What it costs — and how long it takes — to heat your pool, with gas, propane, and electric heat pump compared side by side.
Raising the water 15 °F. Most swimmers are comfortable at 78–84 °F.
Cost to heat up · Natural gas
$44.68
to raise 20,000 gal by 15 °F · ~11.9 hrs · $2.98/°F
Natural gas
$44.68
~11.9 hrs
Propane
$97.66
~11.9 hrs
Electric heat pump
$22.67
~22.7 hrs
Cheapest
Comparison uses typical U.S. rates (gas $1.50/therm, propane $3.00/gal, electricity $0.17/kWh). Your selected estimate above uses the price you entered.
energy = gal × 8.34 × ΔT = 20,000 × 8.34 × 15 = 2,502,000 BTU
therms = BTU ÷ efficiency ÷ 100,000 = 29.8 therms
cost = 29.8 therms × $1.50 = $44.68
8.34 = lb per gallon of water; 1 BTU raises 1 lb of water 1 °F. Net energy to the water — real-world use is higher due to heat loss during heat-up.
This is the one-time cost to heat from your current temperature to your target. Holding that temperature costs more over the season — the pool keeps losing heat to evaporation and air.
A solar or thermal cover is the biggest saver: it cuts evaporation loss 50–70%, so the heat you pay for stays in the pool overnight.
The cheapest fuel to run is almost always an electric heat pump, because it moves heat from the air into the water instead of burning fuel — a COP of 5 means it delivers about five units of heat for every unit of electricity it draws. The catch is speed and weather: heat pumps heat slowly and lose output as the air gets cold.
Natural gas costs more per BTU but heats fast and works in any temperature — the right pick when you want the pool warm tonight or you’re heating a spa. Propanebehaves like gas but is usually the most expensive per BTU, so it mainly makes sense where natural gas isn’t available.
A 20,000-gallon pool at 70 °F, heated to 85 °F (a 15 °F rise):
energy = 20,000 × 8.34 × 15 = 2,502,000 BTU. With a natural-gas heater at 84% efficiency and $1.50/therm, that’s 2,502,000 ÷ 0.84 ÷ 100,000 ≈ 29.8 therms ≈ $45. The same heat-up on a heat pump (COP 5.5, $0.17/kWh) is about 133 kWh ≈ $23 — roughly half the cost, but it takes nearly twice as long.
It depends on pool size, how many degrees you’re raising the water, your heater type, and local fuel prices. As a benchmark, heating a 20,000-gallon pool by 15 °F costs roughly $45 with natural gas, about $23 with an electric heat pump, and near $98 with propane — at typical U.S. rates. Enter your own numbers above for an exact figure, and remember holding that temperature costs more on top of the initial heat-up.
Per unit of heat, an electric heat pump is almost always cheaper to operate because it moves heat instead of burning fuel — a COP around 5 means it delivers roughly five times the energy it draws. Gas costs more per BTU but heats far faster and works in any weather. Propane is usually the priciest per BTU. The trade-off: heat pumps are slow and lose output in cold air; gas wins when you want the pool warm today.
Time = energy needed ÷ heater output. A 250,000 BTU gas heater warms a 20,000-gallon pool about 15 °F in roughly 12 hours; a 110,000 BTU heat pump doing the same takes closer to 23 hours. Bigger pools, bigger temperature jumps, and cold or windy conditions all stretch that out. The calculator estimates your heat-up time from the heater size you enter.
Yes — it’s the single biggest saver. Most of a pool’s heat loss is evaporation, and a solar or thermal cover cuts that by 50–70%. The U.S. Department of Energy reports covers can reduce heating energy by up to 70%. Whatever it costs to heat your pool, a cover keeps that heat in overnight so your heater isn’t fighting the same battle every morning.
First the energy: BTU = gallons × 8.34 × temperature rise in °F. Then the fuel: for gas or propane, divide by the heater’s efficiency and by the BTU per fuel unit (100,000 per therm; 91,500 per gallon of propane). For a heat pump, divide by its COP and by 3,412 BTU per kWh. Multiply the fuel used by your local price and you’ve got the cost. This tool does all of that and shows the math.
This calculator estimates the one-time energy to raise the water to your target. Once it’s warm, the pool constantly loses heat to evaporation, wind, and cooler air, so the heater keeps cycling to hold temperature — that ongoing maintenance is where most seasonal heating dollars go. A cover, wind breaks, and not overheating the water are the levers that cut it.
How many gallons (or liters) your pool holds — any shape, sloped depths, spas included. The number you need before dosing anything.
Open toolHow much liquid chlorine, bleach, or shock to add to hit your target free chlorine.
Open toolDial in cyanuric acid and the ideal FC/CYA ratio so your chlorine actually works.
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