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Raising a 20,000-gallon pool by 10°F takes about 8 to 10 hours with a 250,000 BTU gas heater, 14 to 20 hours with a 120,000 BTU heat pump, and 3 to 5 sunny days with a solar system or cover. The physics is fixed: it takes 8.34 BTU to raise one gallon of water one degree, so a 10-degree rise needs gallons × 83.4 BTU. For 20,000 gallons that is 1,668,000 BTU. Divide by your heater's output times its efficiency to get hours. Add 20 to 40 percent if the pool is uncovered, because you are heating against continuous evaporative loss.
What is the calculation?
Two steps, and both are simple arithmetic.
Step 1 — energy needed: BTU = gallons × 8.34 × degrees of rise. For a 10°F rise, BTU = gallons × 83.4.
Step 2 — time: hours = BTU ÷ (heater output BTU/hr × efficiency).
Gas heaters sold in the US must meet a federal minimum thermal efficiency, and most current models sit between 82 and 84 percent, with condensing units above 90. Heat pumps are different: they move heat rather than make it, and their rated output already reflects the transfer, so use the rated BTU directly but derate it in cool or humid conditions.
| Pool | BTU for +10°F | 250k BTU gas | 400k BTU gas | 120k BTU heat pump |
|---|---|---|---|---|
| 10,000 gal | 834,000 | ~4 hr | ~2.5 hr | ~7 hr |
| 13,500 gal (24 ft round) | 1,125,900 | ~5.4 hr | ~3.4 hr | ~9.4 hr |
| 15,000 gal | 1,251,000 | ~6 hr | ~3.7 hr | ~10.4 hr |
| 20,000 gal | 1,668,000 | ~8 hr | ~5 hr | ~14 hr |
| 25,000 gal | 2,085,000 | ~10 hr | ~6.2 hr | ~17.4 hr |
| 30,000 gal | 2,502,000 | ~12 hr | ~7.4 hr | ~20.9 hr |
Gas figures assume 84 percent thermal efficiency. Heat pump figures assume the unit holds its rated output, which it will at around 80°F air temperature and will not at 55°F.
Why does it always take longer than the math says?
Because the pool is losing heat the entire time you are adding it. An uncovered pool sheds heat through four routes, and evaporation dominates. Every gallon that evaporates carries away roughly 8,100 BTU of latent heat, which is why a breezy, dry evening cools a pool faster than a still, humid one at the same air temperature. It is why a cover is the highest-value efficiency measure available to a pool owner: it stops the evaporation that is doing most of the cooling.
| Loss route | Share of total, uncovered outdoor pool | What reduces it |
|---|---|---|
| Evaporation | Largest single share | Any cover; windbreak; lower set temperature |
| Radiation to night sky | Significant on clear nights | Cover, especially an opaque one |
| Convection from the surface | Rises sharply with wind | Fence, hedge or screen upwind |
| Conduction to ground and walls | Small in most pools | Insulated shell, above-ground insulation kits |
Practically: if you heat overnight without a cover in cool weather, you can spend the entire night's output replacing what evaporates and wake up barely warmer. Put a cover on and the same heater delivers its full rise.
How do the three heater types compare?
| Type | Rise per hour, 20,000 gal | Best for | Limitation |
|---|---|---|---|
| Gas, 400,000 BTU | ~2.0°F | Fast on-demand heating, spas, cold snaps | Highest fuel cost, especially propane |
| Gas, 250,000 BTU | ~1.25°F | Typical residential in-ground | Slow on a large pool |
| Heat pump, 140,000 BTU | ~0.84°F | Holding a set temperature all season | Output collapses below about 50°F air |
| Heat pump, 100,000 BTU | ~0.60°F | Warm-climate season extension | Cannot recover a large drop quickly |
| Electric resistance | ~0.10 - 0.25°F | Small spas only | Prohibitively expensive on a pool |
| Solar panels, 50% of pool area | ~0.3 - 0.5°F per sun hour | Lowest running cost, season extension | Weather dependent, no output at night |
The pattern is consistent: gas buys speed, heat pumps buy cheap steady-state heat, and solar buys almost-free heat you cannot schedule. Many owners in the South run a heat pump for the season and a cover overnight, and never touch a gas heater.
How can I heat the pool faster?
- Fit a cover and use it every night. The single largest lever, and it works with any heat source.
- Heat during the day in sun. Solar gain and a smaller air-to-water gap both work in your favor.
- Run the pump continuously while heating. Heaters only transfer heat while water flows through them, so pump schedule and heat schedule must overlap — see how many hours a day to run a pool pump.
- Block the wind. Convective and evaporative loss both scale with wind speed across the surface.
- Lower the target. Every degree above ambient costs more than the last. Most swimmers are comfortable at 82 to 86°F, and setting 90°F roughly doubles a typical bill.
- Clean the filter. Restricted flow reduces heat transfer through the exchanger and can trip the heater's flow switch.
Fuel prices, electricity rates and climate all vary by state, so treat the dollar figures in the FAQs as a 2026 benchmark rather than a quote, and check your own utility rates before sizing equipment.
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