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Income Taxes (Tables)
 12 Months Ended
Dec. 31, 2024
Income Tax Disclosure [Abstract]  
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:
202420232022
Federal statutory rate21.0 %21.0 %21.0 %
State income tax on pretax income, net of federal tax effect6.2 6.2 6.2 
Decreases in tax from:
Plant regulatory differences (a)
(4.2)(3.4)(3.8)
Other, net— (1.1)(1.5)
Effective income tax rate23.0 %22.7 %21.9 %
(a)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)202420232022
Current federal tax expense$29 $37 $28 
Current state tax expense10 15 9 
Current change in unrecognized tax expense1 — — 
Deferred federal tax benefit(3)(10)(5)
Deferred state tax expense (benefit)3 (3)3 
Deferred change in unrecognized tax expense — 1 — 
Total income tax expense$40 $40 $35 
Components of deferred income tax expense as of Dec. 31:
(Millions of Dollars)202420232022
Deferred tax expense (benefit) excluding items below$12 $(3)$8 
Amortization and adjustments to deferred income taxes on income tax regulatory assets and liabilities(11)(10)(10)
Other(1)1 — 
Deferred tax benefit$— $(12)$(2)
Schedule of Deferred Tax Assets and Liabilities
Components of the net deferred tax liability as of Dec. 31:
(Millions of Dollars)20242023
Deferred tax liabilities:
Difference between book and tax bases of property$366 $350 
Regulatory assets17 22 
Pension expense9 10 
Other10 7 
Total deferred tax liabilities$402 $389 
Deferred tax assets:
Regulatory liabilities$40 $32 
Rate refund5 10 
Environmental remediation4 4 
Other employee benefits4 4 
Tax credit carryforward2 3 
Deferred ITCs2 2 
Other3 4 
Total deferred tax assets$60 $59 
Net deferred tax liability$342 $330 
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)20242023
Federal tax credit carryforwards$2 $3 
State NOL carryforward1 2 
Summary of Statute of Limitations Applicable to Open Tax Years [Table Text Block] NSP-Wisconsin is a member of the Xcel Energy affiliated group that files a consolidated federal income tax return. Statute of limitations applicable to Xcel Energy’s consolidated federal income tax returns expire as follows:
Tax Year(s)Expiration
2014 - 2016March 2025
2021October 2025
Reconciliation of Unrecognized Tax Benefits
Unrecognized tax benefits — permanent vs. temporary:
(Millions of Dollars)Dec. 31, 2024Dec. 31, 2023
Unrecognized tax benefit — Permanent tax positions$4 $3 
Unrecognized tax benefit — Temporary tax positions— — 
Total unrecognized tax benefit$4 $3 
Changes in unrecognized tax benefits:
(Millions of Dollars)202420232022
Balance at Jan. 1$3 $3 $3 
Additions for tax positions of prior years2 — — 
Reductions for tax positions of prior years(1)— — 
Balance at Dec. 31$4 $3 $3 
Tax Benefits Associated with NOL and Tax Credit Carryforwards
Unrecognized tax benefits were reduced by tax benefits associated with tax credit carryforwards:
(Millions of Dollars)Dec. 31, 2024Dec. 31, 2023
Tax credit carryforwards$(2)$(2)
State Statute of Limitations Applicable to Open Tax Years NSP-Wisconsin is a member of the Xcel Energy affiliated group that files consolidated state tax returns based on income in its major operating jurisdictions and various other state income-based tax returns. As of Dec. 31, 2024, NSP-Wisconsin’s earliest open tax years (subject to examination by state taxing authorities in its major operating jurisdictions) were as follows:
StateTax Year(s)Expiration
Wisconsin2016-2019May 2025
Wisconsin2020September 2025
Interest Payable related to Unrecognized Tax Benefits
Interest payable related to unrecognized tax benefits:
(Millions of Dollars)202420232022
Payable for interest related to unrecognized tax benefits at Jan. 1$— $— $— 
Interest expense related to unrecognized tax benefits(1)— — 
Payable for interest related to unrecognized tax benefits at Dec. 31$(1)$— $—