Pool Heating Time Calculator
Raising a kilogram of water by one degree takes 4.186 kJ. Treating a litre as a kilogram, the heat needed is volume × 4.186 × temperature rise, and dividing by 3,600 turns that into kilowatt-hours. Lifting 50,000 litres by five degrees is 291 kWh before losses — and losses have to be added, because a pool keeps cooling from its surface the whole time you are heating it. At 20% that becomes 349 kWh, which a 12 kW heater delivers in 29 hours. That is where the answer usually surprises people: pool heating is measured in days, not in an afternoon.
Your numbers
Time to heat
29.1h
Energy needed
348.8kWh
Energy per degree
69.8kWh
What it means
Even running non-stop this takes 29.1 hours — about 1.2 days. What fixes that is a cover, not a bigger heater: most of the heat leaves as evaporation from the surface.
Formula
Energy needed = Litres × 4.186 × Temperature rise ÷ 3600 × (1 + Heat lost while heating ÷ 100), Time to heat = Energy needed ÷ Heater output
The loss figure is really a question about your cover. Evaporation from an open surface accounts for most of the loss, and fitting a cover often cuts it by more than half. For a heat pump, enter the heat output rather than the electrical input printed on the label — the output is typically four to five times larger, and using the wrong one throws the time out by the same factor.
What to enter
| Input | Default | Accepted range |
|---|---|---|
| Litres (L)Capacity in litres; one litre is 1,000 cm³. | 50,000 | 0 and up |
| Temperature rise (°C)The number of degrees to gain — enter the difference, not the target temperature. | 5 | 0 ~ 40 |
| Heater output (kW)The heat the unit puts into the water; for a heat pump this is output, not input. | 12 | 0 and up |
| Heat lost while heating (%)The share lost from the surface while heating; a cover typically more than halves it. | 20 | 0 ~ 300 |
Step by step
Quick reference table
Results when only Litres (L) changes and everything else stays put.
| Litres (L) | Time to heat (h) | Energy needed (kWh) | Energy per degree (kWh) |
|---|---|---|---|
| 25,000 | 14.5 | 174.4 | 34.9 |
| 37,500 | 21.8 | 261.6 | 52.3 |
| 50,000 | 29.1 | 348.8 | 69.8 |
| 75,000 | 43.6 | 523.3 | 104.7 |
| 100,000 | 58.1 | 697.7 | 139.5 |
What each result means
| Result | At default values |
|---|---|
| Time to heat (h)Hours to reach target running non-stop; heating only by day takes longer than this. | 29.1 |
| Energy needed (kWh)The energy actually needed once the losses while heating are included. | 348.8 |
| Energy per degree (kWh)The energy this pool needs per degree — change the target and just multiply. | 69.8 |
Common mistakes
The loss figure is really a question about your cover. Evaporation from an open surface accounts for most of the loss, and fitting a cover often cuts it by more than half. For a heat pump, enter the heat output rather than the electrical input printed on the label — the output is typically four to five times larger, and using the wrong one throws the time out by the same factor.
Glossary
- Litres
- Capacity in litres; one litre is 1,000 cm³.
- Temperature rise
- The number of degrees to gain — enter the difference, not the target temperature.
- Heater output
- The heat the unit puts into the water; for a heat pump this is output, not input.
- Heat lost while heating
- The share lost from the surface while heating; a cover typically more than halves it.
- Time to heat
- Hours to reach target running non-stop; heating only by day takes longer than this.
- Energy needed
- The energy actually needed once the losses while heating are included.
- Energy per degree
- The energy this pool needs per degree — change the target and just multiply.
Frequently asked questions
Q. How is Pool Heating Time Calculator calculated?
Energy needed = Litres × 4.186 × Temperature rise ÷ 3600 × (1 + Heat lost while heating ÷ 100), Time to heat = Energy needed ÷ Heater output — Raising a kilogram of water by one degree takes 4.186 kJ. Treating a litre as a kilogram, the heat needed is volume × 4.186 × temperature rise, and dividing by 3,600 turns that into kilowatt-hours. Lifting 50,000 litres by five degrees is 291 kWh before losses — and losses have to be added, because a pool keeps cooling from its surface the whole time you are heating it. At 20% that becomes 349 kWh, which a 12 kW heater delivers in 29 hours. That is where the answer usually surprises people: pool heating is measured in days, not in an afternoon.
Q. Can you walk through an example?
With Litres 50,000L, Temperature rise 5°C, Heater output 12kW, Heat lost while heating 20%, the answer is Time to heat 29.1h.
Q. What do I need to enter?
Enter Litres, Temperature rise, Heater output, Heat lost while heating. The result recalculates as you type, and an empty box counts as zero.
Q. How much does the answer move if I change a number?
Changing only Litres (L) moves the answer to Litres (L) 25,000 → Time to heat (h) 14.5 and Litres (L) 100,000 → Time to heat (h) 58.1. The table below lays out five steps.
Q. How are the numbers rounded?
Money is shown to the nearest whole unit, percentages to one decimal place and everything else to two. What you see is rounded; the calculation itself carries the unrounded value forward.
Q. Anything to watch out for?
The loss figure is really a question about your cover. Evaporation from an open surface accounts for most of the loss, and fitting a cover often cuts it by more than half. For a heat pump, enter the heat output rather than the electrical input printed on the label — the output is typically four to five times larger, and using the wrong one throws the time out by the same factor.
Related calculators
Keep your length units consistent across inputs — mixing centimetres and metres throws results far off.