Most new owners are surprised by how long the first heat takes. The answer is not a mystery, though. Two numbers decide it: how much water the tub holds and how many watts the heater puts out. If the maker publishes both, you can work it out yourself in a minute.
The one number you need: 1.16 Wh
Heating 1 litre of water by 1 °C takes about 1.16 watt-hours of energy. That is a property of water, the same in every tub.
So the energy you need is:
litres × degrees of rise × 1.16 = watt-hours
And the time is:
watt-hours ÷ heater watts = hours
A worked example: Aoxun 208 cm
The Aoxun holds 1,000 litres and has an 1,800 W heater. Say your tap water comes out at 15 °C and you want 38 °C. That is a rise of 23 °C.
- Energy: 1,000 × 23 × 1.16 = 26,680 Wh, or 26.7 kWh
- Time: 26,680 ÷ 1,800 = 14.8 hours
That is the best case, with every watt going into the water and none escaping. In real life some heat leaks out through the wall, the floor and the lid while it warms, so expect longer.
Five tubs that publish both figures
Only five tubs in our data give both the water volume and the heater wattage. Here is the same sum for each, from 15 °C to 38 °C:
| Hot tub | Litres | Heater | Energy | Best-case time |
|---|---|---|---|---|
| Intex PureSpa Onyx Octagon | 795 | 2,200 W | 21.2 kWh | 9.6 hours |
| Intex PureSpa Jet & Bubble Deluxe | 1,098 | 2,200 W | 29.3 kWh | 13.3 hours |
| Arebos Barcelona 6 | 910 | 1,800 W | 24.3 kWh | 13.5 hours |
| Aoxun 208 cm | 1,000 | 1,800 W | 26.7 kWh | 14.8 hours |
| Intex PureSpa Bubble 8 | 1,340 | 2,200 W | 35.8 kWh | 16.3 hours |
For the others, the maker does not publish one or both figures. Lay-Z-Spa, Wave and MSpa do not give heater wattage for any model we list, so we cannot do the sum for them.
Degrees per hour
Turn the sum round and you get the fastest possible rise per hour:
heater watts ÷ (litres × 1.16) = °C per hour
| Hot tub | Best case per hour | What the maker quotes |
|---|---|---|
| Intex PureSpa Onyx Octagon | 2,200 ÷ (795 × 1.16) = 2.4 °C | 1.5 to 2.5 °C |
| Intex PureSpa Jet & Bubble Deluxe | 2,200 ÷ (1,098 × 1.16) = 1.7 °C | 1 to 2 °C |
| Arebos Barcelona 6 | 1,800 ÷ (910 × 1.16) = 1.7 °C | Not published |
| Aoxun 208 cm | 1,800 ÷ (1,000 × 1.16) = 1.6 °C | 1 to 2 °C |
| Intex PureSpa Bubble 8 | 2,200 ÷ (1,340 × 1.16) = 1.4 °C | 0.8 to 1.5 °C |
The makers' ranges line up well with the physics. Their top figures sit a touch above our best case, which fits a part-filled tub or a warm day. The makers do not say how they measured.
Winter makes it slower
Two things change in winter. Your tap water is colder, so you need a bigger rise, and the cold air pulls heat out faster while the tub warms.
Here is the Aoxun again, best case, at three starting temperatures:
| Start | Rise to 38 °C | Energy | Time on 1,800 W |
|---|---|---|---|
| 15 °C | 23 °C | 26.7 kWh | 14.8 hours |
| 10 °C | 28 °C | 32.5 kWh | 18.0 hours |
| 5 °C | 33 °C | 38.3 kWh | 21.3 hours |
Add heat loss on top. On a cold, windy night a plain inflatable wall can lose a large share of what the heater adds. That is why the first heat in January can run well past a day.
What the first heat costs
Energy × your unit price. At 25p per kWh, heating the Aoxun from 15 °C costs 26.7 × 25p = about £6.70. From 5 °C it costs 38.3 × 25p = about £9.60, plus whatever leaks out on the way. Our running costs guide covers the cost of keeping it warm afterwards.
How to heat faster
- Lid on and strapped down from the moment you start. Most heat leaves through the top.
- Bubbles off. On most tubs the heater pauses while the blower runs. See the 13 A plug guide.
- A mat underneath. The ground pulls heat out all year.
- Fill with warmer water if you can. Every degree you start higher saves 1.16 Wh per litre.
- Shelter it from the wind.
- Keep it warm between uses instead of letting it go cold. Reheating from cold is the slow part.
Quick answers
Why does a bigger heater not always mean faster? Because water volume matters just as much. The Onyx and the Bubble 8 share a 2,200 W heater, but the Bubble 8 holds 545 litres more and takes 6.7 hours longer in our best case.
Can I use hot water from the house? Check the manual first. Some makers set a maximum fill temperature to protect the liner.
How do I keep the heat in afterwards? See using a hot tub in winter and inflatable, foam or frame.
