What size pool heater do I need for an above-ground pool?

For most above-ground pools (5,000–15,000 gallons), a 50,000 to 75,000 BTU heater is plenty. A 100,000 BTU unit will short-cycle, waste gas, and wear out faster. A good rule: for every 10,000 gallons, aim for 50,000 BTUs to raise the temp 10°F in a couple hours. For a common 12,000–15,000 gallon pool, 75,000 BTUs is the sweet spot.