Schedules of rates and charges for the sale, including dispositions to Federal agencies, of all electric power made available to the Administrator pursuant to section
838f of this title or otherwise acquired, and for the transmission of non-Federal electric power over the Federal transmission system, shall become effective upon confirmation and approval thereof by the Secretary of Energy. Such rate schedules may be modified from time to time by the Secretary of Energy, acting by and through the Administrator, subject to confirmation and approval by the Secretary of Energy, and shall be fixed and established