Document


			Wilson Sonsini Goodrich & Rosati Professional Corporation 650 Page Mill Road Palo Alto, CA 94304-1050 o: 650.493.9300 f: 650.493.6811

26440 SW Parkway Ave., Bldg. 83

Re: Registration Statement on Form S-3

Ladies and Gentlemen:

We have acted as counsel to ESS Tech, Inc., a Delaware corporation (the “Company”), in connection with the registration of the offer and sale of 6,400,000 shares (the “Shares”) of the common stock of the Company, par value $0.0001 per share, pursuant to the Company’s Registration Statement on Form S-3 (File No. 333-291506) filed on November 13, 2025 and declared effective by the Securities and Exchange Commission (the “Commission”) on December 11, 2025 (the “Registration Statement”).

The offering and sale of the Shares were made pursuant to the Securities Purchase Agreement (the “Securities Purchase Agreement”), dated as of August 20, 2026, by and among the Company and the purchasers identified on the signature pages thereto.

We have examined copies of the Securities Purchase Agreement, the Registration Statement, the base prospectus that forms a part thereof (the “Base Prospectus”) as supplemented by the prospectus supplement to be dated August 20, 2026, in the form filed with the Commission pursuant to Rule 424(b) under the Securities Act of 1933, as amended (the “Securities Act”), together with the documents or portions thereof incorporated by reference therein, as modified or superseded as described therein (the “Prospectus Supplement” and, together with the Base Prospectus, the “Prospectus”). We have also examined instruments, documents and records that we deemed relevant and necessary for the basis of our opinion hereinafter expressed.

In such examination, we have assumed (i) the authenticity of original documents and the genuineness of all signatures, (ii) the conformity to the originals of all documents submitted to us as copies, (iii) the truth, accuracy, and completeness of the information, representations and warranties contained in the records, documents, instruments and certificates we have reviewed, and (iv) the legal capacity of all natural persons. As to any facts material to the opinions expressed herein that were not independently established or verified, we have relied upon oral or written statements and representations of officers and other representatives of the Company. In making our examination of documents executed or to be executed, we have assumed that the counterparties thereto, excluding the Company, have been duly incorporated or organized and are, and will continue to be, validly existing and in good standing, and have or will have the requisite legal status and legal capacity under the laws of their respective jurisdictions of incorporation or organization and had or will have the power, corporate or other, to enter into and perform all obligations thereunder and have also assumed the due authorization by all requisite action, corporate or other, and execution and delivery by such parties of such documents and, except as expressly set forth in the paragraphs below, the validity and binding effect thereof on such parties.

AUSTIN BOSTON BOULDER BRUSSELS HONG KONG LONDON LOS ANGELES NEW YORK PALO ALTO
SALT LAKE CITY SAN DIEGO SAN FRANCISCO SEATTLE SHANGHAI WASHINGTON, DC WILMINGTON, DE

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August 21, 2026

Based upon and subject to the foregoing qualifications, assumptions and limitations and the further limitations set out below, we are of the opinion that

1.The Shares have been duly authorized by the Company and, when issued and delivered by the Company against payment therefor in accordance with the terms of and in the manner contemplated by the Securities Purchase Agreement and the Prospectus, will be validly issued, fully paid and nonassessable.

We express no opinion as to the laws of any other jurisdiction other than the federal laws of the United States of America and the General Corporation Law of the State of Delaware.

Our opinion that any document is legal, valid, binding and enforceable is qualified as to:

(a)limitations imposed by bankruptcy, insolvency, reorganization, arrangement, fraudulent transfer, moratorium or other similar laws relating to or affecting the rights of creditors generally;

(b)rights to indemnification and contribution, which may be limited by applicable law or equitable principles; and

(c)the effect of general principles of equity, including without limitation concepts of materiality, reasonableness, good faith and fair dealing, and the possible unavailability of specific performance or injunctive relief and limitation of rights of acceleration, regardless of whether such enforceability is considered in a proceeding in equity or at law.

We hereby consent to the use of this opinion as an exhibit to the Company’s Current Report on Form 8-K filed on or about the date hereof, for incorporation by reference into the Registration Statement.

In giving this consent, we do not admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act.

/s/ Wilson Sonsini Goodrich & Rosati, P.C.

WILSON SONSINI GOODRICH & ROSATI

Professional Corporation