A method of operating a combined cycle electric power plant (10,20,30), the method comprising: operating a combustion turbine generator to generate electricity and produce hot exhaust gases; storing heat from a heat source (300,350) other than the combustion turbine generator in a thermal energy storage system at a temperature T1; and in a first mode of operation: producing steam in a first boiler by heating feedwater with heat supplied at temperature T1 from the thermal energy storage system by a heat transfer fluid, thereby cooling the heat transfer fluid, and storing heat from the cooled heat transfer fluid in the thermal energy storage system at a temperature T2 < T1; heating the steam from the first boiler with heat from the combustion turbine exhaust gases to produce superheated steam at pressure P1; and expanding the superheated steam at pressure P1 through a steam turbine generator to generate electricity; characterized by: in a second mode of operation, after depleting the thermal energy storage system of heat stored at temperature T1: producing steam in the first boiler by heating feedwater with heat supplied from the thermal energy storage system at a temperature of T2 or less by a heat transfer fluid, without additional heat from the heat source (300, 350); heating the steam from the first boiler with heat from the combustion turbine exhaust gases to produce superheated steam at pressure P2 < P1; and expanding the superheated steam at pressure P2 through the steam turbine generator to generate electricity. operating a combustion turbine generator to generate electricity and produce hot exhaust gases; storing heat from a heat source (300,350) other than the combustion turbine generator in a thermal energy storage system at a temperature T1; and in a first mode of operation: producing steam in a first boiler by heating feedwater with heat supplied at temperature T1 from the thermal energy storage system by a heat transfer fluid, thereby cooling the heat transfer fluid, and storing heat from the cooled heat transfer fluid in the thermal energy storage system at a temperature T2 < T1; heating the steam from the first boiler with heat from the combustion turbine exhaust gases to produce superheated steam at pressure P1; and expanding the superheated steam at pressure P1 through a steam turbine generator to generate electricity; producing steam in a first boiler by heating feedwater with heat supplied at temperature T1 from the thermal energy storage system by a heat transfer fluid, thereby cooling the heat transfer fluid, and storing heat from the cooled heat transfer fluid in the thermal energy storage system at a temperature T2 < T1; heating the steam from the first boiler with heat from the combustion turbine exhaust gases to produce superheated steam at pressure P1; and expanding the superheated steam at pressure P1 through a steam turbine generator to generate electricity; characterized by: in a second mode of operation, after depleting the thermal energy storage system of heat stored at temperature T1: producing steam in the first boiler by heating feedwater with heat supplied from the thermal energy storage system at a temperature of T2 or less by a heat transfer fluid, without additional heat from the heat source (300, 350); heating the steam from the first boiler with heat from the combustion turbine exhaust gases to produce superheated steam at pressure P2 < P1; and expanding the superheated steam at pressure P2 through the steam turbine generator to generate electricity. producing steam in the first boiler by heating feedwater with heat supplied from the thermal energy storage system at a temperature of T2 or less by a heat transfer fluid, without additional heat from the heat source (300, 350); heating the steam from the first boiler with heat from the combustion turbine exhaust gases to produce superheated steam at pressure P2 < P1; and expanding the superheated steam at pressure P2 through the steam turbine generator to generate electricity.