1 Preferred Stock had an original issue price of $455,822.59 and such shares had a liquidation value, payable with the Common Stock, as if such shares were convertible into an aggregate of 31,940,063 shares of Common Stock, subject to adjustment. The Series A-1 Preferred Stock was non-voting and non-convertible into Common Stock. Holders of the Series A-1 Preferred Stock were entitled to receive dividends or distributions on each share of Series A-1 Preferred Stock equal to dividends or distributions actually paid on each share of Common Stock, multiplied by the number of shares of Common Stock represented by the Series A-1 Preferred Stock Liquidation Value (as defined in the Series A-1 Certificate of Designation). The 59 shares of Series A-1 Preferred Stock issued to CCM Denali Equity Holdings, LP converted into shares of Series B-1 Preferred Stock on September 12, 2024. The foregoing summary of the terms of the Series A-1 Preferred Stock is qualified in its entirety by the Series A-1 Certificate of Designation, which is filed as an exhibit to the registration statement of which this prospectus forms a part.

On August 29, 2024, we filed with the Secretary of State of the State of Delaware the Certificate of Designation of Series A-2 Non-Voting Non-Convertible Preferred Stock (the “Series A-2 Certificate of Designation”). Under the terms of the Series A-2 Certificate of Designation, each share of Series A-2 Preferred Stock had an original issue price of $9,555,515.30 and such shares had a liquidation value, payable pari passu with the Common Stock, as if such shares were convertible into an aggregate of 28,806,463 shares of Common Stock, subject to adjustment. The Series A-2 Preferred Stock was non-voting and non-convertible into Common Stock. Holders of the Series A-2 Preferred Stock were entitled to receive dividends or distributions on each share of Series A-2 Preferred Stock equal to dividends or distributions actually paid on each share of Common Stock, multiplied by the number of shares of Common Stock represented by the Series A-2 Preferred Stock Liquidation Value (as defined in the Series A-2 Certificate of Designation). The Series A-2 Preferred Stock terms were substantially identical to the Series A-1 Preferred Stock. On August 29, 2024, pursuant to the terms and conditions of the Credit Agreement, the Applicable Percentage (as defined in the Credit Agreement) increased by 4.9%, and as a result we issued to CCM Denali Equity Holdings, LP 7 shares of Series A-2 Preferred Stock. On September 12, 2024, the 7 shares of Series A-2 Preferred Stock issued to CCM Denali Equity Holdings, LP converted into shares of Series B-2 Preferred Stock. The foregoing summary of the terms of the Series A-2 Preferred Stock is qualified in its entirety by the Series A-2 Certificate of Designation, which is filed as an exhibit to the registration statement of which this prospectus forms a part.

On September 11, 2024, we filed with the Secretary of State of the State of Delaware the Certificate of Designation of Series B-1 Non-Voting Convertible Preferred Stock (the “Series B-1 Certificate of Designation”) and the Certificate of Designation of Series B-2 Non-Voting Convertible Preferred Stock (the “Series B-2 Certificate of Designation”). On November 1, 2024, we filed with the Secretary of State of the State of Delaware the Certificate of Designation of Series B-3 Non-Voting Convertible Preferred Stock (the “Series B-3 Certificate of Designation”). On January 24, 2025, we filed with the Secretary of State of the State of Delaware the Certificate of Designation of Series B-4 Non-Voting Convertible Preferred Stock (the “Series B-4 Certificate of Designation”). Each share of Series B-1 Preferred Stock, Series B-2 Preferred Stock, Series B-3 Preferred Stock and Series B-4 Preferred Stock (collectively, the “Series B Preferred Stock”) has a par value of $0.0001 per share. The table below summarizes our outstanding Series B Preferred Stock as of March 31, 2026.

The Series B Preferred Stock is convertible into Common Stock at a conversion ratio of 1.0 million shares of Common Stock per share of Series B Preferred Stock (“Conversion Ratio”). The Conversion Ratio is subject to antidilution protection that is triggered if we issue equity for a price per share that is less than the conversion price then in effect, subject to certain exceptions.

Holders of the Series B Preferred Stock are entitled to receive dividends or distributions on each share of Series B Preferred Stock equal to dividends or distributions actually paid on each share of Common Stock on an as-converted basis.

