NATURE OF OPERATIONS AND FINANCIAL STATEMENT PRESENTATION (Policies)	3 Months Ended
Mar. 31, 2026
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	BASIS OF PRESENTATION TEP's Condensed Consolidated Financial Statements and disclosures are presented in accordance with GAAP, including specific accounting guidance for regulated operations and the SEC's interim reporting requirements. The Condensed Consolidated Financial Statements include the accounts of TEP and its subsidiaries. In the consolidation process, accounts of TEP and its subsidiaries are combined, and intercompany balances and transactions are eliminated. TEP jointly owns several generation facilities and transmission systems with both affiliated and non-affiliated entities. TEP records its proportionate share of: (i) jointly-owned facilities in Utility Plant on the Condensed Consolidated Balance Sheets; and (ii) operating costs associated with these facilities on the Condensed Consolidated Statements of Income. These Condensed Consolidated Financial Statements exclude some information and footnotes required by GAAP and the SEC for annual financial statement reporting and should be read in conjunction with the Consolidated Financial Statements and footnotes in TEP's 2025 Annual Report on Form 10-K. The Condensed Consolidated Financial Statements are unaudited, but, in management's opinion, include all normal, recurring adjustments necessary for a fair statement of the results for the interim periods presented. Because weather and other factors cause seasonal fluctuations in sales, TEP's quarterly operating results are not indicative of annual operating results. To conform with the current period presentation, TEP combined four prior period Operating Expense line items: (i) Fuel; (ii) Purchased Power; (iii) Transmission and Other PPFAC Recoverable Costs; and (iv) Increase (Decrease) to Reflect PPFAC Recovery Treatment, into a single line item titled Fuel and Purchased Power on the Condensed Consolidated Statements of Income. Additionally, TEP has reclassified Income Taxes Receivable/Payable in the prior period from a separately disclosed line into Other, Net on the Condensed Consolidated Statements of Cash Flows to conform with the current period presentation. Both the change in presentation and reclassification had no impact on TEP’s results of operation, financial position, or cash flows.
Variable Interest Entities	Variable Interest Entities A VIE is an entity in which equity investors lack the characteristics of a controlling financial interest or do not have sufficient equity investment at risk for the entity to finance its activities without additional subordinated financial support. TEP regularly reviews contracts to determine if it has a variable interest in an entity, if that entity is a VIE, and if TEP is the primary beneficiary of the VIE. The primary beneficiary is required to consolidate the VIE when it has: (i) the power to direct activities that most significantly impact the economic performance of the VIE; and (ii) the obligation to absorb losses or the right to receive benefits that could potentially be significant to the VIE. TEP has entered into long-term renewable PPAs with various entities. Some of these entities are VIEs due to the long-term fixed price component in the agreements. These PPAs effectively transfer commodity price risk to TEP, the buyer of the power, creating a variable interest. TEP has determined it is not a primary beneficiary of these VIEs as it lacks the power to direct the activities that most significantly impact the economic performance of the VIEs. TEP regularly evaluates its primary beneficiary conclusions to determine if changes have occurred that impact its VIE assessment. As of March 31, 2026, the carrying amounts of assets and liabilities in the balance sheet that relate to variable interests under long-term renewable PPAs are predominantly related to working capital accounts and generally represent the amounts owed by TEP for the deliveries associated with the current billing cycle. TEP's maximum exposure to loss is limited to the cost of replacing the power if the providers do not meet the production guarantee. However, the exposure to loss is mitigated as the Company would likely recover these costs through cost recovery mechanisms.
Reportable Segments	REPORTABLE SEGMENTS TEP's principal business operations include generating, transmitting, and distributing electricity to its retail customers. In addition to retail sales, TEP sells electricity, transmission, and ancillary services to other utilities, municipalities, and energy marketing companies on a wholesale basis. TEP has one operating segment, its regulated utility operations. TEP’s CODM is Susan M. Gray who holds the position of Chief Executive Officer of TEP and its parent company, UNS Energy. The CODM uses net income to assess performance and decide how to allocate resources for UNS Energy overall (including employees and financial or capital resources) predominantly in the annual budget and forecasting process. Net income is reported on the Condensed Consolidated Statements of Income. Operations and Maintenance expense includes expenses reimbursed by third-parties and expenses related to customer-funded RES and DSM programs. Operations and Maintenance expense excluding these reimbursable and customer funded expenses totaled $92 million and $82 million for the three months ended March 31, 2026 and 2025, respectively. Total assets, the measure of segment assets, is reported on the Condensed Consolidated Balance Sheets. Capital expenditures are reported on the Condensed Consolidated Statements of Cash Flows.
