A 150,000 BTU heater is about right for a 15,000-gallon pool.

Yes, that size will heat efficiently in most climates without overpaying or waiting forever.

Pool heater sizing isn’t that complicated once you ignore the marketing. The general rule: you want about 10 BTUs per gallon for decent heating in average conditions. 15,000 gallons x 10 = 150,000 BTUs. That’s enough to raise the water a couple of degrees per hour in moderate weather.

If you live somewhere cold (think: nights below 60°F even in summer) or you want to swim in spring/fall, bump it to 200,000 BTUs. Anything over 250k for a 15k pool is wasting gas and money—you’ll flash-heat the water, short-cycle the heater, and shorten its life.

One more thing: a heat pump works differently—it’s slower but cheaper to run. If you go that route, size it for your pool surface area and desired temperature, not BTUs. But for a standard gas heater? 150k is the sweet spot.

Don’t overthink it.