A joint agency may sell or exchange the excess capacity or output of a project not then required by any of its members for such consideration and for such period and upon such other terms and conditions as may be determined by the parties to any municipality in this state or any other state owning electric distribution facilities, to any political subdivisions, agencies or instrumentalities of any other state, to other joint agencies organized pursuant to this article, to any electric power association, to any public or private utility, or to any other state, federal or municipal agency which owns electric generation, transmission or distribution facilities either within or without this state. Provided, however, that sales of excess capacity or output of a project to electric power associations, public or private utilities, and other persons the interest on whose securities and other obligations are not exempt from taxation by the federal government shall not be made in such amounts, for such periods of time, and under such terms and conditions as will cause the interest on bonds issued to finance the cost of a project to become taxable by the federal government.