How the calculation works
Start with the energy: BTU = gallons × 8.34 × temperature rise. Then convert to whatever you buy, allowing for how much of it actually reaches the water.
One therm of natural gas is 100,000 BTU. One gallon of propane is about 91,500 BTU. One kWh of electricity is 3,412 BTU. Divide by the heater's efficiency, multiply by your price, and that is the bill.
Heat pump efficiency looks wrong at first — 500% is normal, because a heat pump moves heat from the air rather than making it. That is why it costs a fraction of resistance heating to run.
Worked example
A 20,000 gallon pool going from 68°F to 82°F needs 2.33 million BTU. On natural gas at 84% efficiency and $1.40 a therm, that is 2.33m ÷ 100,000 ÷ 0.84 = 27.8 therms, or about $39 — plus the heat lost while warming up, which takes it nearer $53 uncovered.
Assumptions
- Prices are whatever you entered — no national average is assumed anywhere in this calculation.
- 100,000 BTU per therm, 91,500 BTU per gallon of propane, 3,412 BTU per kWh.
- A 35% allowance for heat loss uncovered, 5% covered.
- Holding a temperature for a month assumes moderate weather and is a rough guide only.
Limitations
- Weather drives heating cost more than anything else here. A cold, windy week can double the figure; a warm still one can halve it.
- Heat pump efficiency falls as air temperature drops, so spring and autumn cost more per degree than the rated figure implies.
- Standing charges, tiered rates and time-of-use pricing are not included — only the unit rate you entered.
Questions people ask
How much does it cost to heat a pool?
It depends on your fuel price far more than on your pool. Enter your own rate above rather than trusting a national figure — gas prices vary several-fold between regions, and propane is typically two to three times the cost of natural gas for the same heat.
What is the cheapest way to heat a pool?
A cover, before any heater at all. After that, solar is cheapest to run but weather-dependent, a heat pump is cheap per degree but slow, and gas is fastest but most expensive per BTU.
Why is a heat pump over 100% efficient?
Because it is not making heat, it is moving it. For every unit of electricity it uses, it shifts four to six units of heat out of the air and into the water. That is why its running cost is so much lower than an electric element.
Does lowering the temperature by one degree really matter?
More than most people expect. Heat loss rises with the gap between water and air, so the last few degrees are the most expensive ones you buy. Dropping from 84°F to 81°F is often a noticeable cut in the bill.