(a) The accounting treatments specified in this Code section shall apply in any proceeding before the commission to determine the rates to be charged by an electric utility.

(b) In any proceeding to determine the rates to be charged by an electric utility, the electric utility shall file jurisdictionally allocated cost of service data on the basis of a test period, and the commission shall utilize a test period, consisting of actual data for the most recent 12 month period for which data are available, fully adjusted separately to reflect estimated operations during the 12 months following the utility's proposed effective date of the rates. After the initial filing and until new rates go into effect, the utility shall file actual cost of service data as they become available for each month following the actual data which were filed. The utility shall have the burden of explaining and supporting the reasonableness of all estimates and adjustments contained in its cost of service data.

(c) In any case after March 2, 1981, in which an electric utility transfers partial or total ownership of any electric plant, the electric utility shall return to its ratepayers, in such manner as the commission may prescribe, the ratepayers' cash contribution to the cost of construction, plus the income taxes paid, accrued, and collected by the electric utility in respect thereof, and a portion of the profit, if any, on such transfer.

(1) The ratepayers' contribution to the cost of construction shall be determined by first multiplying the amount of construction and preconstruction expenditures included in the rate base by the authorized return allowed by the commission for the electric utility on the jurisdictional rate base during the period or periods such expenditures were included in the rate base, and then subtracting therefrom any carrying costs capitalized in respect of such construction and preconstruction expenditures included in the rate base. In the event an electric utility transfers partial or total ownership of any electric plant after it has commenced commercial operation, the ratepayers' contribution to the cost of construction shall be reduced in proportion to the remaining life of the electric plant.

(2) The profit, if any, on such transfer shall be allocated between the electric utility and its ratepayers in proportion to their respective contributions to the cost of construction. For the purposes of this calculation, the ratepayers' contribution shall be determined as provided by paragraph (1) of this subsection. The electric utility's contribution shall be the cost, including carrying costs, of the transferred electric plant as recorded on the books and records of the electric utility. The amount of profit, if any, shall be determined by deducting the sum of the ratepayers' and the electric utility's contributions to the cost of construction from the gross proceeds of the transfer.