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Rate Matters (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Fuel Rules [Abstract]              
Return of electric fuel credit, total [1]         $ (3.3) $ 3.2 $ 1.5
Deferred fuel rules monitored costs     $ 7.2 $ (8.8) [1]      
Electric Rate Proceeding [Member] | PSCW [Member] | MGE [Member]              
Rate Proceedings [Abstract]              
Authorized rate increase (decrease), percentage     9.01% [2] 8.81% [3] 0.00% [4]    
Authorized return on equity, percentage     9.80% 9.80% 9.80%    
Approved equity capital structure, percentage     55.60% 55.60% 55.80%    
Fuel Rules [Abstract]              
Fuel rules, bandwidth     2.00% 1.00%      
Effects Of The Tax Cuts And Jobs Act [Abstract]              
Return of unprotected excess deferred taxes         $ 18.2    
Electric Rate Proceeding [Member] | PSCW [Member] | Rate Matters For Future Periods [Member] | MGE [Member]              
Rate Proceedings [Abstract]              
Proposed rate increase (decrease), percentage [5],[6] 4.17% 1.54%          
Proposed return on equity, percentage 9.70% 9.70%          
Proposed equity capital structure, percentage 56.10% 56.10%          
Gas Rate Proceeding [Member] | PSCW [Member] | MGE [Member]              
Rate Proceedings [Abstract]              
Authorized rate increase (decrease), percentage     0.96% [3] 2.15% [3] 4.00% [4]    
Authorized return on equity, percentage     9.80% 9.80% 9.80%    
Approved equity capital structure, percentage     55.60% 55.60% 55.80%    
Gas Rate Proceeding [Member] | PSCW [Member] | Rate Matters For Future Periods [Member] | MGE [Member]              
Rate Proceedings [Abstract]              
Proposed rate increase (decrease), percentage [5],[6] 1.32% 2.44%          
Proposed return on equity, percentage 9.70% 9.70%          
Proposed equity capital structure, percentage 56.10% 56.10%          
[1] There was no change to the recovery (refund) in the fuel rules proceedings from the amount MGE deferred.
[2] The electric rate increase was driven by generation assets including our investments in Badger Hollow II (solar), Paris (solar and battery), Red Barn (wind), and West Riverside (natural gas). In addition, the reopener request included an increase in fuel costs and the recovery of deferred 2021 fuel costs. The reopener also revised the depreciation schedule for Columbia Unit 2 and shared equipment to 2029 to align with the depreciation schedule for Columbia Unit 1.
[3] The electric and gas rate increases were driven by an increase in rate base including our investments in Badger Hollow I and a new customer information system. Also driving the requested electric increase were higher fuel and purchased power costs as well as the completion in 2021 of the one-time return of the electric excess deferred tax credit related to the 2017 Tax Act not restricted by IRS normalization rules. Included in the electric residential rate is a reduction in the customer fixed charge.
[4] The electric rate settlement included an increase in rate base but the associated rate increase was primarily offset by lower fuel and purchased power costs and a one-time $18.2 million return to customers of the portion of excess deferred taxes related to the 2017 Tax Act not restricted by IRS normalization rules. The gas rate increase covered infrastructure costs and technology improvements. The settlement agreement also included escrow accounting treatment for pension and other postretirement benefit costs, bad debt expense, and customer credit card fees. Escrow accounting treatment allows MGE to defer any difference between estimated costs in rates and actual costs incurred until a future rate filing. Any difference would be recorded as a regulatory asset or regulatory liability.
[5] In accordance with the 2024/2025 rate order from the PSCW, MGE will have an earnings sharing mechanism, under which, if MGE earns above the authorized ROE: (i) the utility will retain 100.0% of earnings for the first 15 basis points above the authorized ROE; (ii) 50.0% of the next 60 basis points will be required to be refunded to ratepayers; and (iii) 100.0% of any remaining excess earnings will be required to be refunded to ratepayers.
[6] The electric increase was driven by an increase in rate base including our investments made in West Riverside, local solar, and continued investment in grid modernization, as well as higher costs for transmission, pension and OPEB, and uncollectible costs (including costs previously deferred from prior years). This increase in electric costs is offset by a decrease in fuel costs and benefit from lower tax expense (including impacts from the Inflation Reduction Act). MGE will file an updated 2025 fuel forecast with the PSCW in 2024 which may impact rates in 2025, depending on any variance between the forecast submitted as part of the rates and updated forecast. In addition, the PSCW authorized MGE to defer a recovery of and a return on costs associated for any change in the in service date for Paris and Darien and force majeure costs for Badger Hollow II, Paris, and Darien that were not reflected in this rate filing. The PSCW also approved deferral of any differential in PTC tax credits reflected in rates and actual credits produced. These deferrals will be reflected in MGE's next rate case filing. The gas rate increases were also driven by our investment made in grid modernization and higher pension and OPEB and uncollectible costs (including costs previously deferred from prior years). This increase in gas costs is offset by a tax benefit related to excess deferred taxes. Included in the gas residential rate is a reduction in the customer fixed charge.