How should I size my pool heat pump — by volume or surface area?

Size your pool heat pump by surface area, not volume. Heat pumps replace heat lost, and most loss happens at the surface through evaporation. Multiply your pool’s surface area in square feet by 10 to 14 BTUs — 10 for mild climates with a 5°F rise, 14 for cooler climates or a 10°F+ rise.