What it costs to keep a hot tub hot
Most of a hot tub's electricity goes into replacing heat that leaks through the cover and shell while it sits covered at temperature — its standby loss — plus the filtration cycles that run on their own. California's Title 20 appliance standard limits that standby power for every portable spa sold there, and manufacturers list each model's tested figure with the California Energy Commission. The limit for a hard-shell spa is 3.75 × V^(2/3) + 40 watts, where V is the fill volume in gallons (inflatable spas: 7 × V^(2/3)). It's measured with the cover on and water at 102°F and normalized to a 37°F gap between water and air.
Worked example — 400 gallons: 400^(2/3) = 54.3; × 3.75 + 40 = 244 W at a 37°F gap. Heat loss scales with the gap, so in a 30°F month (a 72°F gap) the tub averages about 474 W — 353 kWh over 31 days. In a 75°F month it averages 178 W. Summed over a U.S.-average year that's 2,656 kWh, about $483 at 18.19¢/kWh.
Yearly cost by size
| Fill volume | Max standby (hard shell) | Per year | Per month | Inflatable, per year |
|---|---|---|---|---|
| 250 gal | 189 W | $375 | $31.21 | $551 |
| 300 gal | 208 W | $413 | $34.39 | $622 |
| 350 gal | 226 W | $449 | $37.40 | $690 |
| 400 gal | 244 W | $483 | $40.27 | $754 |
| 450 gal | 260 W | $516 | $43.02 | $815 |
| 500 gal | 276 W | $548 | $45.67 | $875 |
U.S.-average outdoor climate, water at 102°F, at the legal maximum standby power and the U.S. average price for Jan–Aug 2026. A tub that tests below the cap costs proportionally less; enter its listed standby watts in the calculator.
Climate matters most
Because loss follows the temperature gap, the same tub costs far more in Minnesota than in Florida — and most of the year's bill lands in the three coldest months. The calculator uses each state's normal monthly temperatures, derived from NOAA's population-weighted degree days, and its EIA electricity price. An indoor or enclosed tub sees a small, steady gap all year.
How to run one for less
- Keep the cover on, latched and dry. A waterlogged or sagging cover loses far more heat than the one the tub was tested with.
- Lower the setpoint when you won't use the tub for a few days; every degree narrows the gap.
- On a time-of-use plan, set filtration and heating cycles outside peak hours.
- Check the listing before you buy: two tubs of the same size can be rated very differently.
Common mistakes
- Using the heater's kW rating times 24 hours. The heater cycles; standby power is the honest average.
- Ignoring winter. A summer bill tells you little about January.
- Forgetting the soaks. Each session with the cover off, jets and blower running, adds heat-up time on top of standby.
Setting up a pool too? See the pool pump cost calculator. Pouring a pad first? The concrete for a 10×10 hot tub pad is worked out here.