PUC § 13132

The board shall fix rates for service from the electric system to which the indebtedness pertains, and collect charges for such service, such as to provide revenues at least sufficient in the aggregate to pay, as the same shall become due, the principal and interest on all such bonds so outstanding payable out of those revenues, including premiums, if any, due upon the redemption of any thereof, in addition to paying, as the same shall become due, the necessary expenses of operating and maintaining such system, and all other obligations and indebtedness payable out of those revenues.