At all times when the holders of the Series B Preferred Stock beneficially own at least 10%, 15% or 30% of our capital stock, the holders of the Series B Preferred Stock, exclusively and voting together as a separate class, will have the right to appoint a maximum of 1, 2 or 3 directors to the Board, respectively. At all times when the holders of the Series B Preferred Stock beneficially own at least 40% of our capital stock, the holders of the Series B Preferred Stock, exclusively and voting together as a separate class, will have the right to nominate and designate a fourth director, who shall be designated by the Board or the nominating committee of the Board to a class of common directors and thereafter stand for election as a common director on the Board. The holders of the Series B Preferred Stock will have the right to nominate a fourth director to the Board only if such appointment does not result in a change of control under our governing documents or violate any applicable laws, including requirements of the SEC and Nasdaq, and any such fourth director appointment shall be subject to and conditioned upon compliance by the holders of the Series B Preferred Stock with the Hart-Scott-Rodino Antitrust Improvements Act of 1976, including the submission of any required filings and the expiration or termination of any applicable waiting periods.

The certificates of designations for the Series B Preferred Stock contain customary preemptive rights that permit the holders of Series B Preferred Stock to participate in certain of our future equity offerings.

In the event of a voluntary or involuntary liquidation, dissolution, or winding up of the Company, the holders of the Series B Preferred Stock are entitled to receive distribution of any of our assets or surplus funds pro rata with the holders of the Common Stock and any other holders of our preferred stock issued pursuant to our Securities

Purchase Agreement (the “Purchase Agreement”) with CCM Denali Equity Holdings, LP and the Credit Agreement, including the Series B Preferred Stock, in an amount equal to such amount per share as would have been payable had all shares of Series B Preferred Stock been converted to Common Stock.

We are prohibited from taking certain actions that could adversely affect the rights of the Series B Preferred Stock without the affirmative vote of a majority of the outstanding shares of Series B Preferred Stock until the later of (i) such time when the holders of Series B Preferred Stock shall no longer beneficially own at least 5% of our outstanding capital stock and (ii) June 21, 2029, in the case of the Series B-1 Preferred Stock, August 29, 2029, in the case of the Series B-2 Preferred Stock, November 1, 2029, in the case of the Series B-3 Preferred Stock, or January 24, 2030, in the case of the Series B-4 Preferred Stock.

At any time after June 21, 2029, in the case of the Series B-1 Preferred Stock, August 29, 2029, in the case of the Series B-2 Preferred Stock, November 1, 2029, in the case of the Series B-3 Preferred Stock or January 24, 2030, in the case of the Series B-4 Preferred Stock, the outstanding shares of Series B Preferred Stock held by any holder become redeemable for cash at the redemption price. The redemption price will be an amount per share equal to the greater of (i) $841,999.99 for the Series B-1 Preferred Stock, $2,322,000 for the Series B-2 Preferred Stock, $3,358,000 for the Series B-3 Preferred Stock or $5,990,000 for the Series B-4 Preferred Stock, as applicable, plus all accrued and unpaid dividends thereon, up to and including the date of redemption and (ii) the number of shares of Common Stock issuable upon conversion of the applicable Series B Preferred Stock multiplied by the average of the closing sale price of the Common Stock for the five (5) business days immediately prior to the date of redemption plus all accrued and unpaid dividends thereon, up to and including the date of redemption.

The foregoing summary of the terms of the Series B Preferred Stock is qualified in its entirety by the terms of the respective Series B Certificate of Designation, each of which is filed as an exhibit to the registration statement of which this prospectus forms a part.

The certificate of incorporation, bylaws and the Delaware General Corporation Law, or DGCL, contain provisions that could have the effect of rendering more difficult, delaying, or preventing an acquisition deemed undesirable by our Board. These provisions could also make it difficult for stockholders to take certain actions, including electing directors who are not nominated by the members of our Board or taking other corporate actions, including effecting changes in our management. For instance, our Board will be empowered to elect a director to fill a vacancy created by the expansion of the Board or the resignation, death, or removal of a director in certain circumstances; and our advance notice provisions in our bylaws will require that stockholders must comply with certain procedures in order to nominate candidates to our Board or to propose matters to be acted upon at a stockholders’ meeting.

Our authorized but unissued Common Stock and preferred stock will be available for future issuances without stockholder approval and could be utilized for a variety of corporate purposes, including future offerings to raise additional capital, acquisitions and employee benefit plans. The existence of authorized but unissued and unreserved Common Stock and preferred stock could render more difficult or discourage an attempt to obtain control of us by means of a proxy contest, tender offer, merger or otherwise.

We, in our certificate of incorporation, have expressly elected not to be governed by Section 203 of the DGCL, and thus the restrictions on business combinations contained in Section 203 of the DGCL do not apply to the Corporation.

