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UTILITY PLANT AND JOINTLY-OWNED FACILITIES
 12 Months Ended
Dec. 31, 2023
Regulated Operations [Abstract]  
UTILITY PLANT AND JOINTLY-OWNED FACILITIES UTILITY PLANT AND JOINTLY-OWNED FACILITIES
UTILITY PLANT
The following table shows Plant in Service on the Consolidated Balance Sheets by major class:
Annual Depreciation Rate (3)
Average Remaining Life in Years (3)
December 31,
($ in millions)20232022
Plant in Service
Generation Plant 3.05%22$3,536 $3,491 
Distribution Plant2.61%462,279 2,149 
Transmission Plant 1.69%331,323 1,295 
General Plant6.13%9685 653 
Intangible Plant, Software Costs, and Other (1)
VariousVarious201 224 
Plant Held for Future Use——11 2 
Total Plant in Service (2)
$8,035 $7,814 
(1)Primarily represents computer software, which is being amortized over three to five years for smaller application software and 10 years for large enterprise software and has an average remaining life of three years.
(2)Includes plant acquisition adjustments of $(206) million as of December 31, 2023 and 2022.
(3)Based on the 2022 depreciation study available for the major classes of Plant in Service, effective September 1, 2023 as approved by the ACC as part of the 2023 Rate Order. TEP implemented new depreciation rates for Transmission Plant based on the 2018 depreciation study, effective August 1, 2019, as approved as part of the 2022 Final FERC Rate Order.
Accumulated Depreciation and Amortization
Depreciation Rates
As part of the 2023 Rate Order, effective September 2023, TEP reclassified $115 million from Accumulated Depreciation and Amortization to Regulatory and Other Liabilities—Regulatory Liabilities on the Consolidated Balance Sheets to reflect the impact of the revised depreciation rates on estimated cost of removal.
Amortization of Intangible Plant
Intangible Plant primarily consists of computer software. Accumulated amortization of computer software costs was $100 million and $110 million as of December 31, 2023 and 2022, respectively. Amortization of computer software costs totaled $27 million in 2023, $30 million in 2022, and $33 million in 2021. Future estimated amortization costs for existing computer software are $24 million in 2024, $19 million in 2025, $16 million in 2026, $11 million in 2027, and $7 million in 2028.
Intangible Plant includes $(4) million in acquisition discounts not subject to amortization as of December 31, 2023 and 2022.
JOINTLY-OWNED FACILITIES
As of December 31, 2023, TEP was a participant in the following jointly-owned generation facilities and transmission systems:
($ in millions)Ownership PercentagePlant in ServiceConstruction Work in ProgressAccumulated DepreciationNet Book Value
Four Corners Units 4 and 57.0%$201 $4 $(97)$108 
Luna 33.3%58 3 — 61 
Gila River Unit 375.0%218 17 (62)173 
Gila River Common Facilities43.8%78 1 (30)49 
Springerville Coal Handling Facilities83.0%208 — (103)105 
Springerville Common Facilities86.0%400 — (228)172 
Transmission FacilitiesVarious555 21 (236)340 
Total$1,718 $46 $(756)$1,008 
As a participant in these jointly-owned facilities, TEP is responsible for its share of operating and capital costs. TEP accounts for its share of operating expenses and utility plant costs related to these facilities using proportionate consolidation.
ASSET RETIREMENT OBLIGATIONS
The liability accrual of AROs is primarily related to generation assets and is included in Other on the Consolidated Balance Sheets. The following table reconciles the beginning and ending aggregate carrying amounts of ARO accruals on the Consolidated Balance Sheets:
December 31,
(in millions)20232022
Beginning of Period$121 $139 
Liabilities Incurred (1)
— 1 
Liabilities Settled (2)
(2)(8)
Regulatory Deferral/Accretion Expense3 5 
Revisions to the Present Value of Estimated Cash Flows (3)
(8)(16)
End of Period$114 $121 
(1)In 2022, TEP incurred an ARO for new photovoltaic generation placed in service.
(2)Primarily related to the retirement of Navajo and San Juan.
(3)Primarily related to revised decommissioning estimates for San Juan.