Pool Heat-Pump Size Calculator
Size a pool heat pump in BTU per hour from your volume and temperature rise, using a realistic run time — heat pumps are efficient but slow, so they are sized differently.
Calculator
A heat pump to raise 15,000 gallons by 10 °F in 12.0 h needs about 127,134 BTU/hr — heat pumps are efficient but slower, so plan longer runs.
A pool heat pump does not burn fuel — it moves heat out of the outside air into your water, which makes it cheap to run but slow to react. The energy needed to warm the pool is exactly the same as for a gas heater (weight of water times degrees times specific heat), but the way you size a heat pump is different: instead of a fast 4-hour heat-up you plan for a long, steady run of 12 to 24 hours, because heat pumps top out around 100,000–140,000 BTU/hr and cannot be pushed harder.
Enter your volume, the temperature rise you want and a realistic run time. The tool returns the output you would need over that window. If the number comes back above roughly 140,000 BTU/hr, no single residential heat pump will hit your target that quickly — stretch the run time (or pair it with a cover) rather than shopping for a bigger unit that does not exist.
Formula
Same energy model as any heater, sized over a longer window:
Required output (BTU/hr) = (gallons × 8.34 × ΔT) ÷ hours ÷ efficiency
- gallons — pool volume.
- 8.34 — pounds per US gallon of water.
- ΔT — temperature rise in °F.
- hours — a realistic run time; 12–24 hours for a heat pump.
- efficiency — delivery efficiency to the water.
Note the difference from a gas heater is the time, not the physics. Heat-pump efficiency is expressed as a COP (how many units of heat it delivers per unit of electricity) and belongs on the running-cost side; here we only size the output.
Worked example
A 15,000-gallon pool, warming 10 °F, over a 12-hour run at 82% delivery:
15,000 × 8.34 × 10 = 1,251,000 BTU
1,251,000 ÷ 12 ÷ 0.82 = 127,134 BTU/hr
That lands right in the range of a large residential heat pump (around 125,000–140,000 BTU). Ask for the same 10 °F in 4 hours and you would need about 381,000 BTU/hr — impossible for a heat pump, which is exactly why they are sized over long, patient runs.
Heat pumps vs. gas heaters
Heat pumps and gas heaters solve the same problem with opposite trade-offs. Gas delivers a lot of BTUs quickly and is ideal for on-demand heating or short seasons in cold climates. A heat pump sips electricity — a COP around 5 means it delivers roughly five units of heat per unit of power — but it produces those BTUs slowly and works best where air temperatures stay above about 50 °F. For steady, season-long heating in a mild climate, the heat pump usually wins on running cost by a wide margin; for a fast warm-up before guests arrive, gas wins on speed.
Because a heat pump cannot simply be made bigger, the lever you control is time. Sizing over 12 hours instead of 4 cuts the required output to a third, which is what makes a heat pump feasible at all. Two habits get the most out of one: run it during the warmest part of the day when the outside air is a richer heat source, and keep a cover on overnight so it maintains rather than rebuilds temperature every morning. A heat pump that only has to top up a covered pool will hold your target comfortably; one asked to reheat a bare pool after a cold night may never catch up.
The output here is an estimate for the heat-up itself and does not include heat lost to the air while the pump runs — another reason a cover matters more with a heat pump than with an oversized gas heater.
Frequently asked questions
How many BTU heat pump do I need?
Size over a long run time. A 15,000-gallon pool warming 10 °F over 12 hours needs about 127,000 BTU/hr — a large residential heat pump. Enter your own volume, ΔT and run time above.
Why size a heat pump over 12 hours instead of 4?
Heat pumps top out around 100,000–140,000 BTU/hr and cannot be pushed faster. Sizing over a longer window brings the required output into a range a real unit can deliver.
Are heat pumps cheaper to run than gas?
Usually, in mild climates. A COP near 5 means about five units of heat per unit of electricity, so running cost is often far lower than gas — check the numbers with the heating cost calculator using your own rates.
Do heat pumps work in cold weather?
They pull heat from the air, so output and efficiency fall as the air cools and they work best above roughly 50 °F. In a cold climate or a short shoulder season, gas or a longer run time may be needed.
Should I still use a cover with a heat pump?
Absolutely — more so than with gas. Because a heat pump adds heat slowly, a cover that stops overnight loss lets it maintain temperature instead of rebuilding it every morning.