The supervisory body of a municipality proceeding under the terms of this article shall devote all moneys derived from any source other than the issuance of bonds to or for the payment of all operating expenses; bond interest and retirement and/or sinking fund payments; the acquisition and improvement of the electric plant; contingencies; the payment of other obligations incurred in the operation and maintenance of the electric plant and the furnishing of electric service; the redemption and purchase of electric plant bonds in which case such bonds shall be canceled; the creation and maintenance of a cash working fund; the payment of an amount to the general funds of the municipality not to exceed a cumulative return of six percent (6%) per annum of the equity or investment, if any, of the municipality; and, if the governing body of the municipality shall by resolution so request, payments to the municipality in lieu of ad valorem taxes on the property of the electric plant within the corporate or county limits of the municipality not to exceed the amount of taxes payable on privately owned property of similar nature. Any surplus thereafter remaining after establishment of proper reserves, if any, shall be devoted solely to the reduction of rates. In computing the equity or investment of the municipality, the value of the electric plant shall be taken as its historical cost, less the accrued depreciation as of the date of the first exercise by the municipality of the powers conferred by this article. The payment of bonds or the acquisition or improvement of property from the receipts derived from electric service or any other operation of the supervisory body as such shall not be considered to increase the equity or investment of the municipality.