A 15,000 gallon pool with 512 square feet of surface, warmed from 68°F to 82°F in 24 hours on a still 65°F night, needs a heater that puts about 136,000 Btu/h (40 kW) into the water. That is a 200,000 Btu/h gas heater at 82% efficiency, or a heat pump rated near 47 kW. Put a cover on and the same job needs 96,600 Btu/h, which drops the gas heater to 150,000 Btu/h.
How the pool heater size is calculated
Two pieces of arithmetic sit under every heater size. The first is the energy to warm the water. One Btu raises one pound of water by 1°F (EIA), and a gallon of water weighs 8.34 pounds (USGS), so:
Btu to heat = 8.34 lb/gal × gallons × temperature rise (°F)
For the example that is 8.34 × 15,000 × 14 = 1,751,400 Btu. Divide by 24 hours and you get 72,975 Btu/h, the figure most online sizing charts stop at. It is too small, because the pool is losing heat to the air the whole time you are heating it.
The second piece is that loss. Trade sizing sheets put it at roughly 10 to 12 Btu/h for every square foot of surface for every °F between the water and the air; the calculator uses 11. For 512 sq ft that is 5,632 Btu/h per °F. Hayward's sizing sheet adds 25% for a 5 mph wind and doubles the load at 10 mph, and a cover takes off 60%, the middle of the 50 to 70% heating saving quoted in the University of Florida energy guide.
Hold load = 11 Btu/h × surface (sq ft) × (target − air, °F)
For the example that is 5,632 × 17 = 95,744 Btu/h, the surface-area rule of thumb. It is what a heater must beat just to keep 82°F on a 65°F night. Finishing in a set number of hours needs the exact heat balance, which counts the loss as the water warms: 136,260 Btu/h here. The result is always above both the no-loss figure and the hold load.
Gas, propane and heat pump sizes
Gas and propane heaters are sold by input Btu/h and lose a share up the flue. The federal minimum thermal efficiency is 82% until May 30, 2028 (10 CFR 430.32), and Brookhaven National Laboratory's survey found gas units from 150,000 to 400,000 Btu/h input plus one 95% condensing model. So 136,260 ÷ 0.82 = 166,171 Btu/h of input, and the next size up is 200,000.
Heat pumps are rated by output and COP, the heat delivered per unit of electricity. Both fall with cold air. The one manufacturer curve in the Brookhaven survey gives COP 5.5 and 101,000 Btu/h at 80°F air, and COP 4.0 and 72,000 Btu/h at 50°F. At 65°F the calculator uses COP 4.75 and 86% of rated output, so the 136,260 Btu/h load needs a unit rated near 46.6 kW. Florida's energy code sets a minimum COP of 4.0. Type your own COP from the spec sheet if you have it.
What it costs to heat
Cost per heat-up is the heat delivered divided by the heater's efficiency and the fuel's energy content. With EIA's 2025 average of $15.34 per thousand cubic feet of gas (about $1.48 per therm), $2.67 per gallon of propane (March 2026) and $0.173 per kWh (2025):
| Heater | Energy for the 24 hour heat-up | Cost |
|---|---|---|
| Natural gas, 82% | 39.9 therms | $59 |
| Propane, 82% | 43.6 gal | $116 |
| Heat pump, COP 4.75 | 202 kWh | $35 |
With a cover the same heat-up is 28.3 therms ($42) on gas and 143 kWh ($25) on a heat pump. The Cal Poly study for the National Plasterers Council measured about 95% less evaporation under solid, foam and bubble covers, and 50% for solar disks. See the pool evaporation calculator for the water side of that saving.
How long a heater takes
A pool follows a simple curve: it warms fast at first, then flattens as its own losses rise toward the heater's output. The drawing above the cost table plots that curve for your numbers. A 250,000 Btu/h gas heater (205,000 Btu/h out) finishes the example in 12.1 hours, about 1.2°F an hour. A heater whose output is less than the losses at the target temperature never gets there, and the calculator says so instead of giving a time.
Heating tips that follow from the numbers
Heat only what you will use: every degree costs the same 125,000 Btu for this pool plus the extra loss of a warmer surface. Comfortable swimming is 78 to 80°F according to the University of Florida guide. Keep the cover on between swims, since evaporation is the largest single loss. Heat pumps are cheapest to run on warm days, gas on cold ones and when speed matters.
Next questions
Water flows through the heater from your pool pump, and its turnover rate says how often the whole pool passes through it. Start from your pool volume in gallons if you have not measured it, add the heater to a pool cost estimate, and see every tool in the equipment category.