Calculator
Hot tub heat-up calculator
Enter the water litres (or pick a tub size), the starting and target temperature, the heater power and the unit rate from your bill. The result is the minimum: real heating takes longer because heat is lost.
In short
Heating water needs about 4.186 kJ per litre per °C. So kWh = litres x 4.186 x (target °C minus start °C) / 3,600, hours = kWh / heater kW and cost in pence = kWh x your p/kWh. Example: 800 L from 10 to 38 °C is about 26.0 kWh, or 11.8 hours at 2.2 kW.
Work out the heat-up
Minimum, ignores heat loss
- Energy
- 26.0 kWh
- Heating time
- 11.8 h
- 11 h 50 min
- Cost to heat
- n/a
- Enter your unit rate
Real heating takes longer: heat escapes through the walls, the cover and the air, more so in cold or windy weather and with the bubbles running.
Minimum energy from 10 °C to 38 °C
The same sum for common tub sizes, heating from a cold 10 °C fill to 38 °C. These are minimum figures: on a cool, breezy evening or with the cover off, heating takes noticeably longer. Multiply the kWh by the unit rate on your bill for the cost in pence.
| Water | Minimum kWh | Hours at 2.0 kW | Hours at 2.2 kW |
|---|---|---|---|
| 600 L | 19.5 | 9.8 | 8.9 |
| 800 L | 26.0 | 13.0 | 11.8 |
| 930 L | 30.3 | 15.1 | 13.8 |
| 1,000 L | 32.6 | 16.3 | 14.8 |
Why real times are longer
A tub loses heat all the time it is warmer than the air around it: through the inflated walls, the floor, the cover and the open surface. The colder and windier it is, the faster it loses heat, and the bubbles carry heat away too. Many tubs also pause the heater while the bubbles run. A close-fitting thermal cover, an insulated floor mat and a sheltered spot all shorten the wait.