How the estimate works
Most of a hot tub’s electricity goes into keeping the water hot with the cover on. That’s called standby power. California requires portable electric spas to be tested for it at 102 °F water and 60 °F air. Spas sold there carry a label with the result.
The calculator:
- Starts from your spa’s tested standby watts. If you don’t know it, it uses the maximum California allows for your size, 3.75 × gallons^(2/3) + 40 watts.
- Adjusts it for your conditions. Heat loss scales with the water–air temperature gap, so standby is multiplied by (water temp − outdoor temp) ÷ 42.
- Adds jet-pump energy for your soaks, if you enter the pump’s watts.
- Multiplies by your electricity rate.
| Spa size | California standby limit |
|---|---|
| 250 gal | about 189 W |
| 350 gal | about 226 W |
| 450 gal | about 260 W |
| 550 gal | about 292 W |
These are the maximums for new spas sold in California, not typical values. Many certified spas test well below them.
What this leaves out
- Cover-off time. An open hot tub loses heat fast through evaporation. Long, frequent soaks in cold weather raise real bills above this estimate.
- Filtration and electronics don’t scale with temperature the way heat loss does, so estimates far from the 60 °F test condition are less precise.
- Older or damaged spas. A waterlogged cover or failed insulation can multiply standby use. Measuring with your utility’s smart-meter data is the best check.
Assumptions and limitations
- Scales tested standby power in proportion to the water–air temperature gap. That's a simplification: filtration and electronics don't scale the same way.
- Heat lost while the cover is off is not included, so frequent long soaks in cold weather raise real use.
- California's standby limit is a maximum for new spas, not a typical value.
Key terms
Common questions
How much does it cost to run a hot tub per month?
It depends mostly on its insulation and cover, your climate and your electricity rate. A spa with a tested standby of 200 W at 102 °F water and 60 °F air uses about 146 kWh a month just holding temperature, which is about $26 at 18¢/kWh. In colder months it can be much more.
Where do I find my hot tub's standby power?
Portable electric spas sold in California since 2019 must be tested and labeled with their standby power, and models are listed in the California Energy Commission's appliance efficiency database. Many manufacturers also publish it on spec sheets.
Does a hot tub use more electricity in winter?
Yes. Heat loss rises with the gap between water and air temperature. Holding 102 °F water at 20 °F outside means roughly double the temperature gap of 60 °F weather, so standby energy roughly doubles too.
Sources and methodology references
- Appliance Efficiency Regulations (Title 20): portable electric spas, California Energy CommissionMaximum standby power for standard spas 3.75 × V^(2/3) + 40 W, tested at 102 °F water and 60 °F air; labeling required since June 1, 2019.
- Modernized Appliance Efficiency Database System (look up your spa's standby watts), California Energy Commission
- Energy Conservation Program: test procedure for portable electric spas (Federal Register, June 13, 2023), US Department of Energy
- Short-Term Energy Outlook: electricity prices, US Energy Information Administration
Published October 2, 2026. Results are estimates. See how our calculators work.
Spotted a wrong result or an outdated source? Report a problem with this calculator.
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