Pool heater cost calculator
Warming a 15,000-gallon pool by 10 °F takes 1,251,000 BTU of heat: about 15.6 therms of gas in an 80% efficient heater.
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At an example rate of $1.50 per therm, that is $23. That is the one-time heat-up. Keeping the pool warm every day usually costs far more, mostly because of evaporation.
About these numbers. The heat-up math is basic physics: one gallon of water weighs about 8.34 lb, and 1 BTU warms 1 lb of water by 1 °F. But our heater and cost formulas are not yet covered by the tested reference cases that back our chemistry calculators. The defaults (80% efficiency, $1.50 per therm, $0.17 per kWh, heat pump COP 5) are example values. Use your heater's rating and the prices on your bills.
Calculator: heat-up cost (8.34 BTU per gallon per °F)
Heat needed (BTU) = gallons × 8.34 × degrees of rise. The fuel you buy depends on the heater:
- Gas: therms = BTU ÷ 100,000 ÷ efficiency. One therm of natural gas is 100,000 BTU (EIA).
- Heat pump: kWh = BTU ÷ 3,412 ÷ COP. COP (coefficient of performance) is heat out per unit of electricity in.
- Electric resistance: kWh = BTU ÷ 3,412, since nearly all the power becomes heat.
| Pool, rise | Heat needed | Gas | Heat pump, COP 5 | Electric resistance |
|---|---|---|---|---|
| 10,000 gal, +10 °F | 834,000 BTU | 10.4 therms, $15.64 | 48.9 kWh, $8.31 | 244.4 kWh, $41.55 |
| 10,000 gal, +20 °F | 1,668,000 BTU | 20.8 therms, $31.28 | 97.8 kWh, $16.62 | 488.8 kWh, $83.10 |
| 15,000 gal, +10 °F | 1,251,000 BTU | 15.6 therms, $23.46 | 73.3 kWh, $12.47 | 366.6 kWh, $62.33 |
| 15,000 gal, +20 °F | 2,502,000 BTU | 31.3 therms, $46.91 | 146.7 kWh, $24.93 | 733.3 kWh, $124.65 |
| 20,000 gal, +20 °F | 3,336,000 BTU | 41.7 therms, $62.55 | 195.5 kWh, $33.24 | 977.7 kWh, $166.21 |
| 25,000 gal, +20 °F | 4,170,000 BTU | 52.1 therms, $78.19 | 244.4 kWh, $41.55 | 1,222.1 kWh, $207.76 |
This is a minimum, not a forecast: real heat-up takes more energy because the pool loses heat while it warms.
Monthly cost: what the estimate can't know
After the first heat-up, a heater mostly replaces heat the pool loses: evaporation from the surface, wind, cool nights, and less sun on cloudy days. Those losses depend on your surface area, climate, and cover habits, which gallons alone cannot capture. So instead of inventing a monthly number, here are the U.S. Department of Energy's (DOE) own season estimates for a 1,000-square-foot outdoor pool kept at 80 °F:
| City (season) | Gas, uncovered | Gas, with cover | Heat pump, uncovered | Heat pump, with cover |
|---|---|---|---|---|
| Phoenix (Mar–Oct) | $2,420 | $229 | $1,192 | $116 |
| Dallas (Apr–Oct) | $2,616 | $382 | $1,321 | $191 |
| Atlanta (Apr–Oct) | $3,063 | $578 | $1,512 | $279 |
| Chicago (May–Sep) | $2,823 | $403 | $1,410 | $204 |
| Seattle (Jun–Aug) | $2,431 | $578 | $1,226 | $293 |
DOE assumptions: gas heater 80% efficient at $1.09/therm; heat pump average COP 5.0 at $0.1301/kWh; pool uncovered 8 hours a day in the "with cover" case. These rates are older than today's, so scale by your own prices.
Two things stand out: the cover matters more than the heater type, and a heat pump cost roughly half as much as gas in every city at those rates.
Gas vs heat pump vs solar
- Gas (natural gas or propane) heats fast and works in any weather, which the DOE says makes it a good fit for pools used only occasionally. Newer units reach 89–95% efficiency.
- Heat pumps move heat from the air into the water. The DOE puts typical COPs at 3.0–7.0 and says they work efficiently while the air stays above roughly 45–50 °F. They cost more to buy, usually less to run, and heat slowly.
- Solar collectors use free heat but depend on sun and roof space, and they usually need more pump time.
Heat pumps heat slowly: bringing 10,000 gallons up 20 °F (1,668,000 BTU) takes at least 16.7 hours with a heat pump that delivers 100,000 BTU/h, versus about 8.3 hours with a 250,000 BTU/h gas heater at 80% (200,000 BTU/h delivered). Real times are longer because of heat loss. If your heater seems undersized, check the pool heater size calculator.
Cover savings
The DOE calls a cover the single most effective way to cut pool heating costs, with savings of 50–70% possible. Evaporation is the largest heat loss, and a cover stops most of it. The same DOE page says covers reduce make-up water by 30–50% and chemical use by 35–60%. One trade-off: a transparent bubble cover can cut the pool's solar gain by 5–15%, and an opaque one by 20–40%, so on sunny days the cover type matters.
Your utility rate
Use the delivered rate from your bill, including distribution charges, not only the supply price.
- Natural gas is billed per therm (100,000 BTU) or per hundred cubic feet. The bill shows the conversion.
- Propane is sold by the gallon, and one gallon holds 91,452 BTU (EIA). To get a price per therm, divide the price per gallon by 0.915.
- Electricity is billed per kWh. A time-of-use plan can make overnight heat-pump runs cheaper.
Quick comparison per 100,000 BTU delivered to the water: gas at $1.50/therm and 80% efficiency costs $1.88; a heat pump at $0.17/kWh and COP 5 costs about $1.00; electric resistance at $0.17/kWh costs about $4.98. Use your own rates the same way. Pump electricity is extra. The pump run time calculator covers it, and the chemical cost calculator covers chlorine.
FAQ
Do pool heaters use a lot of electricity?
Gas heaters use very little electricity. They burn gas. Heat pumps do use electricity: warming 15,000 gallons by 10 °F takes about 73 kWh at COP 5, plus whatever it takes to hold the temperature. Electric resistance heaters use about five times what a COP 5 heat pump uses for the same heat.
How long does it take to heat a 10,000 gallon pool?
For a 20 °F rise, at least 8.3 hours with a 250,000 BTU/h gas heater at 80% efficiency, or 16.7 hours with a heat pump delivering 100,000 BTU/h. Wind, night cooling, and an open surface add time. A cover shortens it.
Is it cheaper to heat a pool with gas or electricity?
With a heat pump, electricity is usually cheaper to run. In the DOE's estimates, a heat pump cost roughly half what gas did in every listed city. Electric resistance heating is the most expensive. Your own rates decide it. Compare the cost per 100,000 BTU delivered as shown above.
Sources
- U.S. DOE Energy Saver, Gas pool heaters (archived copy): energy.gov via web.archive.org
- U.S. DOE Energy Saver, Heat pump swimming pool heaters (archived copy, June 2026): energy.gov via web.archive.org
- U.S. DOE Energy Saver, Swimming pool covers (archived copy, May 2026): energy.gov via web.archive.org
- U.S. EIA, British thermal units (Btu) and energy unit conversions: eia.gov