100,000 BTUs is the sweet spot for above-ground pools.

Yes, that’s the number for most standard above-ground pools up to 24 feet round.

The math is simple: you want to heat the pool from cold to comfortable in a reasonable time (12-24 hours). For an average above-ground pool holding 10,000-15,000 gallons, 100,000 BTUs does the job. It’s not overkill, and it’s not so big that you’re short-cycling on a tiny heater.

If you live somewhere cold (northern climate, lots of wind), or you want to swim in early spring or late fall, bump to 150,000. Otherwise, stick with 100,000. Most people overestimate what they need. A heater that’s too big wastes gas and money because it runs in short bursts. A heater that’s too small never catches up.

Don’t forget the cover. That’s where you actually save heat.

A 100,000 BTU heater with a solar cover is going to feel like cheating.