How pool heating cost is worked out
BTU = gallons × 8.34 × °F rise · cost = BTU ÷ (BTU per unit × efficiency) × price per unit
One BTU warms one pound of water by 1 °F, and a gallon weighs 8.34 lb, so raising 20,000 gallons by 20 °F takes 3.34 million BTU. A therm of natural gas holds 100,000 BTU and an 84 % efficient heater puts 84,000 of them into the water: 39.7 therms, 59.57 USD at 1.50 USD a therm. A heat pump at COP 5 gets 17,060 BTU from each kWh: 195.5 kWh, 29.33 USD at 0.15 USD per kWh.
| Pool | Natural gas, 1.50/therm | Propane, 3.00/gal | Heat pump, 0.15/kWh | Electric, 0.15/kWh |
|---|---|---|---|---|
| 10,000 gal | 30 | 65 | 15 | 73 |
| 15,000 gal | 45 | 98 | 22 | 110 |
| 20,000 gal | 60 | 130 | 29 | 147 |
| 25,000 gal | 74 | 163 | 37 | 183 |
| 30,000 gal | 89 | 195 | 44 | 220 |
| 40,000 gal | 119 | 260 | 59 | 293 |
Warming up versus keeping it warm
The first heat-up is the big bill; after that you pay each day for what the pool loses, mostly from its surface at night. A 20,000-gallon pool that drops 4 °F overnight costs about 11.90 USD a day to bring back with gas at 1.50 USD a therm and about 5.90 USD with a heat pump at 0.15 USD per kWh. Halve the drop with a cover and the daily cost halves too. Gas heats fast in any weather; a heat pump costs less per degree but is slower and loses output in cool air. To size one, use the pool heater calculator or the pool heat pump calculator; for the pump that runs while you heat, the pool pump electricity cost calculator.
Formulas: 1 BTU per lb per °F, 8.34 lb per gallon; 100,000 BTU per therm of natural gas, about 91,500 BTU per gallon of propane, 3,412 BTU per kWh. Gas and propane heater efficiency 84 % and heat pump COP 5 are assumptions; electric resistance is 100 %. The fuel prices are example inputs, not averages: enter the ones on your bill. Read October 23, 2026.
Frequently asked questions
How much does it cost to heat a pool per day?
Once the pool is warm you pay to replace what it loses overnight. A 20,000-gallon pool that drops 4 °F costs about 11.90 USD a day to reheat with natural gas at 1.50 USD a therm, or about 5.90 USD with a heat pump (COP 5) at 0.15 USD per kWh. Heating from cold costs far more.
How much propane does it take to heat a 10,000-gallon pool?
Raising 10,000 gallons by 20 °F takes 1.67 million BTU. A gallon of propane holds about 91,500 BTU and an 84 % efficient heater puts about 77,000 of them into the water, so you need about 21.7 gallons, 65 USD at 3.00 USD a gallon. Each extra 10 °F adds about 11 gallons.
Is a heat pump cheaper to run than a gas pool heater?
Per degree, usually yes. At COP 5 a heat pump delivers about 17,000 BTU per kWh, so at 0.15 USD per kWh a million BTU costs about 8.80 USD; natural gas at 1.50 USD a therm costs about 17.90 USD and propane at 3.00 USD a gallon about 39 USD. Heat pumps are slower and lose output in cool air.
What are the downsides of a gas pool heater?
Running cost: at 1.50 USD a therm it costs about twice as much per degree as a heat pump at 0.15 USD per kWh. It needs a gas line or a propane tank and must be installed with the venting and clearances in its manual. The upside is speed: it heats a cold pool fast in any weather.
How can I lower my pool heating cost?
Cover the pool when no one swims: much of the heat leaves through the surface by evaporation, so a cover reduces the nightly drop. Keep the water a few degrees cooler, shelter the pool from wind, and heat only for the days you swim. Measure the overnight drop with and without the cover and enter it above.