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Income Taxes (Tables)
 12 Months Ended
Dec. 31, 2023
Income Tax Disclosure [Abstract]  
Summary of Statute of Limitations Applicable to Open Tax Years
Tax Year(s)Expiration
2014 - 2016March 2025
2020September 2024
Reconciliation of Unrecognized Tax Benefits
Unrecognized tax benefits — permanent vs temporary:
(Millions of Dollars)Dec. 31, 2023Dec. 31, 2022
Unrecognized tax benefit — Permanent tax positions$5 $5 
Unrecognized tax benefit — Temporary tax positions— 4 
Total unrecognized tax benefit$5 $9 
Changes in unrecognized tax benefits:
(Millions of Dollars)202320222021
Balance at Jan. 1$9 $8 $7 
Additions based on tax positions related to the current year1 1 1 
Reductions for tax positions of prior years(5)— — 
Balance at Dec. 31$5 $9 $8 
Tax Benefits Associated with NOL and Tax Credit Carryforwards
Unrecognized tax benefits were reduced by tax benefits associated with NOL and tax credit carryforwards:
(Millions of Dollars)Dec. 31, 2023Dec. 31, 2022
NOL and tax credit carryforwards$(4)$(8)
Interest Payable related to Unrecognized Tax Benefits
Interest payable related to unrecognized tax benefits:
(Millions of Dollars)202320222021
Payable for interest related to unrecognized tax benefits at Jan. 1$— $(1)$(1)
Interest expense related to unrecognized tax benefits— 1 — 
Payable for interest related to unrecognized tax benefits at Dec. 31$— $— $(1)
NOL and Tax Credit Carryforwards NOL amounts represent the tax loss that is carried forward and tax credits represent the deferred tax asset. NOL and tax credit carryforwards as of Dec. 31 were as follows:
(Millions of Dollars)20232022
Federal NOL carryforward$— $8 
Federal tax credit carryforwards406 294 
Valuation allowances for federal credit carryforwards(2)— 
State NOL carryforwards3 4 
Schedule of Effective Income Tax Rate Reconciliation
Total income tax expense from operations differs from the amount computed by applying the statutory federal income tax rate to income before income tax expense.
Effective income tax rate for years ended Dec. 31:
202320222021
Federal statutory rate21.0 %21.0 %21.0 %
State income tax on pretax income, net of federal tax effect2.4 2.3 2.5 
Increases (decreases) in tax from:
Wind PTCs (a)
(33.5)(35.9)(39.7)
Plant regulatory differences (b)
(3.3)(3.8)(4.8)
Amortization of deficient/(excess) nonplant deferred taxes0.3 (0.9)(1.1)
Other, net(0.2)(0.6)(0.7)
Effective income tax rate(13.3)%(17.9)%(22.8)%
(a)Wind PTCs net of estimated transfer discounts. Gross wind PTCs are credited to customers (reduction to revenue) and do not materially impact net income.
(b)Plant regulatory differences primarily relate to the credit of excess deferred taxes to customers through the average rate assumption method. Income tax benefits associated with the credit are offset by corresponding revenue reductions.
Schedule of Components of Income Tax Expense (Benefit)
Components of income tax expense for years ended Dec. 31:
(Millions of Dollars)202320222021
Current federal tax expense (benefit)$76 $(65)$(11)
Current state tax expense (benefit)12 (5)(1)
Deferred federal tax (benefit) expense(132)3 (57)
Deferred state tax (benefit) expense(1)13 9 
Deferred change in unrecognized tax expense— 1 1 
Total income tax benefit$(45)$(53)$(59)
Components of deferred income tax expense as of Dec. 31:
(Millions of Dollars)202320222021
Deferred tax (benefit) expense excluding items below
$(117)$37 $(23)
Amortization and adjustments to deferred income taxes on income tax regulatory assets and liabilities(16)(20)(24)
Deferred tax (benefit) expense$(133)$17 $(47)
Schedule of Deferred Tax Assets and Liabilities
Components of the net deferred tax liability as of Dec. 31:
(Millions of Dollars)2023
2022 (a)
Deferred tax liabilities:
Differences between book and tax bases of property$1,072 $1,015 
Operating lease assets90 97 
Regulatory assets51 61 
Pension expense32 33 
Total deferred tax liabilities$1,245 $1,206 
Deferred tax assets:
Tax credit carryforward$406 $294 
Regulatory liabilities95 98 
Operating lease liabilities90 97 
Other employee benefits6 6 
Deferred fuel costs5 (35)
NOL carryforward— 2 
Tax credit valuation allowances(2)— 
Other23 5 
Total deferred tax assets623 467 
Net deferred tax liability$622 $739 
(a)Prior periods have been reclassified to conform to current year presentation.
State Summary of Statute of Limitations Applicable to Open Tax Years SPS is a member of the Xcel Energy affiliated group that files consolidated state income tax returns. As of Dec. 31, 2023, SPS’ earliest open tax years that are subject to examination by state taxing authorities under applicable statutes of limitations are as follows:
StateTax Year(s)Expiration
Texas2016, 2018May 2024
Texas2017July 2025
Texas2019August 2024