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)			2021		2020
Plant in Service
Generation Plant	3.11%	18	$	3,753	$	3,279
Distribution Plant	1.93%	33	2,024		1,906
Transmission Plant	1.69%	35	1,210		1,090
General Plant	6.01%	7	540		503
Intangible Plant, Software Costs, and Other (1)	Various	Various	268		291
Plant Held for Future Use	—	—	3		4
Total Plant in Service (2)			$	7,798	$	7,073

(1)Primarily represents computer software, which is 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.

(3)Based on the 2018 depreciation study available for the major classes of Plant in Service, effective January 1, 2021, as approved as part of the 2020 Rate Order. Transmission Plant depreciation rates are based on the 2018 depreciation study, effective August 1, 2019, as approved as part of the 2019 FERC Rate Case.

In 2019, TEP entered into a BTA to develop Oso Grande. In May 2021, Oso Grande was placed in service, adding 250 MW of wind-powered electric generation, increasing TEP's total renewable nominal generation capacity, including PPAs and owned utility-scale generation, to over 700 MW. As of December 31, 2021, there was $444 million in costs related to Oso Grande in Plant in Service on the Consolidated Balance Sheets.

In 2020, due to expiring leases, TEP purchased 32.2% in undivided interests in facilities at Springerville used in common with Springerville Unit 1 and Unit 2 (Springerville Common Facilities) at a total fixed purchase price of $68 million. The transaction resulted in an increase in Plant in Service and a decrease in Utility Plant Under Finance Leases on the Consolidated Balance Sheets.

As a condition of the purchase, Salt River Project Agricultural Improvement and Power District (SRP), the owner of Springerville Unit 4, purchased a 14% undivided interest in the Springerville Common Facilities for $30 million in 2020. Also, Tri-State, the lessee of Springerville Unit 3, was obligated to either: (i) buy a 14% undivided interest in the facilities for

$30 million by December 31, 2021; or (ii) continue to make payments to TEP for the use of these facilities. Tri-State did not exercise its purchase option and therefore will continue to make payments to TEP.

Under the air permit approved by the Pima County Department of Environmental Quality, TEP placed into service 10 natural gas RICE units with a total nominal generation capacity of 188 MW in 2020. As of December 31, 2021, there was $187 million related to the Sundt RICE units recorded in Plant in Service on the Consolidated Balance Sheets.

As part of the 2020 Rate Order, effective January 2021, TEP reclassified $33 million from Regulatory and Other Liabilities—Regulatory Liabilities to Accumulated Depreciation and Amortization on the Consolidated Balance Sheets to reflect the impact of the revised depreciation rates on estimated cost of removal.

Intangible Plant primarily consists of computer software. Accumulated amortization of computer software costs were $169 million and $199 million as of December 31, 2021 and 2020, respectively. Amortization of computer software costs totaled $33 million in 2021, $29 million in 2020, and $26 million in 2019. Future estimated amortization costs for existing computer software are $26 million in 2022, $19 million in 2023, $12 million in 2024, $7 million in 2025, and $4 million in 2026.

Intangible Plant includes $(4) million in acquisition discounts not subject to amortization as of December 31, 2021, and none in 2020.

As of December 31, 2021, TEP was a participant in the following jointly-owned generation facilities and transmission systems:


($ in millions)	Ownership Percentage	Plant in Service		Construction Work in Progress		Accumulated Depreciation		Net Book Value
San Juan Unit 1	50.0%	$	285	$	1	$	(269)	$	17
Four Corners Units 4 and 5	7.0%	188		4		(81)		111
Luna	33.3%	59		1		(3)		57
Gila River Unit 3	75.0%	206		—		(65)		141
Gila River Common Facilities	43.8%	75		1		(26)		50
Springerville Coal Handling Facilities	83.0%	209		—		(95)		114
Springerville Common Facilities	86.0%	399		—		(207)		192
Transmission Facilities	Various	462		20		(229)		253
Total		$	1,883	$	27	$	(975)	$	935

As a participant in these jointly-owned facilities, TEP is responsible for its share of operating and capital costs. The Company accounts for its share of operating expenses and utility plant costs related to these facilities using proportionate consolidation.

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)	2021		2020
Beginning of Period	$	96	$	107
Liabilities Incurred (1)	14		—
Liabilities Settled (2)	(2)		(5)
Regulatory Deferral/Accretion Expense	4		4
Revisions to the Present Value of Estimated Cash Flows (3)	27		(10)
End of Period	$	139	$	96

(3)Primarily related to changes due to revised estimates of the timing of cash flows required to settle future liabilities of San Juan and changes in ownership of Springerville Common Facilities.