Size a pool heater with the DOE formula and compare gas and heat pump running cost at your rates
Updated October 2026
Pick the pool’s shape and enter its dimensions in feet; surface area, not volume, drives heat loss. Enter the temperature you want and the average air temperature of the coldest month you plan to swim (a local weather service or climate normals give this). Choose a faster heat-up only if you heat the pool on demand. For running cost, enter your gas heater’s efficiency, your gas price per therm, the heat pump’s COP and your electricity price; the electricity default is the EIA U.S. residential average for July 2026, and the gas price default is only an example.
Energy Saver’s method: (1) choose the pool temperature; (2) find the average temperature of the coldest month of use; (3) subtract to get the temperature rise; (4) find the surface area; (5) multiply area × rise × 12. For the 512 sq ft pool: 512 × 30 × 12 = 184,320 Btu/h. The formula assumes the water rises 1 to 1¼°F per hour with a 3½ mph average wind at the surface, so heating from cold takes roughly 24 to 30 hours for a 30°F rise; for a 1½°F or 2°F hourly rise multiply by 1.5 or 2. Surface area: length × width for a rectangle, 0.785 × diameter² for a round pool, 0.785 × length × width for an oval.
| Pool | Area (sq ft) | Rise | Output needed (Btu/h) | Gas, per hour | Heat pump, per hour |
|---|---|---|---|---|---|
| 16 × 32 ft rectangle, spring opening (82°F wanted, 52°F coldest month) | 512 | 30°F | 184,320 | $3.07 | $1.98 |
| Same pool, faster 1.5°F/hour heat-up | 512 | 30°F | 276,480 | $4.61 | $2.97 |
| 24-ft round pool in a mild climate (82°F, 65°F) | 452 | 17°F | 92,287 | $1.54 | $0.99 |
| 8-ft round spa to 102°F in 50°F weather | 50 | 52°F | 31,366 | $0.52 | $0.34 |
Costs are per hour of running at full output with the default prices; actual monthly cost depends on how many hours the heater runs, which depends mostly on weather and whether the pool is covered.
Gas heaters heat quickly in any weather; Energy Saver notes models with 89% to 95% efficiency. Heat pumps move heat from the air, with coefficients of performance usually between 3.0 and 7.0 (300% to 700% efficiency), and work efficiently while the outdoor air stays above about 45°F to 50°F; they cost more to buy but usually less to run. Solar heaters have the lowest running cost but depend on sun and collector area. To compare an upgrade, DOE’s savings formula is: current annual cost × [1 − (current efficiency ÷ new efficiency)].
Much of a pool’s heat leaves through the water surface. In DOE’s own cost tables, a covered pool costs a fraction of an uncovered one to heat: for Miami at 78°F, its gas-heater table shows $2,910 a year uncovered and $567 with a cover. A cover often does more than a bigger or more efficient heater.
Sizing by gallons instead of surface area. Using the warmest month instead of the coldest month you want to swim. Comparing input with output: if a gas heater’s rating is its input, multiply by its efficiency to get output before comparing it with this result. Ignoring wind: windy, dry or high sites lose heat faster, and DOE says those pools need a larger heater. Running a heat pump in cold weather, when its output and efficiency drop.
It does not model wind exposure, shade, evaporation by humidity, seasonal hours of operation, propane, solar heaters, or gas line and electrical sizing. DOE recommends having a trained pool professional size a heat pump, and a licensed installer should check gas supply, venting and electrical service.