Restricted Cash	RESTRICTED CASH Restricted cash includes cash balances restricted with respect to withdrawal or usage based on contractual or regulatory considerations. Restricted cash primarily represents cash contractually required to be set aside to pay TEP's share of final mine reclamation and decommissioning costs at San Juan.
Investment Tax Credits	Investment Tax Credits In March 2026, TEP entered into an agreement to transfer ITCs related to Roadrunner Reserve II in the second half of 2026. In connection with ITCs expected to be generated by the Roadrunner Reserve II project, TEP recorded a $10 million deferred tax asset in Deferred Income Taxes, Net on the Condensed Consolidated Balance Sheets as of March 31, 2026, with a corresponding regulatory liability in Regulatory Liabilities on the Condensed Consolidated Balance Sheets to reflect the anticipated return of benefits to customers. As of March 31, 2026, TEP recorded a $1 million valuation allowance against the deferred tax asset reflecting the expected discount in proceeds from a third-party sale. Roadrunner Reserve I was placed in service in July 2025 and generated $98 million of ITCs. In December 2025, TEP entered into an agreement to transfer ITCs, under which it transferred $49 million of ITCs and received $46 million in cash proceeds. On April 9, 2026, TEP transferred the remaining $49 million of ITCs related to Roadrunner Reserve I and received $46 million in cash proceeds.
New Accounting Standards Issued, and Not Yet Adopted	NEW ACCOUNTING STANDARDS ISSUED AND NOT YET ADOPTED Standards Recently Issued by the FASB The following new authoritative accounting guidance issued by the FASB has not yet been adopted and reflected in TEP’s financial statements. TEP is assessing the impact such guidance may have on TEP’s financial position, results of operations, cash flows, and disclosures. Government Grants In December 2025, the FASB issued accounting guidance that establishes recognition, measurement, and presentation requirements for government grants for business entities. The update applies to transfers of monetary or tangible nonmonetary assets from a government to a business entity, excluding certain transaction types such as tax incentives. The recognition, measurement, and presentation of a government grant is dependent on the classification of the grant as either related to an asset or related to income. Grants cannot be recognized until it is probable that the entity will comply with the conditions attached to the grant and that the grant will be received. The amendments are effective for annual and interim reporting periods beginning after January 1, 2029, for public business entities. The guidance may be applied using a modified prospective, modified retrospective, or full retrospective approach. Early adoption is permitted. Targeted Improvements to the Accounting for Internal-Use Software In September 2025, the FASB issued accounting guidance that updates the recognition criteria for internal-use software. The update removes references to software development project stages and requires capitalization of software costs once: (i) management has authorized and committed to funding the project; and (ii) it is probable that the project will be completed and the software will perform its intended function. Evaluating the probability that the project will be completed includes an assessment of whether significant development uncertainty exists. The amendment requires entities to apply the disclosure requirements applicable to property, plant, and equipment to all capitalized internal-use software costs, irrespective of financial statement presentation. The amendments are effective for annual and interim reporting periods beginning after January 1, 2028. The guidance may be applied prospectively, retrospectively, or using a modified transition approach. Early adoption is permitted. Disaggregation of Income Statement Expenses In November 2024, the FASB issued accounting guidance that requires disaggregation of income statement expenses into specified categories in the footnotes to the financial statements. In January 2025, the FASB issued accounting guidance clarifying the effective date of this standard. The amendments are effective for annual periods beginning January 1, 2027, and interim reporting periods beginning after January 1, 2028. The guidance is to be applied on a prospective basis with the option to apply the standard retrospectively. Early adoption is permitted. SEC Climate-Related Disclosures The following final SEC rules regarding climate-related disclosures are pending further action by the SEC. In March 2024, the SEC issued final rules requiring the disclosure of climate‑related risks and greenhouse gas emissions, which were subsequently stayed pending judicial review. In September 2025, the Court of Appeals for the Eighth Circuit paused its consideration of legal challenges against the rules, pending further action by the SEC. TEP awaits the SEC's decision whether it will defend, amend, or withdraw the rules, the timing of which TEP cannot currently predict.
