How much heat does a pool need?
Water takes a lot of energy to warm up. One BTU raises one pound of water by 1 °F, and a gallon of water weighs about 8.34 lb. So:
heat (BTU) = gallons × 8.34 × temperature rise (°F)
Dividing by 3,412 converts BTU to kilowatt-hours. Pools also lose heat while they warm, mostly through evaporation, so the calculator adds an allowance for losses, 20% by default.
Heater size
The heater output you need is the total heat divided by the time you allow:
output (BTU/h) = total heat ÷ hours
Worked example
A 20,000-gallon pool needs to warm by 20 °F in 48 hours, with 20% extra for losses:
| Result | Value |
|---|---|
| Heat to deliver, including losses | 1,173 kWh (about 4 million BTU) |
| Heater output needed | 83,400 BTU/h (24.4 kW) |
| Electricity used by a heat pump (COP 5) | 235 kWh |
| Cost at $0.17/kWh | $40 |
The same heat-up would cost:
- about $72 with an 84%-efficient gas heater at $1.50 per therm;
- about $199 with an electric resistance heater at $0.17/kWh.
To do it in 24 hours instead, the heater would need about 167,000 BTU/h.
Choosing a heater type
| Type | Strengths | Weaknesses |
|---|---|---|
| Heat pump | Lowest running cost, moves 4 to 6 units of heat per unit of electricity | Slower; loses efficiency below about 50 °F air temperature |
| Gas or propane | Heats fast, works in any weather | Higher running cost; needs a gas line or tank |
| Electric resistance | Cheap to buy, compact | Most expensive to run; best for spas |
| Solar | Free heat once installed | Depends on sun and collector area |
Cut the losses first
- A pool cover is the single most effective step: the U.S. Department of Energy estimates it can cut heating costs by 50% to 70%.
- Wind speeds up evaporation; a fence or hedge helps.
- Lowering the target by a couple of degrees saves a noticeable share of the energy each season.
Tips
- Heat pumps are rated at mild conditions; their COP drops in cool weather, so allow extra time in spring and fall.
- Keeping a pool warm usually costs less than repeatedly heating it from cold, as long as it is covered.
- Check your utility’s rates: gas is often billed in therms (100,000 BTU) and propane by the gallon (about 91,500 BTU).