The revenue of the corporation shall be devoted, first, to the payment of operating and maintenance expenses and the principal and interest on outstanding obligations, and thereafter to such reserves for improvement, new construction, depreciation and contingencies, as the board may from time to time prescribe. Revenue not required for the purposes set forth in this section shall be returned from time to time to the users of the services or products of such corporation on a pro rata basis according to the amount of business done with each during the period, either in cash, in abatement of current charges for electric energy, or otherwise, as the board determines; but such return may be made by way of general rate reduction to such users, if the board so elects.