Pool Heating Cost Calculator

What it costs to heat your pool

Heat needed, run time and running cost for your own heater. Leave the price blank and you still get the energy and the hours.

Your pool

Volume gal Water now °F Target °F Up to 104°F. This calculator covers pool and spa heating.

Your heater

Input rating Btu/h The input rating off the plate, not the output. Reading one as the other makes the job look about a fifth quicker than it is. Thermal efficiency % 82% is the federal minimum for heaters made since April 2013. Older units can be well below it.

Fuel Price per therm $

Count the filter pump that runs during the heat-up

Your filter pump has to run for the whole heat-up. AHRI 1160 section 6.1.1 counts only a pump inside the unit toward COP, so published efficiency figures exclude it. It is shown on its own line here, never folded into the heater.

Pump power while heating kW Read the watts off the pump display and divide by 1,000. Pricing this line needs an electricity rate, including on the gas path.

Example calculation, shown until you change the figures above.

2,332,400 Btu into the water

7.11 hours of run time

28.44 therms of gas

Enter your fuel price above and this becomes a cost. We will not guess a national average on your behalf.

Measure what your pool is actually losing

Two thermometer readings beat any weather model, because they describe your pool on the night you measured it rather than an average one. This works on its own: no heater needs to be selected.

Heater off for the whole interval.
No fill, no backwash, no draining, and no significant rain.
Overnight and dark, or the figure includes solar gain and is not a loss rate.
The same thermometer for both readings. That matters more than an accurate one, because only the difference is used, so a constant offset cancels.
The same spot and the same depth, away from a return, the heater outlet or the skimmer. A still pool separates into layers overnight and the surface cools fastest, so do not read it shallow.

Temperature drop °F Over how long hours Surface area optional sq ft

The filter pump was running during the measurement That night reasonably matched my heat pump rating condition

What this will not tell you

How much heat your pool loses to the weather. Published splits between evaporation, convection, radiation and conduction disagree with each other, so we measure instead of modelling.
What a cover saves, as a percentage. The widely quoted figures are not reproducible from primary sources. Measure a covered night against an uncovered one.
What size heater to buy. That needs the loss model we refuse to build.
A heat pump output at any air temperature other than the three it was certified at. No interpolation law is established for this equipment.
A 1161 COP converted into a 1160 one, or any COP with no rating condition attached.
Solar output, payback against another heater, or cooling.

Full methodology, every constant and its primary source, and the complete refusal list are in docs/heating-v1.md.