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.
Heat up energy versus holding energy
Warming a pool involves two very different jobs. The first is the heat up: raising a large volume of water from its current temperature to your target, a one time cost this calculator estimates from gallons, rise, and efficiency. The second is holding that temperature against constant losses to the air, ground, and evaporation, which continue for as long as you keep the pool warm. The heat up is a fixed number you can compute cleanly, while the holding load depends on weather, wind, humidity, and whether you use a cover. Sizing a heater well means covering both, not just the initial climb.
Where pool heat is lost
Most pool heat leaves through the surface, and evaporation is usually the biggest single path, often more than half of the total loss on a breezy or dry day. Every gallon that evaporates carries away a large amount of energy as it turns to vapor. Radiation to a cold night sky, convection to moving air, and conduction into the surrounding ground and walls add the rest. Because these losses scale with surface area and with wind and dryness, a large shallow pool in a windy spot bleeds heat much faster than a small sheltered one, which is why a heater that seems oversized on paper can still struggle.
Gas heaters versus heat pumps
Gas and propane heaters deliver a large BTU per hour output, so they heat a pool up quickly and work in any weather, which suits pools used on short notice. Their downside is fuel cost every hour they run. Electric heat pumps move heat from the air rather than burning fuel, so they are far cheaper per BTU, but their output falls as the air cools and they heat slowly, which favors steady season long use in mild climates. The kWh and BTU outputs from this calculator let you compare running costs directly against your local electric and gas prices.
Using this calculator to plan
Start by finding your pool volume, then set the rise as the gap between the water today and your target, often 78 to 82 degrees F for swimming. The BTU and kWh results show the raw energy, and the heating time shows how long a candidate heater takes for that climb. To account for the real world, remember the tool ignores ongoing losses, so add a margin and strongly consider a cover to cut evaporation. Rerun with a smaller rise to see how much faster the pool reaches a modest temperature versus a warm one.