The revenues of an electric cooperative 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 of directors from time to time may prescribe. Revenues not required for the purposes set forth in this section shall be returned from time to time to the members of the cooperative in cash, in abatement of current charges for electrical energy, or otherwise as the board of directors may determine on a pro rata basis according to the amount of business done with each during the period. Such return may be made by way of general rate reduction to the members if the board of directors so elects.