How many BTU do I need to heat a 20,000 gallon pool?

Heating a 20,000-gallon pool requires roughly 500,000–600,000 BTU to raise water temperature by 20–25°F from a cold start, and a continuous heater output of 100,000–150,000 BTU/hr to maintain your target temperature in moderate outdoor conditions.

The BTU math for a 20,000-gallon pool breaks down this way: one BTU raises one pound of water by 1°F, and a gallon of water weighs approximately 8.34 lbs — so 20,000 gallons is roughly 166,800 lbs of water. To raise that volume 1°F takes about 166,800 BTU. Translating that into a heater size depends on how fast you want to heat up and how much heat the pool loses overnight to ambient air temperature. A 100,000 BTU/hr electric resistance heater gains approximately 1°F every 1.7 hours on a 20,000-gallon pool with no heat loss factored in.

• Total energy to raise a 20,000-gallon pool 1°F: approximately 166,800 BTU.
• Recommended continuous heater output for a 20,000-gallon pool: 100,000–150,000 BTU/hr.
• Cold-start heat-up (20°F rise) at 150,000 BTU/hr: approximately 22–24 hours with minimal heat loss.
• Electric resistance heaters in the 11–18KW range (37,500–61,400 BTU/hr) are undersized for 20,000-gallon pools; gas or heat pump units are the practical options at this volume.
• Pool surface area and overnight air temperature are the primary variables that increase the required BTU/hr beyond the base calculation.