How the math works
Chlority mathBTU per hour = gallons × 8.34 (pounds per gallon) × temperature rise ÷ hours. One BTU raises one pound of water by 1°F. Heaters are listed when their maker-stated output is at least that; the last column shows how long each would take for your rise.
LimitsThe formula ignores heat lost to air and evaporation while heating, so real times are longer, especially at night and without a cover. Heat pump ratings are measured in warm air and fall as the air cools.
Questions
Heat pump or gas pool heater?
Heat pumps cost much less to run and suit holding a temperature through a warm season; gas heaters heat fast in any weather and suit occasional use or cold shoulder seasons.
How long does it take to heat a pool?
A 20,000-gallon pool rising 15°F needs about 2.5 million BTU; a 100,000 BTU heat pump takes roughly 25 hours before losses, a 400,000 BTU gas heater about 6 hours.