Heat-Pump Water Heaters Ready for Prime Time

Rather than using electricity directly for heating the water, heat pumps use electricity to move heat from one place to another. By alternating cycles of compression and expansion (evaporation) in a closed loop, a heat-pump water heater is able to extract heat from the surrounding air and deliver it to an insulated tank of water even though the stored water is at a higher temperature than the surrounding air.

While a storage-type electric water heater has a theoretical maximum energy factor (a measure of efficiency) of 1.0, a heat-pump water heater can achieve an energy factor of 2.0 or higher. The performance of some products is measured using