Home / The guide / Heating and cooling

Add a heat pump water heater or wood stove wetback as backup

A heat pump water heater (HPWH) uses far less electricity than a resistance element and can run on daytime solar. A wetback uses stove heat in winter when solar is weakest.

How

1. HPWHs are often 2 to 3 times more efficient than resistance heaters (DOE).
2. Install one where there is at least 1,000 cu ft (about 28 m3) of air and temperatures stay roughly 4 to 32 C (40 to 90 F) for best efficiency. It cools the room it sits in.
3. Schedule it to run in solar hours and keep the tank as your energy store.
4. For a wetback, use a stove designed for one, with a correctly sized heat exchanger, pressure and temperature relief, and pipework that cannot be isolated from the tank.
5. Remember an HPWH falls back to its resistance element in high demand mode, which can overload a small inverter; use economy mode.
6. In earthquake areas, strap the water heater so it cannot move, and fit flexible gas connections, as FEMA advises.

Safety: Never block or cap a relief valve on any hot water system.

Do these first

• power-1

Sources

1. Decarbonization Tech Snapshot: Heat Pump Water Heaters. U.S. Department of Energy, accessed 2026-11
2. Protect your property from earthquakes. FEMA, accessed 2026-11

Open in the interactive guide