Pool Heating Cost Calculator

How much does it cost to heat a pool per month?

Every "$200–500 a month" figure you've read is somebody's pool in somebody's climate. The bill has a shape, though, and once you see it you can price your own: an uncovered pool loses about 12 BTU an hour for every square foot of surface, for every degree it sits above the air; a cover cuts that to about 5. Multiply by the hours in a month and you have BTU; divide by what your fuel delivers per dollar and you have the bill. A 15 × 30 pool (450 ft²), priced at $1.50/therm gas, $3.00/gal propane and $0.17/kWh electricity, with a heat pump running at 65 °F air:

Holding cost only, 24 hours a day, at the industry sizing factors (12 and 5 BTU/hr·ft²·°F). Gas and propane at 82% efficiency; heat pump COP 4.4 at 65 °F air. Two honest caveats: the sizing factors describe a cool, breezy night, so a sheltered pool in a still month runs under these numbers — and the sun pays part of the bill by day, most of it in high summer, which is why the same heater costs several times more in April than in July.

Read the rows, not the columns, first: the rise is the bill. Holding 82 °F in 72 °F air costs less than half of holding it in 62 °F air, on any fuel. That's the arithmetic behind the advice to pick a temperature you'll actually swim at and to let the season come to you rather than fighting April.

Gas vs heat pump: which is cheaper to run?

Put the fuels on the same footing — dollars per million BTU actually delivered into the water — and the ranking is stable almost everywhere in the US:

At these prices, gas would have to fall to about $0.94/therm before it matched the heat pump per BTU; the tool above works the break-even for your rates and your air temperature.

So why does anyone buy gas? Because cost per BTU is only half of the question. A heat pump is cheap heat delivered slowly and only when the air is warm. Its output and its COP both sag as the air cools — the tool derates it for you — and a 110k-BTU unit takes a day or two to bring a pool up. Gas is dear heat delivered fast, regardless of weather — a 400k-BTU heater does the same heat-up overnight, and it doesn't care that it's 50 °F outside. That makes gas the right answer for weekend heat-ups, spas, and cold shoulder seasons, and the heat pump the right answer for a pool held warm through a long season in mild air. Toggle "heat up for weekends" above to see the two schedules priced against each other on your pool.

What does the heat-up cost?

Raising the water is a separate bill from holding it there, and it's the one people forget when they "just heat it for the party." Water takes 8.34 BTU per gallon per degree, so bringing 16,000 gallons up costs:

One heat-up, no losses during the climb (a real one loses a little more while it runs, and an uncovered pool over a cold night loses a lot). Same rates as above.

That table decides the schedule question. Heat-ups on gas are cheap enough that a weekend-only pool — heat Friday, hold two days, let it drift back — usually beats holding temperature all week, if the pool is covered between times so it doesn't fall all the way back. Heat-ups on a heat pump are cheap too, but slow: a 15-degree climb can take a day and a half, which is fine for "warm by Saturday" and useless for "warm by tonight." Holding all season makes sense when you swim most days, when the air is mild (a small rise is a small bill), or when the heat-up would take longer than the gap between swims. The tool prices both on your pool; the answer is often closer than you'd guess.

The cover is the cheapest heater you can buy

Every column in the first table falls by 58% when the cover goes on, because a cover stops the evaporation that carries most of the heat away. On a gas-heated pool holding a 15 °F rise, that's a saving of several hundred dollars a month at typical rates — the payback on a solar blanket is measured in days. If a heating quote is scaring you, price the cover before the heater: a smaller heater plus a cover routinely beats the bigger heater alone, on both bills. The cover savings calculator works it in gallons and dollars for your weather.

Propane, electric resistance, and solar

Propane runs the same heaters as natural gas, at roughly twice the cost per BTU at typical prices; if you're on propane and heating a pool, a heat pump is usually the payback story of the house. Electric resistance heaters are cheap to buy, easy to install and the dearest heat there is — every BTU is bought at the retail electric rate with no COP to help. They make sense for a spa or a very small pool with short heat-ups, and almost never for a pool held warm. Solar panels deliver free BTU when the sun is out and nothing when it isn't; sized at 50–100% of the pool's surface area they extend a season by weeks and take the edge off the bill, but they don't hold a temperature on demand. We don't model them here because their output is a weather series, not a rate — but note that a cover captures much of the same benefit for a fiftieth of the price.

Upfront cost and lifespan: the other half of the comparison

Total cost is the operating table above times the years you'll own it, plus the purchase. For a pool held warm through a season, the heat pump's higher price is typically recovered in two to four seasons of lower bills against gas, and faster against propane or electric. For a pool heated a dozen weekends a year, the cheaper gas heater usually wins outright — its bill is small because it runs so little. Size whichever you choose with the heater sizing calculator; an undersized heat pump is the classic regret, and an oversized gas heater just finishes sooner.