In this guide: Gas vs heat pump vs solar · Sizing the BTUs · Why a cover matters · Which to choose · Calculators
The right heater depends on how fast you need heat, your climate, and your budget for running costs. All three technologies raise water temperature; they differ enormously in speed and cost per degree.
Gas heaters (natural gas or propane) heat fast regardless of the weather — ideal for spas and for people who want heat on demand. They have the lowest purchase price but the highest running cost, and that cost tracks fuel prices.
Heat pumps move heat from the air into the water rather than burning fuel, so they're several times more efficient to run. They cost more upfront and heat more slowly, and they lose efficiency as the air gets cold — best where you want steady, economical warmth through a long season.
Solar circulates pool water through roof or ground collectors. Running cost is essentially the pump energy, so it's the cheapest to operate, but output depends on sun and collector area, and it can't heat on demand. Often paired with another heater for cloudy stretches.
Heating water is pure thermodynamics: it takes 1 BTU to raise 1 pound of water 1°F, and a gallon of water weighs about 8.34 lb. So the energy to warm your pool is gallons × 8.34 × the temperature rise you want — before accounting for surface heat loss. That's why volume drives heater sizing and heat-up time.
Raising 20,000 gallons by 10°F needs about 20,000 × 8.34 × 10 = 1,668,000 BTU of delivered heat. A 400,000 BTU/hr gas heater at ~80% efficiency supplies that in roughly 5–6 hours; a heat pump takes longer but costs far less to run.
An uncovered pool loses most of its heat through evaporation at the surface — the U.S. Department of Energy calls a pool cover the single most effective way to cut heating costs. Whatever heater you buy, a cover keeps the heat you paid for from escaping overnight.
Want heat now, or heat a spa? Gas. Want the lowest running cost over a long season and can wait a bit? Heat pump. Sunny site, patient, and cost-focused? Solar, ideally with a backup. Size any of them to your volume and target temperature with the heater calculator.
These calculators turn the steps above into your exact numbers:
This guide is built on published health codes, national standards, and government energy guidance. For the exact equations behind the linked calculators, see our formula reference.
This guide is general information, not a substitute for your local health code or a licensed professional. Always follow the dosing instructions on your chemical labels and confirm readings with a test kit.