Allowance for Credit Losses	ALLOWANCE FOR CREDIT LOSSES TEP separately evaluates retail, wholesale, and other accounts receivable for credit losses and has not recorded an allowance for credit losses for non-retail accounts receivable.
Fair Value Measurement	TEP categorizes financial instruments into the three-level hierarchy based on inputs used to determine the fair value. Level 1 inputs are unadjusted quoted prices for identical assets or liabilities in an active market. Level 2 inputs include quoted prices for similar assets or liabilities, quoted prices in non-active markets, and pricing models whose inputs are observable, directly or indirectly. Level 3 inputs are unobservable and supported by little or no market activity. TEP has no financial instruments categorized as Level 3.
Derivative Instruments	DERIVATIVE INSTRUMENTS TEP enters into various derivative and non-derivative contracts to reduce exposure to energy price risk associated with its natural gas and purchased power requirements. The objectives for entering into such contracts include: (i) creating price stability; (ii) meeting load and reserve requirements; and (iii) reducing exposure to price volatility that may result from delayed recovery under the PPFAC mechanism. In addition, TEP enters into derivative and non-derivative contracts to optimize the system's generation resources by selling power in the wholesale market for the benefit of TEP's retail customers. TEP primarily applies the market approach for recurring fair value measurements. When TEP has observable inputs for substantially the full term of the asset or liability or uses quoted prices in an inactive market, it categorizes the instrument in Level 2. TEP categorizes derivatives in Level 3 when an aggregate pricing service or published prices that represent a consensus reporting of multiple brokers is used. For both purchased power and natural gas prices, TEP obtains quotes from brokers, major market participants, exchanges, or industry publications and relies on its own price experience from active transactions in the market. TEP primarily uses one set of quotations each for purchased power and natural gas and then validates those prices using other sources. TEP believes that the market information provided is reflective of market conditions as of the time and date indicated. Published prices for energy derivative contracts may not be available due to the nature of contract delivery terms such as non-standard time blocks and non-standard delivery points. In these cases, TEP applies adjustments based on historical price curve relationships, transmission costs, and real power line losses. TEP also considers the impact of counterparty credit risk using current and historical default and recovery rates, as well as its own credit risk using credit default swap data. The inputs and TEP's assessments of the significance of a particular input to the fair value measurements require judgment and may affect the valuation of fair value assets and liabilities and their placement within the fair value hierarchy levels. TEP evaluates the assumptions underlying its price curves monthly.
Credit Risk	CREDIT RISK The use of contractual arrangements to manage the risks associated with changes in energy commodity prices creates credit risk exposure resulting from the possibility of non-performance by counterparties pursuant to the terms of their contractual obligations. TEP enters into contracts for the physical delivery of power and natural gas which contain remedies in the event of non-performance by the supply counterparties. In addition, volatile energy prices can create significant credit exposure from energy market receivables and subsequent measurements at fair value. TEP has contractual agreements for energy procurement and hedging activities that contain provisions requiring TEP and its counterparties to post collateral under certain circumstances. These circumstances include: (i) exposures in excess of unsecured credit limits due to the volume of trading activity; (ii) changes in natural gas or power prices; (iii) credit rating downgrades; or (iv) unfavorable changes in parties' assessments of each other's credit strength. If such credit events were to occur, TEP, or its counterparties, could have to provide certain credit enhancements in the form of cash, LOCs, or other acceptable security to collateralize exposure beyond the allowed amounts. TEP considers the effect of counterparty credit risk in determining the fair value of derivative instruments that are in a net asset position, after incorporating collateral posted by counterparties, and then allocates the credit risk adjustment to individual contracts. TEP also considers the impact of its credit risk on instruments that are in a net liability position, after considering the collateral posted, and then allocates the credit risk adjustment to individual contracts.

