There are two costs: heating the water the first time, and keeping it hot.
Heating from cold
Heating 1 litre of water by 1 °C takes about 1.16 Wh. So heating 1,000 litres from 15 °C tap water to 38 °C (23 degrees) takes about 26.7 kWh.
At 25p per kWh that is roughly £6.70 for the first heat. Check your own tariff, because unit prices differ by supplier and by region.
On a 2,200 W heater that first heat takes 12 hours or more, and longer in winter when the air pulls heat out while it warms. Most makers quote 1 to 2.5 °C an hour.
Keeping it hot
This is where the wall and the cover matter. A plain inflatable on a cold night can lose several degrees, which the heater then has to win back. Foam-wall tubs lose much less. Wave Spas quotes about £2 a day to hold 40 °C in its foam-wall tubs. That is the maker's figure, not ours, and your cost depends on the weather and how often you lift the lid.
Five ways to cut the bill
- Keep the lid on and strapped down. Most heat leaves through the top.
- Put a mat under the tub. The ground is cold all year.
- Heat on cheap electricity. With app control you can run the heater on an off-peak tariff.
- Drop the temperature when you go away instead of switching off and reheating from cold.
- Shelter it from wind. A fence or screen on the windward side helps.
See our tubs with app control and foam-wall tubs.
