How to size a pool heat pump
BTU per hour = surface sq ft × (pool °F − air °F) × 12 ÷ output share at your air temperature
This is the US Department of Energy method: a 16x32 pool (512 sq ft) kept at 82 °F in a month that averages 65 °F needs 512 × 17 × 12 = 104,448 BTU per hour of output. A heat pump makes its rated output only in warm air. Hayward's 140,000 BTU HeatPro, for example, is rated 140,000 at 80 °F air but 85,000 at 50 °F, about 61 %. If you heat in 65 °F air, a unit rated near 130,000 is the honest match.
| Pool | Air 75 °F | Air 70 °F | Air 65 °F | Rated size, 65 °F air |
|---|---|---|---|---|
| 12x24 (288 sq ft) | 24,000 | 41,000 | 59,000 | 73,000 |
| 14x28 (392 sq ft) | 33,000 | 56,000 | 80,000 | 100,000 |
| 16x32 (512 sq ft) | 43,000 | 74,000 | 104,000 | 131,000 |
| 18x36 (648 sq ft) | 54,000 | 93,000 | 132,000 | 165,000 |
| 20x40 (800 sq ft) | 67,000 | 115,000 | 163,000 | 204,000 |
What it draws and when it stops paying
Electric input in kW = BTU per hour ÷ (COP × 3,412). A heat pump giving 140,000 BTU at a COP of 5.7 draws about 7.2 kW, which is the figure on Hayward's sheet. The DOE puts heat pump COPs at 3 to 7 and says they work efficiently while the outside air stays above 45 to 50 °F; standard sizes include 75,000, 100,000 and 125,000 BTU. Below that, or to heat a cold pool in a day, a gas heater fits better: compare with the pool heater calculator. Not sure of the area? The pool volume calculator shows it with the gallons.
Sizing formula, COP range, 45-50 °F limit and standard sizes: US Department of Energy, Energy Saver, "Heat Pump Swimming Pool Heaters". 80 °F and 50 °F outputs and the 7.2 kW input: Hayward HeatPro series sell sheet (AHRI 1160 ratings). The 65 °F share (80 %) is our estimate between the two ratings. Read October 23, 2026.
Frequently asked questions
How do I calculate the size of a pool heat pump?
Multiply the pool surface in sq ft by the gap between the water temperature you want and the average air temperature of your coolest swimming month, then by 12. A 16x32 pool held at 82 °F in 65 °F air needs about 104,000 BTU per hour. Raise that for cool air, since rated output assumes 80 °F.
What size heat pump for a 20,000 gallon pool?
Heat pumps are sized by surface, not gallons. A 20,000-gallon 16x32 pool held 17 °F above the air needs about 104,000 BTU per hour, which in 65 °F air means a unit rated around 130,000 at 80 °F. In warmer climates a 100,000 BTU unit is often enough.
How much electricity does a pool heat pump use?
Electric input in kW is the heat output in BTU per hour divided by (COP × 3,412). A 140,000 BTU heat pump at COP 5.7 draws about 7.2 kW, so 8 hours a day at 0.15 USD per kWh costs about 8.60 USD a day. Covering the pool cuts the hours it runs.
At what temperature does a pool heat pump stop working well?
The US Department of Energy says heat pump pool heaters work efficiently while outdoor air stays above the 45 to 50 °F range. Output falls well before that: one 140,000 BTU unit is rated 85,000 BTU at 50 °F air. For cold-weather heating, a gas heater is usually the better fit.
Is a heat pump or a gas heater cheaper to run?
Usually the heat pump. With a COP of 5 it delivers five units of heat per unit of electricity, while a typical gas heater turns about 84 % of its fuel into heat. A heat pump costs more to buy and heats slowly, so gas wins for occasional quick heat-ups.