Refrigerators are similar to tank system, rather than on-demand hot water system. With smaller heat pump, tank is needed because it can't heat water immediately

Refrigerators are similar to tank system, rather than on-demand hot water system. With smaller heat pump, tank is needed because it can't heat water immediately. Refrigerators have small heat pump so that's similar thing. Both should have good insulation. Heat pump heater can be run when solar panel generates so much energy (energy price is low). That's a similar thing as batteries for energy use time shift.