A heat reservoir for storing the energy necessary for powering heat engines, particularly steam engines, is disclosed. The heat reservoir converts electrical energy into thermal energy through the use of electrical resistance heating coils bathed in one or more high heat capacity metals. Loss of heat through thermal conduction is minimized through the use of an insulating jacket surrounding the reservoir. The stored heat can be quickly transferred to a working fluid, such as steam, by using both a heat storage metal and a heat exchanger metal that possess high thermal conductivity.