Any city of the state of Idaho owning or controlling a power plant may sell its excess power to persons and corporations for any lawful purpose. The term "excess power" means all electricity not needed by the city or the inhabitants thereof. All charges or rates for the excess power shall be fixed by ordinance and shall be uniform and fair to all consumers and no discrimination shall be allowed or practiced by any city; provided, that any city which may desire to take advantage of the provisions of this section may only contract with consumers as to excess power. Under this section all contracts with consumers are to be drafted subject to the foregoing provision and no contract shall be for a period longer than five (5) years.