NATURE OF OPERATIONS AND FINANCIAL STATEMENT PRESENTATION (Policies)

3 Months Ended

Mar. 31, 2026

Organization, Consolidation and Presentation of Financial Statements [Abstract]

BASIS OF PRESENTATION

TEP's Condensed Consolidated Financial Statements and disclosures are presented in accordance with GAAP, including specific accounting guidance for regulated operations and the SEC's interim reporting requirements.

The Condensed Consolidated Financial Statements include the accounts of TEP and its subsidiaries. In the consolidation process, accounts of TEP and its subsidiaries are combined, and intercompany balances and transactions are eliminated. TEP jointly owns several generation facilities and transmission systems with both affiliated and non-affiliated entities. TEP records its proportionate share of: (i) jointly-owned facilities in Utility Plant on the Condensed Consolidated Balance Sheets; and (ii) operating costs associated with these facilities on the Condensed Consolidated Statements of Income. These Condensed Consolidated Financial Statements exclude some information and footnotes required by GAAP and the SEC for annual financial statement reporting and should be read in conjunction with the Consolidated Financial Statements and footnotes in TEP's 2025 Annual Report on Form 10-K.

The Condensed Consolidated Financial Statements are unaudited, but, in management's opinion, include all normal, recurring adjustments necessary for a fair statement of the results for the interim periods presented. Because weather and other factors cause seasonal fluctuations in sales, TEP's quarterly operating results are not indicative of annual operating results. To conform with the current period presentation, TEP combined four prior period Operating Expense line items: (i) Fuel; (ii) Purchased Power; (iii) Transmission and Other PPFAC Recoverable Costs; and (iv) Increase (Decrease) to Reflect PPFAC Recovery Treatment, into a single line item titled Fuel and Purchased Power on the Condensed Consolidated Statements of Income. Additionally, TEP has reclassified Income Taxes Receivable/Payable in the prior period from a separately disclosed line into Other, Net on the Condensed Consolidated Statements of Cash Flows to conform with the current period presentation. Both the change in presentation and reclassification had no impact on TEP’s results of operation, financial position, or cash flows.

Variable Interest Entities

A VIE is an entity in which equity investors lack the characteristics of a controlling financial interest or do not have sufficient equity investment at risk for the entity to finance its activities without additional subordinated financial support. TEP regularly reviews contracts to determine if it has a variable interest in an entity, if that entity is a VIE, and if TEP is the primary beneficiary of the VIE. The primary beneficiary is required to consolidate the VIE when it has: (i) the power to direct activities that most significantly impact the economic performance of the VIE; and (ii) the obligation to absorb losses or the right to receive benefits that could potentially be significant to the VIE.

TEP has entered into long-term renewable PPAs with various entities. Some of these entities are VIEs due to the long-term fixed price component in the agreements. These PPAs effectively transfer commodity price risk to TEP, the buyer of the power, creating a variable interest. TEP has determined it is not a primary beneficiary of these VIEs as it lacks the power to direct the activities that most significantly impact the economic performance of the VIEs. TEP regularly evaluates its primary beneficiary conclusions to determine if changes have occurred that impact its VIE assessment.

As of March 31, 2026, the carrying amounts of assets and liabilities in the balance sheet that relate to variable interests under long-term renewable PPAs are predominantly related to working capital accounts and generally represent the amounts owed by TEP for the deliveries associated with the current billing cycle. TEP's maximum exposure to loss is limited to the cost of replacing the power if the providers do not meet the production guarantee. However, the exposure to loss is mitigated as the Company would likely recover these costs through cost recovery mechanisms.

Reportable Segments

REPORTABLE SEGMENTS

TEP's principal business operations include generating, transmitting, and distributing electricity to its retail customers. In addition to retail sales, TEP sells electricity, transmission, and ancillary services to other utilities, municipalities, and energy marketing companies on a wholesale basis. TEP has one operating segment, its regulated utility operations. TEP’s CODM is Susan M. Gray who holds the position of Chief Executive Officer of TEP and its parent company, UNS Energy. The CODM uses net income to assess performance and decide how to allocate resources for UNS Energy overall (including employees and

financial or capital resources) predominantly in the annual budget and forecasting process. Net income is reported on the Condensed Consolidated Statements of Income. Operations and Maintenance expense includes expenses reimbursed by third-parties and expenses related to customer-funded RES and DSM programs. Operations and Maintenance expense excluding these reimbursable and customer funded expenses totaled $92 million and $82 million for the three months ended March 31, 2026 and 2025, respectively. Total assets, the measure of segment assets, is reported on the Condensed Consolidated Balance Sheets. Capital expenditures are reported on the Condensed Consolidated Statements of Cash Flows.

