How the cost is worked out
Heating water takes a fixed amount of energy: 1 Btu raises 1 pound of water by 1°F, and a gallon weighs about 8.34 lb. The heater type only changes how much fuel or electricity you buy to deliver that heat.
Heat needed (Btu) = gallons × 8.34 × temperature rise (°F)
Energy bought = heat ÷ efficiency (gas, propane) or ÷ COP (heat pump)
One therm of natural gas is 100,000 Btu. A gallon of propane holds about 91,500 Btu. One kWh is 3,412 Btu.Keeping the pool warm is the bigger cost over a season. Rather than guess at heat losses, the calculator asks for the one number you can measure yourself: how many degrees the pool drops overnight with the heater off. Note the temperature at sunset and again at sunrise. The heater has to put that back every day.
Worked example
15,000 gallons from 65 to 82°F, cooling 3°F a night
- Heat-up = 15,000 × 8.34 × 17 = 2.13 million Btu
- Daily top-up = 15,000 × 8.34 × 3 = 0.38 million Btu
| Heater (example prices) | Heat-up | Per month | With a cover |
|---|---|---|---|
| Heat pump, COP 5, $0.184/kWh | $23 | $121 | $61 |
| Natural gas, 84%, $1.50/therm | $38 | $201 | $101 |
| Propane, 84%, $2.50/gal | $69 | $366 | $183 |
| Electric resistance, $0.184/kWh | $115 | $607 | $304 |
Monthly cost to hold 82°F, uncovered
Default prices
- Electricity: 18.4 cents per kWh, the U.S. residential average for June 2026 from EIA data. In metric mode the default is 26.3p per kWh, the UK Ofgem price cap rate for October to December 2026.
- Natural gas: $1.50 per therm. EIA’s 2024 U.S. residential average was $14.50 per thousand cubic feet, about $1.40 per therm, and prices have risen since.
- Propane: $2.50 per gallon as an example.
Replace them with the rates on your own bills. Prices vary a lot by state and season.
Cutting the bill
- Use a cover. The DOE and EPA both put heating savings from a cover at 50 to 70%. The calculator shows the 50% case.
- Lower the set point. Each degree you don’t ask for saves heat every single day.
- Cut wind. A fence, hedge or screen reduces evaporation, which the DOE calls the largest source of heat loss.
- Run a heat pump in the warm part of the day. Its COP is higher when the air is warmer.
To size the heater itself, use the pool heater size calculator. To see how much water and heat your pool loses to evaporation, try the evaporation calculator.
Frequently asked questions
How much does it cost to heat a pool per month?
It depends on how much heat the pool loses and on your heater. Holding a 15,000 gallon pool that drops 3°F a night costs about $121 a month with a heat pump and $201 with natural gas at the example prices.
Is a heat pump cheaper to run than gas?
Usually. With a COP of 5 a heat pump delivers 5 kWh of heat per kWh of electricity. Whether it beats gas depends on your local electricity and gas prices, so try both in the calculator.
How much does it cost to heat a pool from cold?
Raising 15,000 gallons by 17°F costs about $23 with a heat pump and $38 with natural gas at the example prices.
How much does a pool cover save on heating?
The DOE and EPA both say 50 to 70%, because covers stop most evaporation, the largest source of heat loss.
What is COP?
Coefficient of performance: units of heat delivered per unit of electricity. The DOE says pool heat pumps usually run between 3 and 7. It falls as the air gets cooler.
Why does my pool lose so much heat at night?
Evaporation and radiation to a clear night sky both pull heat out of the surface. Wind and dry air make it worse. A cover traps most of that heat.
Sources
- Gas Pool Heaters. U.S. Department of Energy, Energy Saver.
- Heat Pump Swimming Pool Heaters. U.S. Department of Energy, Energy Saver.
- Pool Water Efficiency (WaterSense guide EPA-832-F-22-015), September 2022. U.S. Environmental Protection Agency.
- U.S. residential electricity rates, June 2026 (EIA Electric Power Monthly). ElectricRates.org summary of EIA data.
- Energy price cap, 1 October to 31 December 2026. Uswitch, reporting Ofgem’s cap.
- EIA Projects Electricity to Cost 4x Natural Gas in 2027 (2024 U.S. residential gas price). American Gas Association, March 2026.
Reviewed November 7, 2026. We check the formulas against the sources above and re-run the worked examples whenever we update this page. Found something wrong? Tell us.