Our Board is divided into three classes, each of which will generally serve for a term of three years with only one class of directors being elected in each year. This system of electing directors may tend to discourage a third party from making a tender offer or otherwise attempting to obtain control of us, because it generally makes it more difficult for stockholders to replace a majority of the directors.

Our certificate of incorporation limits the liability of our directors to the fullest extent permitted by the DGCL and provides that we will provide them with customary indemnification. We entered into customary indemnification agreements with each of our executive officers and directors that provide them, in general, with customary indemnification in connection with their service to us or on our behalf.

Our Common Stock is listed on Nasdaq under the symbol “EOSE”.

We may issue debt securities, which may be secured or unsecured and may be exchangeable for and/or convertible into other securities, including our ordinary shares. The debt securities will be issued under one or more separate indentures between us and a designated trustee. The terms of each series of debt securities being offered, including the terms, if any, on which a series of debt securities may be convertible into or exchangeable for other securities, and the material terms of the indenture will be set forth in the applicable prospectus supplement.

The applicable prospectus supplement will set forth, to the extent required and as applicable, the following terms (non-exhaustive) of the debt securities in respect of which the prospectus supplement is delivered:

We may issue warrants to purchase our debt or equity securities or securities of third parties or other rights, including rights to receive payment in cash or securities based on the value, rate or price of one or more specified commodities, currencies, securities or indices, or any combination of the foregoing. Warrants may be issued independently or together with any other securities and may be attached to, or separate from, such securities. Each series of warrants will be issued under a separate warrant agreement to be entered into between us and a warrant agent. The terms of any warrants to be issued and a description of the material provisions of the applicable warrant agreement will be set forth in the applicable prospectus supplement.

The applicable prospectus supplement will describe the following terms of any warrants in respect of which this prospectus is being delivered:

As specified in the applicable prospectus supplement, we may issue units consisting of one or more warrants, debt securities, shares of preferred stock, shares of common stock or any combination of such securities. The applicable supplement will describe:

This section describes the general terms and provisions of the rights to purchase certain of our securities that we may issue to holders of our securities by this prospectus. The applicable prospectus supplement will describe the specific terms of the rights then issued, and the terms and provisions described in this section will apply only to the extent not superseded by the terms of the applicable prospectus supplement.

We may issue, as a dividend at no cost, to holders of record of our securities or any class or series thereof on the applicable record date, rights to purchase shares of our common stock or preferred stock, to purchase warrants exercisable for shares of our common stock or preferred stock, or to purchase units consisting of two or more of the foregoing. In this prospectus, we refer to such rights as “stockholder rights.” If stockholder rights are so issued to existing holders of securities, each stockholder right will entitle the registered holder thereof to purchase the securities issuable upon exercise of the rights pursuant to the terms set forth in the applicable prospectus supplement.

If stockholder rights are issued, the applicable prospectus supplement will describe the terms of such stockholder rights including, but not limited to, the following, where applicable:

In addition to the terms of the stockholder rights and the securities issuable upon exercise thereof, the prospectus supplement may describe, for a holder of such stockholder rights who validly exercises all stockholder rights issued to such holder, how to subscribe for unsubscribed securities, issuable pursuant to unexercised stockholder rights issued to other holders, to the extent such stockholder rights have not been exercised.

Holders of stockholder rights will not be entitled by virtue of being such holders, to vote, to consent, to receive dividends, to receive notice with respect to any meeting of stockholders for the election of our directors or any other matter, or to exercise any rights whatsoever as our stockholders, except to the extent described in the related prospectus supplement.

We are registering the offer and sale from time to time by us of our common stock, preferred stock, debt securities, warrants, units or rights in one or more offerings.

We will receive proceeds from the issuance and sale of our common stock, preferred stock, debt securities, warrants, units or rights. We will pay any underwriting discounts and commissions and expenses incurred by us in connection with the sale of securities by us.

We will bear all other costs, fees and expenses incurred in effecting the registration of the securities covered by this prospectus, including, without limitation, all registration and filing fees, Nasdaq listing fees and fees and expenses of our counsel and our independent registered public accountants.

The securities to be offered and sold by us covered by this prospectus may be offered and sold from time to time.

Such sales may be made on one or more exchanges or in the over-the-counter market or otherwise, at prices and under terms then prevailing or at prices related to the then current market price or in negotiated transactions. We reserve the right to accept and, together with its respective agents, to reject, any proposed purchase of securities to be made directly or through agents. We and any of its permitted transferees may sell their securities offered by this prospectus on any stock exchange, market or trading facility on which the securities are traded or in private transactions. If underwriters are used in the sale, such underwriters will acquire the shares for their own account. These sales may be at a fixed price or varying prices, which may be changed, or at market prices prevailing at the time of sale, at prices relating to prevailing market prices or at negotiated prices. The securities may be offered to the public through underwriting syndicates represented by managing underwriters or by underwriters without a syndicate. The obligations of the underwriters to purchase the securities will be subject to certain conditions. The underwriters will be obligated to purchase all the securities offered if any of the securities are purchased.