Restricted Cash

RESTRICTED CASH

Restricted cash includes cash balances restricted with respect to withdrawal or usage based on contractual or regulatory considerations.

Restricted cash primarily represents cash contractually required to be set aside to pay TEP's share of final mine reclamation and decommissioning costs at San Juan.

Investment Tax Credits

In March 2026, TEP entered into an agreement to transfer ITCs related to Roadrunner Reserve II in the second half of 2026. In connection with ITCs expected to be generated by the Roadrunner Reserve II project, TEP recorded a $10 million deferred tax asset in Deferred Income Taxes, Net on the Condensed Consolidated Balance Sheets as of March 31, 2026, with a corresponding regulatory liability in Regulatory Liabilities on the Condensed Consolidated Balance Sheets to reflect the anticipated return of benefits to customers. As of March 31, 2026, TEP recorded a $1 million valuation allowance against the deferred tax asset reflecting the expected discount in proceeds from a third-party sale.

Roadrunner Reserve I was placed in service in July 2025 and generated $98 million of ITCs. In December 2025, TEP entered into an agreement to transfer ITCs, under which it transferred $49 million of ITCs and received $46 million in cash proceeds. On April 9, 2026, TEP transferred the remaining $49 million of ITCs related to Roadrunner Reserve I and received $46 million in cash proceeds.

New Accounting Standards Issued, and Not Yet Adopted

NEW ACCOUNTING STANDARDS ISSUED AND NOT YET ADOPTED

Standards Recently Issued by the FASB

The following new authoritative accounting guidance issued by the FASB has not yet been adopted and reflected in TEP’s financial statements. TEP is assessing the impact such guidance may have on TEP’s financial position, results of operations, cash flows, and disclosures.

Government Grants

In December 2025, the FASB issued accounting guidance that establishes recognition, measurement, and presentation requirements for government grants for business entities. The update applies to transfers of monetary or tangible nonmonetary assets from a government to a business entity, excluding certain transaction types such as tax incentives. The recognition, measurement, and presentation of a government grant is dependent on the classification of the grant as either related to an asset or related to income. Grants cannot be recognized until it is probable that the entity will comply with the conditions attached to

the grant and that the grant will be received. The amendments are effective for annual and interim reporting periods beginning after January 1, 2029, for public business entities. The guidance may be applied using a modified prospective, modified retrospective, or full retrospective approach. Early adoption is permitted.

Targeted Improvements to the Accounting for Internal-Use Software

In September 2025, the FASB issued accounting guidance that updates the recognition criteria for internal-use software. The update removes references to software development project stages and requires capitalization of software costs once: (i) management has authorized and committed to funding the project; and (ii) it is probable that the project will be completed and the software will perform its intended function. Evaluating the probability that the project will be completed includes an assessment of whether significant development uncertainty exists. The amendment requires entities to apply the disclosure requirements applicable to property, plant, and equipment to all capitalized internal-use software costs, irrespective of financial statement presentation. The amendments are effective for annual and interim reporting periods beginning after January 1, 2028. The guidance may be applied prospectively, retrospectively, or using a modified transition approach. Early adoption is permitted.

Disaggregation of Income Statement Expenses

In November 2024, the FASB issued accounting guidance that requires disaggregation of income statement expenses into specified categories in the footnotes to the financial statements. In January 2025, the FASB issued accounting guidance clarifying the effective date of this standard. The amendments are effective for annual periods beginning January 1, 2027, and interim reporting periods beginning after January 1, 2028. The guidance is to be applied on a prospective basis with the option to apply the standard retrospectively. Early adoption is permitted.

SEC Climate-Related Disclosures

The following final SEC rules regarding climate-related disclosures are pending further action by the SEC.

