A corporation formed under this chapter shall have power to charge reasonable fees, rents, tolls, prices and other charges for service rendered which shall be sufficient at all times to pay all operating and maintenance expenses necessary or desirable for the prudent conduct and operation of its business and the principal of and interest on such obligations as the corporation may have issued or assumed in the performance of the purpose for which it was formed, and the revenues and receipts of a corporation shall first be devoted to such operating and maintenance expenses and to the payment of such principal and interest and thereafter to such reserves for improvement, new construction, depreciation and contingencies as the board may from time to time prescribe. Revenues and receipts not needed for these purposes shall be returned to the members, either in cash or in payment of past due or current charges for energy, as the board may decide, in proportion to the gross operating revenues received from each, or such return may be made by way of a general rate reduction to members, if the board so elects.