The hydroelectric plant or plants and transmission lines constructed pursuant to Section 25720 may be leased for operation to, or power generated may be sold to, a public utility or public agency engaged in the distribution, use, or sale of electricity. The power generated may be used by the county for its own purposes, or for the production or transmission of water, but shall not be offered for sale directly by the county to customers other than a public utility or public agency. The power to acquire works and facilities shall not include, and nothing in this article shall be construed to allow, the acquisition of property already employed in the generation of hydroelectric energy for public utility purposes, except by mutual agreement between the county and the owner and operator of that property.