The easy half of the calculation
Raising water one degree Fahrenheit takes one BTU per pound. A US gallon of water at pool temperature weighs about 8.34 pounds, so a gallon takes 8.34 BTU per degree. Twenty thousand gallons is 166,800 pounds, which means 166,800 BTU for every degree and 1.67 million BTU for a ten degree lift.
Divide that by a heater output and you get a time. A 250,000 BTU/h gas heater running at 84 percent efficiency puts 210,000 BTU/h into the water, so ignoring losses the ten degrees takes just under eight hours. That is the number most sizing charts give you, and it is optimistic for a reason that has nothing to do with the heater.
The surface is working against you the whole time
A pool loses heat almost entirely through its top surface, and the dominant mechanism is evaporation rather than conduction. Every pound of water that leaves as vapour takes roughly 1,050 BTU with it, which is why a pool can lose heat steadily on a night when the air is the same temperature as the water. There is no temperature difference driving that loss; there is a vapour pressure difference, and wind refreshes the air above the surface and keeps it going.
This page lumps all of it into one coefficient in BTU per hour per square foot per degree of water-to-air difference, which is a crude model chosen because it is transparent. The default of 6 describes reasonably sheltered water. Exposed water in wind can be double that or more, and the physical reason is that the wind term in the real evaporation relationship is large. If you want the losses modelled honestly, with humidity and wind speed as separate inputs, that is what the heat loss and cover savings page does.
Subtract the loss from the heater output and the arithmetic changes shape. On a 512 square foot pool with the water 15 degrees above the air and the coefficient at 6, the loss is 46,000 BTU/h. Against 210,000 BTU/h of heater that turns the eight hour heat-up into roughly ten. Put a cover on and the same heat-up drops back towards eight and a half. The cover is worth more than a heater upgrade, and it costs a fraction as much.
Sizing on time, not on gallons
| Situation | What drives the size |
|---|---|
| Pool held at temperature all season | Standing losses. A heater only slightly larger than the loss rate will hold it, and a cover cuts the requirement sharply. |
| Weekend pool, warmed on demand | Heat-up time. This is where large outputs earn their cost, because the whole rise has to happen in a day or two. |
| Spa or small pool | Heat-up time again, but the volume is small enough that even a modest heater moves it quickly. |
| Heat pump on a long season | Standing losses plus air temperature. Output falls as the air cools, and below a point the unit stops being useful at all. |
Enter a target heat-up time in the last field and the page runs the arithmetic backwards: energy divided by hours, plus the loss rate, divided by efficiency, gives the rated output that would meet the deadline. That is the honest way to pick a size, because it forces you to state what you are actually buying.
Gas heaters and heat pumps are not interchangeable numbers
A gas heater is normally sold by input rating, and its efficiency converts that to what reaches the water. A heat pump is sold by output at a stated set of conditions, so its rating is already the water-side figure and the efficiency field should be set to 100 to avoid discounting it twice. The catch with a heat pump is that its output is a function of air temperature and humidity, and the rating conditions are usually warm. On a cool morning it can deliver considerably less than the label, which stretches heat-up times in exactly the weather that made you want the heat.
The other difference is the shape of the running cost, which is not modelled on this page at all. A gas heater burns a lot of energy for a short time; a heat pump moves several units of heat per unit of electricity but does it slowly. Comparing them means comparing fuel prices and hours, not BTU ratings.
Questions people ask
How long does it take to heat a pool 10 degrees?
Take the gallons, multiply by 8.34 to get pounds, multiply by the degrees to get BTU, then divide by the heater output after efficiency and after subtracting surface losses. For 20,000 gallons and 10 degrees that is 1.67 million BTU. A 250,000 BTU/h gas heater at 84 percent puts in 210,000 BTU/h, so before losses it is about eight hours, and with a typical uncovered surface loss it is closer to ten. Covering the pool during the heat-up takes an hour or more off that, which is the largest single lever available.
What size heater do I need for my pool?
It depends entirely on whether you are holding a temperature or reaching one. If the pool sits at temperature all season, the heater only has to match the standing loss rate, and a cover can halve that requirement. If you warm the pool for a weekend and let it fall back, the size is set by how fast you want the rise, and that is the calculation in the last field on this page. Sizing by gallons alone, which is what most charts do, answers neither question and just produces a number that is usually somewhere in the right region by accident.
Does a solar cover really make that much difference?
Yes, and the reason is that it stops evaporation rather than that it insulates. Evaporation carries around 1,050 BTU with every pound of water that leaves, and it happens whenever the air above the surface is drier than saturated, regardless of whether the air is warmer or cooler than the water. A cover puts a barrier over that. It also cuts the convection and the radiation to the night sky, but those are the smaller terms. The commonly quoted reduction sits in the region of half to three quarters of the total loss, which is why it appears as an adjustable input here rather than a fixed factor.
Is a heat pump or a gas heater better for a pool?
They solve different problems. A gas heater produces a large output immediately and does not care much about the weather, which suits a pool that is warmed on demand for a weekend. A heat pump moves several units of heat per unit of electricity, which makes the running cost much lower, but the output is modest and falls as the air cools, so it suits a pool held at temperature over a long season. The mistake is comparing the BTU numbers directly: a 125,000 BTU/h heat pump and a 125,000 BTU/h gas heater put similar heat in the water per hour but cost very different amounts to run and behave very differently in cool weather.
Why is my heater not raising the temperature at all?
If the heater is firing and the water is not gaining, the losses are matching or beating the input, and this page will show that as a negative or near-zero net output. Wind over an uncovered surface is the usual cause, because the evaporation term climbs steeply with air movement. Other possibilities are flow related rather than thermal: a heater below its minimum flow will short-cycle or refuse to fire, and a bypass valve left open sends most of the water around the heater instead of through it. Check flow first, because it is quick, then look at the surface.