Any electric or heat pump water heater that can have its thermostat digitally controlled could be combined with a thermostatic mixing valve hooked to the cold w

Any electric or heat pump water heater that can have its thermostat digitally controlled could be combined with a thermostatic mixing valve hooked to the cold water line and used as a heat battery. When electricity is cheap/solar is producing have set the thermostat to the highest value it allows. Otherwise set it down to the the lowest comfortable value(~120F). The thermostatic mixing valve will mix the hot water with cold water to meet the comfortable value regardless of where it's at in the tank and the insulation will keep the water hot for hours after it's heated to the maximum.