For purposes of economy, efficiency, and conserving oil and natural gas, the head of a federal agency, when practicable and consistent with other laws and requirements applicable to that agency, shall endeavor to purchase electric energy from a non-federal person for consumption in Alaska by a facility of that agency when (taking into account the remaining useful life of any facility available to that agency to generate electric energy for that agency and the cost of maintaining the facility on a standby basis) the purchase will result in—