Where the electricity goes
A hot tub uses power in two ways. Most of the time it sits covered, and the heater keeps topping up the heat that leaks out through the cabinet and cover. Then, when you soak, the cover is off, the jets run and the hot water steams heat into the air. In cold weather the standby part dominates; with heavy use, the soaking part can catch up.
Standby: the legal limit as a starting point
California’s Title 20 rules cap the standby power of portable electric spas at 5 × V2/3 watts, where V is the volume in gallons, measured with 102°F water and air no warmer than 60°F, cover on. For a 400 gallon spa that’s 271 W. Your spa may test below the limit; if you know your spa’s rated standby power from its spec sheet or the California Energy Commission database, enter it.
Heat loss through the cover and walls scales with the difference between water and air temperature, so we adjust the rated standby to your temperatures:
Standby watts = rated watts × (water °F − air °F) ÷ 42
42°F is the 102°F water and 60°F air the rating is measured at.Soaking: heat lost with the cover off
With the cover off, evaporation carries away most of the heat. We estimate it with the Smith et al. outdoor evaporation equation (the same one behind our pool evaporation calculator) for the tub’s surface, then add the jet pump’s power for the time you’re in. Jets and air blowers churn the surface and raise evaporation further, so treat this part as a floor rather than a ceiling.
Worked example
400 gallons at 102°F, 78 × 78 inch water surface, 50°F average outside, four 30-minute soaks a week
- Standby = 271 W × (102 − 50) ÷ 42 = 336 W, or 8.1 kWh a day
- Soaking = 8.5 kWh a week for lost heat and the jets
- Total ≈ 282 kWh a month: $52 a month at 18.4¢ per kWh, or £74 at the UK price cap rate of 26.32p
Monthly cost by outdoor temperature
Show the numbers
| Outdoor °F | kWh a month | Cost a month |
|---|---|---|
| 20 | 426 | $78 |
| 30 | 378 | $70 |
| 40 | 330 | $61 |
| 50 | 282 | $52 |
| 60 | 234 | $43 |
| 70 | 185 | $34 |
| 80 | 136 | $25 |
How to cut the running cost
- Keep a good cover on. A waterlogged, cracked or badly fitting cover can lose far more heat than the standby rating assumes. Covers get heavier as they soak up water, which is the sign to replace one.
- Lower the set point a degree or two, especially for weeks you won’t use it.
- Shelter it from wind. A screen or enclosure cuts heat loss on cold, windy days.
- Use economy or sleep modes and filter cycles timed for cheaper electricity if your tariff has them.
- Keep the cover on while the jets run between soaks. Running jets with the cover off on a cold night throws heat away.
Pool owners can compare heater options with the pool heating cost calculator.
Frequently asked questions
How much does it cost to run a hot tub per month?
It depends on the weather, your spa’s insulation, how often you use it and your electricity price. The worked example comes to about $52 a month at US average prices with 50°F outside.
How much does it cost to run a hot tub in the UK?
At the October to December 2026 price cap rate of 26.32p per kWh, the same example costs about £74 a month. Switch the calculator to metric to use UK units and prices.
How many kWh does a hot tub use?
The example uses about 282 kWh a month, most of it keeping the water hot under the cover. Well-insulated spas in mild weather can use much less.
Is it cheaper to leave a hot tub on or turn it off?
For regular use, it’s usually cheaper to leave it on at a slightly lower temperature. Reheating 400 gallons from cold takes a lot of energy, and repeated cold cycles can be hard on equipment.
Does a hot tub cover really make a difference?
Yes, it’s the biggest factor. Almost all of the standby estimate assumes a sound, dry cover. Without one, losses rise toward the soaking rate all day.
Where can I find my spa’s standby power?
Check the spec sheet, the energy label, or search the California Energy Commission’s appliance efficiency database for your model.
Sources
- California Title 20 Section 1602 for portable electric spas and hot tubs. Hot Tub Insider.
- Methods for Calculation of Evaporation from Swimming Pools and Other Water Surfaces (reviews the Smith et al. outdoor-pool equation). M. M. Shah, ASHRAE Transactions, 2014.
- U.S. residential electricity rates, June 2026 (EIA Electric Power Monthly). ElectricRates.org summary of EIA data.
- Energy price cap, 1 October to 31 December 2026. Uswitch, reporting Ofgem’s cap.
Reviewed November 7, 2026. We check the formulas against the sources above and re-run the worked examples whenever we update this page. Found something wrong? Tell us.