Hot Tub Running Cost Calculator
Rules and price data last checked September 30, 2026.
Running a 400-gallon standard hot tub at 102 F in 50 F air costs up to about $46 a month in electricity at $0.18 per kWh, if it just meets California's standby limit for new tubs.
Electricity, per month$46about 250 kWh at $0.18 per kWhWorked out from the formula and price data listed in the sources below the calculator, for the tub and electricity price shown in the form.
1. Your tub and your winter
Listed on the spec sheet as water capacity or fill volume.
The monthly average where the tub sits, not the coldest night.
State figures are EIA's average residential price for July 2026. Your own bill (total charges divided by kWh used) is better.
2. How well does your tub hold heat?
100% means the tub just meets the California limit for its size (244 W at a 37 F difference between water and air). Better insulation and a tight cover push it lower; an old tub, a thin cover or a cracked seal push it higher. Moving this slider is the way to model those.
- Water over air52 F
- Standby heat loss, continuous342 W (250 kWh a month)
- Total per month$46
- Total per year at this air temperature$549
Read this as a ceiling for a new tub that meets the California limit, scaled by your slider setting. It is a legal limit, not a measured average, so a real tub can come in under it, and an old or poorly covered one over it.
3. Optional extras
How we got this
- California's maximum standby power for a new standard spa is 3.75 × V^(2/3) + 40 watts, measured at a 37 F difference between water and air, where V is the fill volume in gallons: 400 gallons to the 2/3 power: 54.288; 3.75 × 54.288 = 203.58 W; 203.58 W + 40 W = 243.58 W (California Title 20, Table G-5)
- Your heat-loss slider is at 100%, so the limit is used as it is. Standby power rises in proportion to the gap between water and air, the scaling the California rule uses to normalize its test results. water minus air: 102 F − 50 F = 52 F; 243.58 W × 52 F ÷ 37 F = 342.33 W (California Title 20, Table G-5)
- A tub at its set point draws standby power around the clock, so count every hour of an average month. Watts times hours, divided by 1,000, gives kilowatt-hours: 365 days × 24 hours ÷ 12 months = 730 hours; 342.33 W × 730 hours ÷ 1,000 = 249.90 kWh (U.S. Department of Energy formula)
- No jet pump use is counted (tick "Add jet pump use" and fill in its numbers to include it), so the month's electricity is the standby alone: electricity use: 249.90 kWh
- Times the electricity price, the U.S. average from EIA, in cents per kWh, divided by 100: 249.90 kWh × 18.31 cents per kWh ÷ 100 = $45.76 a month (rounded to the cent) (EIA, July 2026 data)
- No water or chemicals are counted (tick "Add water and chemicals" and fill in their numbers to include them), so the total per month is the electricity: total per month: $45.76 a month
The idea behind it: A hot tub draws standby power all day to stay hot, so the bill follows the standby watts, the gap between your water and the air, and the 730 hours in a month. This calculator starts from California's standby limit for a new tub of your size and scales it in proportion to that gap, the same way the rule normalizes its test results.
4. Work backward from a budget
5. Same tub, different weather
| Average outside temperature | Total per month |
|---|---|
| 20 F | $72 |
| 35 F | $59 |
| 50 F | $46 |
| 65 F | $33 |
| 80 F | $19 |
Heat loss scales with the gap between water and air, the same normalization the California rule uses. Wind, sun and a bare tub can move the real figure.
Shopping in November? What to compare on a hot tub deal, including this running cost.
Estimate what a hot tub costs to keep hot each month, for your tub size, water temperature, outside weather and electricity price. Then turn it around: enter a monthly budget and see the warmest water it allows.
How this is calculated
The part of a hot tub's electricity you pay whether or not anyone uses it is standby: the heater replacing heat that leaks through the shell, cover and plumbing all day. California's Energy Commission rules give a hard number for that. Portable electric spas made on or after June 1, 2019 may not exceed, for a standard tub, 3.75 x V^(2/3) + 40 watts of standby power measured at a 37 F difference between water and air, where V is the fill volume in gallons. We start from that ceiling, scale it by the gap between your water and your outside air (the same scaling the regulation uses to normalize its tests), and multiply by the hours in a month (730) and your price per kWh.
The honest limit of this method is in the wording: a legal ceiling is not an average. A good tub beats it, an old or badly covered one does not. Two controls let you correct for that, a slider for how your tub compares with the limit and a box for the standby watts on your spec sheet. Use the spec sheet if you have it.
Worked examples: what a tub costs to keep hot, by size and weather
Water held at 102 F, the ceiling for a new standard tub, at the U.S. average residential price (18.31 cents per kWh, EIA, July 2026). Calculated by the same code as the tool.
| Water capacity | Standby limit at 37 F gap | Per month, 35 F air | Per month, 50 F air | Per month, 65 F air |
|---|---|---|---|---|
| 250 gal | 189 W | $46 | $35 | $25 |
| 400 gal | 244 W | $59 | $46 | $33 |
| 550 gal | 292 W | $71 | $55 | $39 |
| 700 gal | 336 W | $81 | $63 | $45 |
What this estimate leaves out
- Sun, wind and whether the tub sits in a sheltered spot. The model uses air temperature only.
