A municipality is hereby authorized to fix, charge and collect rents, rates, fees and charges for electric power and energy and other services, facilities and commodities sold, furnished or supplied through the facilities of its electric system or its interest in any joint project. For so long as any bonds of a municipality are outstanding and unpaid, the rents, fees and charges shall be so fixed as to provide revenues sufficient to pay all costs of and charges and expenses in connection with the proper operation and maintenance of its electric system, and its interest in any joint project, and all necessary repairs, replacements or renewals thereof, to pay when due the principal of, premium, if any, and interest on all bonds and other evidences of indebtedness payable from said revenues, to create and maintain reserves as may be required by any resolution authorizing and securing bonds, to pay when due the principal of, premium, if any, and interest on all bonds heretofore or hereafter issued to finance additions, improvements and betterments to its electric system, and to pay any and all amounts which the municipality may be obligated to pay from said revenues by law or contract. Nothing herein contained shall be construed to prohibit any municipality from expending any revenues in excess of that required herein in any manner otherwise permitted by law.