How do I size a pool heater correctly?

Calculate your pool’s gallons first, then pick a heater that can raise the temperature by about 1°F per hour. For a 20,000-gallon pool, that means roughly 100,000 BTU for gas or a heat pump sized to your climate. Don’t go too small—you’ll waste energy waiting forever—and avoid way too big, which short-cycles and wears out faster. Use the manufacturer’s guidelines for heat pumps, and read the actual BTU rating, not the model number.