Match BTUs to your pool volume
Undersize a heat pump and it runs forever without getting warm; oversize it and you overpay. The single biggest factor is your pool's water volume in gallons, followed by how cold your climate and target temperature are.
Small and mid-size pools
For pools up to roughly 5,000 gallons, a compact 16,000 BTU heat pump is usually enough — and if it also cools, you can drop water temperature during heat waves. Efficiency (COP) matters here since it runs a lot.
Large pools
Bigger above-ground and in-ground pools (up to ~21,000 gallons) need serious output — a 65,000 BTU inverter unit heats far faster and holds temperature on cool nights. Expect a higher price and a heavier electrical draw.
Cut heating costs
A pool cover is the cheapest upgrade you can pair with any heater: it slashes evaporation, which is where most heat is lost overnight.