VFD heat pump water heater?

I just thought of something. Why isn't there a 10 gallon or so water heater (equivalent to roughly a 40 gallon or so regular water heater) that uses a VFD heat pump? When the heater is not being used, a 500w or so heating element keeps it warm. As soon as it's used, the controller senses the decrease in temperature and starts the heat pump. The compressor speed is modulated to maintain temperature. For extra efficiency gain, the VFD circuitry is thermally coupled to the water inlet pipe. For further efficiency gain, the temperature is turned down during off-peak hours.