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Pool Heater Running-Cost Estimator

Enter your pool volume, current and target water temperature, and your heater's specs, to estimate the run time and cost to heat up — for gas, heat-pump, and resistance heaters.

US gallons, not UK gallons: a UK gallon is 1.2 US gallons.
CDC: hot tub water should not be higher than 104°F; targets above it are refused.
The gas input rating from the heater's data plate.
Typical gas heaters run 78-95% efficient.

Heat-up estimate

Run time
9.15 hrs
Estimated cost
$36.61
Therms used
22.88
BTU needed
1,876,500

How the estimate works

Water takes 1 BTU of heat to raise 1 pound by 1°F, and a gallon of water weighs about 8.34 lb, so the total heat needed is pool gallons × 8.34 × the temperature rise in °F. Dividing that by how fast your heater actually delivers usable heat — its input rate multiplied by its efficiency (or, for a heat pump, its COP) — gives the hours it will take. Multiplying the input rate by that run time gives the fuel or electricity consumed, and its cost at your local price.

This is a heat-up estimate, not a full seasonal running cost. Once the pool is at temperature, it keeps losing heat every day, and the U.S. Department of Energy says “evaporation is by far the largest source of energy loss”: “each pound of 80ºF water that evaporates takes a whopping 1,048 Btu of heat out of the pool.” A quarter inch a day off a 16 × 32 ft pool is about 79.7 US gallons, roughly 697,000 BTU, so under four days of it equals a 15°F heat-up of the whole 19,200-gallon pool (2,402,280 BTU). Over a season, holding the temperature usually costs far more than the first heat-up; the DOE calls a cover “the single most effective means of reducing pool heating costs.”

Frequently Asked Questions

Why does a heat pump cost less to run than a gas heater?

A gas heater burns fuel directly to make heat, so it can't exceed 100% efficiency. A heat pump instead moves existing heat from the surrounding air into the water, so for every unit of electricity it consumes it can deliver several units of heat — expressed as its coefficient of performance (COP), which the DOE says usually ranges from 3.0 to 7.0. That's why a heat pump's 'efficiency' in this calculator can be entered as 400-700%, while a gas heater tops out well under 100%.

Why does a heat pump take longer to heat a pool than a gas heater?

Heat pumps typically have a lower total heat output (BTU/hr) than a similarly priced gas heater, even though they're cheaper to run per BTU delivered. That trade-off — slower to heat, cheaper to maintain temperature — is why heat pumps suit pools kept at a steady temperature all season, while gas heaters suit pools that need to heat up quickly for occasional use.

Does this estimate include ongoing heat loss?

No. This is a one-time heat-up estimate: the energy needed to raise the water from a starting temperature to a target temperature. Keeping a pool at that temperature afterward costs more, since heat is lost every day, mostly to evaporation (DOE: each pound of 80°F water that evaporates takes 1,048 Btu out of the pool). A cover is the single most effective way to cut that loss.

Where do I find my heater's input rate and efficiency?

Both are usually printed on the heater's data plate or spec sheet: gas heaters list a BTU/hr input rating and an AFUE or thermal-efficiency percentage; electric heat pumps list a power draw in kW/amps and a COP rating (sometimes given at a specific air temperature). If you can't find the COP, the manufacturer's product page or manual usually lists it.

Planning estimate only, for a one-time heat-up. Actual cost varies with weather, wind, evaporation, whether a cover is used, and your heater's real-world performance — check your utility's current rates and your heater's rated specs for the most accurate figure.

Pool safety comes first