Pool heating
What it costs to keep your pool warm
Give this page the two measurements you can take with a tape and the temperature you actually want to swim in. It works out the surface area, the water volume, how much heat that water loses every month of your season, how big a heater has to be to replace it, and what four different fuels charge you for the same job.
You are never asked for a BTU figure, an annual energy number, or a cost. Those are the answers. Handing them to a page that multiplies them by a price is not a calculation, it is a receipt for arithmetic you already did.
Your pool, your climate, your season
Every field below is something you can answer by standing next to the pool or looking at a calendar. Change any of them and the whole table underneath moves.
Surface area 512 sq ftthe area that loses the heat
Water volume 19,150 gallons159,815 lb of water to warm up
Heat the water loses across the season 404.88 million Btubefore any sunlight is credited back
Sunlight the pool collects 208.70 million Btufree heat, already subtracted below
Heat your heater must actually supply 196.18 million Btuover 214 days
Heater output required 182,840 Btu/hsized on April, the hardest month in this season
What the season costs on each fuel
Same pool, same water temperature, same number of delivered BTUs. The only thing that changes is what the equipment charges to deliver them.
| Heater type | Fuel used | Season cost | Why it lands there |
|---|---|---|---|
| Heat pump | 10,912 kWh | $1,662 | season average COP 5.32 |
| Natural gas | 2,392 therms | $4,291 | at $1.79 per therm |
| Propane | 2,616 gal | $6,995 | at $2.674 per gallon |
| Electric resistance | 57,498 kWh | $8,757 | coefficient of performance is exactly 1 |
The spread between the top and bottom rows is $7,095 for a single season on this example pool. That gap is the entire reason the equipment choice matters more than the thermostat setting.
Electricity is priced at 15.23 cents per kilowatthour and natural gas at $18.58 per thousand cubic feet, both for AZ. Those two figures come from U.S. Energy Information Administration and are listed with their retrieval dates on the energy prices page, where you can also substitute the numbers off your own bill.
Month by month, for the default pool
| Month | Mean air | Heat lost (MMBtu) | Sun collected (MMBtu) | Net to supply (MMBtu) | Heat-pump cost |
|---|---|---|---|---|---|
| April | 69.1 °F | 79.6 | 30.1 | 49.5 | $415 |
| May | 78.7 °F | 72.1 | 34.9 | 37.1 | $311 |
| June | 89.5 °F | 58.0 | 35.2 | 22.8 | $191 |
| July | 93.0 °F | 38.6 | 32.0 | 6.6 | $55 |
| August | 91.3 °F | 36.4 | 29.6 | 6.8 | $57 |
| September | 85.9 °F | 49.2 | 25.5 | 23.7 | $198 |
| October | 74.2 °F | 71.1 | 21.4 | 49.6 | $416 |
Look at the two end months against the middle ones. In the warmest part of the season the sun is covering a large share of the loss on its own and the heater barely runs. At the shoulders the air is cooler, the sky is clearer at night, and the sun is lower, so the same pool at the same temperature costs several times as much per week. That asymmetry is the subject of the season length page.
What this page assumes, in one place
Three numbers behind the answer above are judgements rather than measurements, and each one is exposed as a control rather than buried:
- Wind reaching the water. The climate record measures wind over open ground two metres up. Your pool is behind a fence, and its surface is lower still. The default keeps 35% of the regional figure, which puts the water surface at about 1.7 mph in April for Phoenix, AZ, against a regional 4.9 mph. Evaporation is more sensitive to this than to anything else on the form, and the regional figure itself is unreliable — see the climate page for why.
- How much sunlight the pool keeps. Default 85% of what lands on the surface. A dark interior finish keeps more, a bright white one less.
- Freeform surface area. Only if you pick the kidney or freeform shape, and only because a curve has no formula. Measure it and type it in if precision matters to you.
Everything else on this page is either arithmetic on your inputs or a figure with a publisher and a retrieval date. The methodology page lists all of them, including the ones we could not source and had to reason about instead.
Where to go next
- Where the heat actually goes — evaporation, convection and night-sky radiation, separated, with your pool's daily water loss in gallons.
- How big a heater — the two different sizing questions, and why the answer to one is often four times the answer to the other.
- What a cover is worth — per hour it is actually on, not per hour it is theoretically on.
- Heat pump against gas — the install premium, the operating gap, and how long the swap takes to repay.