The COP of a heat pump hot water heater is strongly dependent on the ambient and current water temperature. You get the best COP heating from cold, but it's no

The COP of a heat pump hot water heater is strongly dependent on the ambient and current water temperature. You get the best COP heating from cold, but it's not often that you have to reheat an entire tank of cold water. The actual achieved COP in normal use is more like 2-2.5.