We may use any one or more of the following methods when selling the securities offered by this prospectus:

There can be no assurance that we will sell all or any of the securities offered by this prospectus. In addition, we may also sell securities under Rule 144 under the Securities Act, if available, or in other transactions exempt from registration, rather than under this prospectus. We have the sole and absolute discretion not to accept any purchase offer or make any sale of securities if we or they deem the purchase price to be unsatisfactory at any particular time.

With respect to a particular offering of the securities by us, to the extent required, an accompanying prospectus supplement or, if appropriate, a post-effective amendment to the registration statement of which this prospectus is part, will be prepared and will set forth the following information:

In order to facilitate the offering of the securities, any underwriters or agents, as the case may be, involved in the offering of such securities may engage in transactions that stabilize, maintain or otherwise affect the price of our securities. Specifically, the underwriters or agents, as the case may be, may overallot in connection with the offering, creating a short position in our securities for their own account. In addition, to cover overallotments or to stabilize the price of our securities, the underwriters or agents, as the case may be, may bid for, and purchase, such securities in the open market. Finally, in any offering of securities through a syndicate of underwriters, the underwriting syndicate may reclaim selling concessions allotted to an underwriter or a broker-dealer for distributing such securities in the offering if the syndicate repurchases previously distributed securities in transactions to cover syndicate short positions, in stabilization transactions or otherwise. Any of these activities may stabilize or maintain the market price of the securities above independent market levels. The underwriters or agents, as the case may be, are not required to engage in these activities, and may end any of these activities at any time.

We may solicit offers to purchase the securities directly from, and may sell such securities directly to, institutional investors or others. In this case, no underwriters or agents would be involved. The terms of any of those sales, including the terms of any bidding or auction process, if utilized, will be described in the applicable prospectus supplement.

It is possible that one or more underwriters may make a market in our securities, but such underwriters will not be obligated to do so and may discontinue any market making at any time without notice. We cannot give any assurance as to the liquidity of the trading market for our securities. Our common stock is listed on the Nasdaq under the symbol “EOSE.”

We may authorize underwriters, broker-dealers or agents to solicit offers by certain purchasers to purchase the securities at the public offering price set forth in the prospectus supplement pursuant to delayed delivery contracts providing for payment and delivery on a specified date in the future. The contracts will be subject only to those conditions set forth in the prospectus supplement, and the prospectus supplement will set forth any commissions we pay for solicitation of these contracts.

In effecting sales, broker-dealers or agents engaged by us may arrange for other broker-dealers to participate. Broker-dealers or agents may receive commissions, discounts or concessions from us in amounts to be negotiated immediately prior to the sale.

If at the time of any offering made under this prospectus a member of FINRA participating in the offering has a “conflict of interest” as defined in FINRA Rule 5121 (“Rule 5121”), that offering will be conducted in accordance with the relevant provisions of Rule 5121.

Underwriters, broker-dealers or agents may facilitate the marketing of an offering online directly or through one of their affiliates. In those cases, prospective investors may view offering terms and a prospectus online and, depending upon the particular underwriter, broker-dealer or agent, place orders online or through their financial advisors.

The underwriters, broker-dealers and agents may engage in transactions with us, or perform services for us, in the ordinary course of business.

In order to comply with the securities laws of certain states, if applicable, the securities must be sold in such jurisdictions only through registered or licensed brokers or dealers. In addition, in certain states the securities may not be sold unless they have been registered or qualified for sale in the applicable state or an exemption from the registration or qualification requirement is available and is complied with.

Agents, broker-dealers and underwriters may be entitled to indemnification by us against certain civil liabilities, including liabilities under the Securities Act, or to contribution with respect to payments which the agents, broker-dealers or underwriters may be required to make in respect thereof.

Davis Polk & Wardwell LLP has passed upon the validity of the securities of the Company offered by this prospectus. Any underwriters, dealers or agents will be advised by their own legal counsel concerning matters relating to any offering.

The financial statements of Eos Energy Enterprises, Inc., incorporated by reference in this prospectus, have been audited by Deloitte & Touche LLP, an independent registered public accounting firm, as stated in their report. Such financial statements are incorporated by reference in reliance upon the report of such firm given their authority as experts in accounting and auditing.