In March 2024, the SEC issued final rules requiring the disclosure of climate‑related risks and greenhouse gas emissions, which were subsequently stayed pending judicial review. In September 2025, the Court of Appeals for the Eighth Circuit paused its consideration of legal challenges against the rules, pending further action by the SEC. TEP awaits the SEC's decision whether it will defend, amend, or withdraw the rules, the timing of which TEP cannot currently predict.

Allowance for Credit Losses

ALLOWANCE FOR CREDIT LOSSES

TEP separately evaluates retail, wholesale, and other accounts receivable for credit losses and has not recorded an allowance for credit losses for non-retail accounts receivable.

Fair Value Measurement

TEP categorizes financial instruments into the three-level hierarchy based on inputs used to determine the fair value. Level 1 inputs are unadjusted quoted prices for identical assets or liabilities in an active market. Level 2 inputs include quoted prices for similar assets or liabilities, quoted prices in non-active markets, and pricing models whose inputs are observable, directly or indirectly. Level 3 inputs are unobservable and supported by little or no market activity. TEP has no financial instruments categorized as Level 3.

Derivative Instruments

DERIVATIVE INSTRUMENTS

TEP enters into various derivative and non-derivative contracts to reduce exposure to energy price risk associated with its natural gas and purchased power requirements. The objectives for entering into such contracts include: (i) creating price stability; (ii) meeting load and reserve requirements; and (iii) reducing exposure to price volatility that may result from delayed recovery under the PPFAC mechanism. In addition, TEP enters into derivative and non-derivative contracts to optimize the system's generation resources by selling power in the wholesale market for the benefit of TEP's retail customers.

TEP primarily applies the market approach for recurring fair value measurements. When TEP has observable inputs for substantially the full term of the asset or liability or uses quoted prices in an inactive market, it categorizes the instrument in Level 2. TEP categorizes derivatives in Level 3 when an aggregate pricing service or published prices that represent a consensus reporting of multiple brokers is used.

For both purchased power and natural gas prices, TEP obtains quotes from brokers, major market participants, exchanges, or industry publications and relies on its own price experience from active transactions in the market. TEP primarily uses one set of quotations each for purchased power and natural gas and then validates those prices using other sources. TEP believes that the market information provided is reflective of market conditions as of the time and date indicated.

Published prices for energy derivative contracts may not be available due to the nature of contract delivery terms such as non-standard time blocks and non-standard delivery points. In these cases, TEP applies adjustments based on historical price curve relationships, transmission costs, and real power line losses.

TEP also considers the impact of counterparty credit risk using current and historical default and recovery rates, as well as its own credit risk using credit default swap data.

The inputs and TEP's assessments of the significance of a particular input to the fair value measurements require judgment and may affect the valuation of fair value assets and liabilities and their placement within the fair value hierarchy levels. TEP evaluates the assumptions underlying its price curves monthly.

Credit Risk

CREDIT RISK

The use of contractual arrangements to manage the risks associated with changes in energy commodity prices creates credit risk exposure resulting from the possibility of non-performance by counterparties pursuant to the terms of their contractual obligations. TEP enters into contracts for the physical delivery of power and natural gas which contain remedies in the event of non-performance by the supply counterparties. In addition, volatile energy prices can create significant credit exposure from energy market receivables and subsequent measurements at fair value.

TEP has contractual agreements for energy procurement and hedging activities that contain provisions requiring TEP and its counterparties to post collateral under certain circumstances. These circumstances include: (i) exposures in excess of unsecured credit limits due to the volume of trading activity; (ii) changes in natural gas or power prices; (iii) credit rating downgrades; or (iv) unfavorable changes in parties' assessments of each other's credit strength. If such credit events were to occur, TEP, or its counterparties, could have to provide certain credit enhancements in the form of cash, LOCs, or other acceptable security to collateralize exposure beyond the allowed amounts.

TEP considers the effect of counterparty credit risk in determining the fair value of derivative instruments that are in a net asset position, after incorporating collateral posted by counterparties, and then allocates the credit risk adjustment to individual contracts. TEP also considers the impact of its credit risk on instruments that are in a net liability position, after considering the collateral posted, and then allocates the credit risk adjustment to individual contracts.