An energy storage system converts variable renewable electricity (VRE) to continuous heat at over 1000° C. Intermittent electrical energy heats a solid medium. Heat from the solid medium is delivered continuously on demand. Heat delivery via flowing gas establishes a thermocline which maintains high outlet temperature throughout discharge. The delivered heat which may be used for processes including power generation and cogeneration. In one application, the energy storage system provides higher-temperature heat to a conventional lower-temperature heat source to boost the temperature of a thermal power cycle working fluid to a turbine, thereby increasing efficiency of the power cycle.