A 400-gallon hot tub indoors at 68°F that meets California's standby limit costs about $29.72 a month to keep at 102°F — $357 a year (1,961 kWh) at the U.S. average 18.19¢/kWh.
At 18.19¢/kWh — U.S. average, Jan–Aug 2026 (EIA).
| Water | Per year | vs 102°F |
|---|---|---|
| 98°F | $315 | −$41.96 |
| 100°F | $336 | −$20.98 |
| 102°F | $357 | — |
| 104°F | $378 | +$20.98 |
| State | ¢/kWh | kWh a year | Per year | Coldest month |
|---|---|---|---|---|
| Minnesota | 16.44 | 1,961 | $322 | $27.38 |
| Colorado | 16.8 | 1,961 | $329 | $27.98 |
| New York | 29.26 | 1,961 | $574 | $48.73 |
| Ohio | 18.81 | 1,961 | $369 | $31.32 |
| Washington | 14.42 | 1,961 | $283 | $24.01 |
| Texas | 16.03 | 1,961 | $314 | $26.69 |
| California | 32.58 | 1,961 | $639 | $54.26 |
| Arizona | 15.36 | 1,961 | $301 | $25.58 |
| Florida | 15.32 | 1,961 | $300 | $25.51 |
400^(2/3) = 54.3; × 3.75 + 40 = 244 W at a 37°F gap. Heat loss through the cover and shell scales with the gap, so each month we multiply by (102°F − room temperature) ÷ 37 and by the hours in the month.
Prices: EIA average residential rates for Jan–Aug 2026. Outdoor temperatures: NOAA Climate Prediction Center population-weighted normals.
The full method, material data and tips are on the main hot tub cost calculator page.