Pool Heat Pump Size and Cost to Run (With the Math)

Size by surface, not gallons. A typical 15,000 gallon pool is about 16 x 32 ft, 512 sq ft: 10 F warmer needs about 61,000 BTU, 15 F needs about 92,000.

Hours x kW x your rate. A 100,000 BTU unit at COP 6.4 (4.6 kW) running 8 hours a day for 30 days uses about 1,100 kWh, about $190 at 17.3 cents. With a cover it runs fewer hours.

Usually, because a heat pump moves heat instead of making it: at COP 6 it delivers about 6 units of heat per unit of electricity. It heats more slowly and loses output in cold air, which is why gas still suits quick weekend heating.

It takes 1 BTU to warm 1 lb of water by 1 F, and a gallon weighs 8.34 lb. A 15,000 gallon pool needs about 125,100 BTU per degree, so a 100,000 BTU unit adds about 1 F every 1.25 hours, about 12.5 hours for 10 F before heat losses. Expect longer in practice, especially without a cover or in cool air.

Only for small pools. The 120V units in our list run 14,800 to 30,000 BTU. At 30,000 BTU and the surface x rise x 12 rule, that covers up to 250 sq ft at a 10 F rise or about 167 sq ft at 15 F, so it suits a 15 ft round pool (177 sq ft) but not an in-ground pool.

Use the minimum circuit ampacity and maximum breaker size on the unit's nameplate, as NEC Article 440 requires, on a dedicated circuit with a disconnect within sight of the unit. For scale, a 100,000 BTU unit at COP 6.4 draws about 4.6 kW, about 20 A at 230V while running. Have a licensed electrician do pool wiring.

The US Department of Energy says pool covers can cut heating costs by 50 to 70 percent. Most heat leaves through the surface, so covering it overnight means the heat pump runs fewer hours.

Yes, cost per hour falls in proportion to COP. A 100,000 BTU unit at COP 5 draws about 5.9 kW, about $1.01 an hour at 17.3 cents, versus about 4.6 kW and $0.79 an hour at COP 6.4. Rated COP in our list runs from 4.1 to 6.6.

If you swim often, yes, set it to hold your target temperature. Reheating a cold pool takes a long time, about 1.25 hours per degree for a 15,000 gallon pool with a 100,000 BTU unit, so letting it drop and recovering saves little. Use a cover so holding the temperature takes fewer hours.

It works, but output and COP fall as the air cools, because it pulls heat from the outside air. Check the minimum operating air temperature on the maker's spec sheet. For heating early in spring or late in fall, size for the larger temperature rise those months need.