New heat pump water heaters consume half the energy of conventional electric-resistance ones.

by John J. Tomlinson

About half of all residential water heaters in the United States use resistance heating elements inside a tank to convert electric energy into heat. The most efficient of these electric resistance water heaters (ERWH) is about 93% efficient, so it would seem that there is little room for further efficiency improvements to electric water heaters. But in fact, a heat pump can greatly increase a water heater’s efficiency (see “Measuring Efficiency”).

Today’s residential heat pump water heater (HPWH) brings the efficiency of water heating to a level not possible with a conventional ERWH. Like an air conditioner or refrigerator, the heat pump water heater uses a compressor, an evaporator, a condenser, and an expansion device to transfer heat from one region to another. A refrigerator moves heat from inside the refrigerator to the house air. An air conditioner moves heat from inside the house to outside. In the case of a HPWH, heat is moved from the house, garage, or outside air into the water tank.

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