When revenue bonds are outstanding the director shall establish, maintain, and collect rates or charges for electric power and energy, and other services, facilities and commodities sold and supplied by the director which shall be fair and nondiscriminatory and adequate to provide revenue sufficient to pay the principal of and interest on revenue bonds outstanding, and all payments which the director is obligated to make to the state treasurer for deposit in any special fund or funds created for such purpose, and for the proper operation and maintenance of the utility and all necessary repairs, replacements and renewals thereof.