Pool Heater Energy and Time Calculator
This calculator estimates the energy needed to raise a pool's temperature and how long a given heater takes to do it. Enter the water volume in gallons, the temperature rise you want, and optionally your heater output and efficiency. It returns the heat energy in both BTU and kilowatt hours, plus the heating time when you supply a heater output. It sizes the heat up only and does not model ongoing surface heat loss.
How to calculate pool heating
- Enter your pool volume in gallons (use a pool volume calculator if you are unsure).
- Use the F or C toggle, then set the temperature rise you want.
- Enter your heater output in BTU per hour, or leave it blank to skip the time estimate.
- Set the efficiency percentage (100 gives the raw heat energy in the water).
- Read the heat energy in BTU and kWh at the top, and the heating time card below.
- Click Copy result to grab the summary line.
Examples
20,000 gallons, +10 F, 400,000 BTU/hr heater at 100 percent
20,000 gallons, +10 F rise, 400,000 BTU/hr, 100 percent efficiency
1,669,081 BTU (489.2 kWh), about 4.17 hours
10,000 gallons, +20 F, 250,000 BTU/hr heater at 85 percent
10,000 gallons, +20 F rise, 250,000 BTU/hr, 85 percent efficiency
1,963,624 BTU (575.5 kWh), about 7.85 hours
25,000 gallons, +12 F, 300,000 BTU/hr heater at 80 percent
25,000 gallons, +12 F rise, 300,000 BTU/hr, 80 percent efficiency
3,129,527 BTU (917.2 kWh), about 10.43 hours
Frequently asked questions
How much energy does it take to heat a pool?
One BTU raises one pound of water by one degree Fahrenheit, and a US gallon of water weighs about 8.345 pounds. So the base energy is gallons times 8.345404 times the temperature rise in degrees F. A 20,000 gallon pool needs roughly 1.67 million BTU for a 10 degree rise.
How long will my heater take?
The tool divides the total BTU needed by your heater output in BTU per hour. A 400,000 BTU per hour heater warming 20,000 gallons by 10 degrees F takes about 4.17 hours at full efficiency. Leave the heater output blank and the time card shows add heater output instead.
How does the efficiency setting work?
Efficiency is entered as a percent from 0 to 100 and divided into the energy, because no heater turns all its fuel into pool heat. At 80 percent the input energy is 25 percent higher than the pure heat in the water, and at 100 percent you see the raw thermodynamic minimum.
How do I convert BTU to kWh?
One kilowatt hour equals 3,412.142 BTU, so the tool divides the BTU figure by that number. The kWh view is handy for comparing against an electric heat pump or reading your power bill in the same units.
Can I enter the temperature rise in Celsius?
Yes. Toggle to C and the value is treated as a temperature difference, scaled by 9/5 with no 32 degree offset before the calculation. A 5 degree C rise is a 9 degree F rise.
Does this include heat loss from the pool?
No. It calculates the one time energy to raise the water from its current temperature to the target. It does not model evaporation, convection, radiation, or conduction to the ground, which are the losses that keep drawing energy while you hold that temperature.
Does covering the pool help?
Yes, a great deal. Evaporation is usually the single largest pool heat loss, and a cover sharply reduces it. This tool sizes the heat up energy, while a cover mainly cuts the energy needed to hold the temperature once you reach it, so real world runtime is often longer than the raw estimate.
Which is cheaper, a gas heater or a heat pump?
A gas heater heats fast but burns fuel every hour, while an electric heat pump moves heat slowly and cheaply per BTU in mild weather. Compare the kWh figure here against your electric rate and the BTU figure against your gas price to see which wins for your climate and schedule.
Does my data stay private?
Yes. The calculation runs entirely in your browser with client side JavaScript. Nothing you enter is uploaded or stored on a server, so the tool works offline once the page has loaded.
Learn more
- Pool Heater Sizing and Heat Loss
Why heating a pool up is only half the story, where pool heat actually goes, and how to size a gas heater or heat pump for your climate and schedule.
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