- Heater recovery after people get in and out, and the energy to warm a full refill.
- Any time-of-use or tiered rate. Enter your blended price, or run the tool twice for on-peak and off-peak.
- Tubs older than the 2019 California rule, which are not covered by the limit at all.
Insulation and cover: what the slider stands for
Insulation, the cover and the seals decide how much heat a tub loses, and standby power is the measurement of exactly that. The table shows what a tub that loses more heat than the limit costs over time, for a 400-gallon tub in 50 F air. The 130% and 160% rows are illustrations, not measured tubs.
| Heat loss compared with the limit | Per month | Per year | Extra over 5 years |
|---|---|---|---|
| 100% | $45.76 | $549 | baseline |
| 130% | $59.48 | $714 | $824 |
| 160% | $73.21 | $879 | $1,647 |
This is the number to hold up against a sale price. A tub that costs a few hundred dollars less and loses 60% more heat gives some of that back every winter. Ask any dealer for the standby power of the exact model and the test conditions behind it.
Working backward from a budget
Most calculators stop at "it will cost about this". If you already know what you are willing to pay, the budget box turns the question around. Cost changes in a straight line with the water-over-air gap, so the tool can solve for the highest set point that fits your number at today's air temperature. If even 60 F is over budget, it says so, which usually means the tub is too big for the budget, the cover needs work, or the electricity price is the problem.
Keep going
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Questions people ask
How much does it cost to run a hot tub each month?
For a 400-gallon standard tub held at 102 F with 50 F outside air and the U.S. average residential price of 18.31 cents per kWh, the calculator gives about $46 a month for keeping it hot, as a ceiling for a new tub that just meets California's standby limit. At 35 F outside it is about $59 and at 65 F about $33. Jets, water and chemicals are extra and optional in the tool. Your own price per kWh and your own tub move the answer a lot, so enter them.
Where do the standby numbers come from?
California Code of Regulations Title 20, section 1605.3 and its Table G-5 set a maximum standby power for portable electric spas made on or after June 1, 2019. For a standard spa it is 3.75 times the fill volume in gallons raised to the 2/3 power, plus 40 watts, measured at a 37 F difference between water and air. Exercise spas use the same formula at a 22 F difference, and inflatable spas are limited to 7 times the volume to the 2/3 power at 37 F. The calculator uses that limit as its starting point.
Is that limit what my tub actually uses?
No. It is a legal ceiling for new tubs, not a measured average. A well-built tub can come in under it, and a tub made before 2019, a worn cover or a leaky seal can use more. That is why the calculator has a slider for how your tub compares with the limit and a box for the standby watts printed on your spec sheet. If you have the spec-sheet number, use it: it replaces the limit-based figure.
Why does cold weather raise the bill?
Heat leaves the tub in proportion to the gap between the water temperature and the air around it. The California rule itself normalizes every test to a standard difference by scaling standby power with that gap, and the calculator uses the same scaling. With 102 F water, a 37 F gap is 65 F air. In 28 F air the gap is twice as large, so standby power is twice as high.
How do I measure what my own tub uses?
The U.S. Department of Energy says plug-in electricity monitors measure devices that run on 120 volts, and cannot be used on large 220-volt appliances, where a whole-house energy monitor is the option. If your tub runs on 220 volts, check its voltage first, then use the spec sheet or compare your bill for the same month with and without the tub running. Energy in kWh is watts times hours divided by 1,000, and cost is kWh times your rate, per the same DOE page.
Does an inflatable hot tub cost more to run?
California's limit for inflatable spas is higher for the same size. At 300 gallons it works out to about 314 watts for an inflatable against about 208 watts for a standard spa. Inflatables are allowed more standby loss, so at the ceiling they cost more to keep hot. Pick "Inflatable spa" in the calculator to see it for your size.
What electricity price should I use?
Use your own: total charges on your bill divided by kWh used. As a starting point the calculator offers the U.S. Energy Information Administration's average residential price by state for July 2026, which ranged from 12.72 cents per kWh in Louisiana to 48 in Hawaii, with the U.S. average at 18.31. Oklahoma was 14.35. It is one summer month, so a winter rate or a tiered plan can differ.
What does the budget box do?
It works backward. Enter the most you want to spend per month and the calculator finds the highest water temperature that stays within that budget at your outside temperature, counting jets, water and chemicals if you turned them on. Cost rises in a straight line with the water-over-air gap, so the answer is exact for the model, and it tells you plainly when even the lowest setting is over budget.
Does the estimate include jets, water and chemicals?
Only if you switch them on, and only with numbers you enter. Jet use is pump nameplate watts times hours, using the Department of Energy's watts-times-hours method. Water is your refills per year times the tub's volume times your water price per 1,000 gallons, and chemicals are a monthly dollar amount you type in. We do not add the energy to heat fresh water after a refill.
Does the calculator store what I type?
No. It runs in your browser, nothing is sent anywhere, and nothing is remembered after you leave.
Sources and references
- California Code of Regulations, Title 20, section 1605.3(g)(7) and Table G-5, portable electric spas (via Cornell LII; checked Sep 30, 2026)
- U.S. Energy Information Administration: Electric Power Monthly, Table 5.6.A, average price of electricity by state, July 2026 (released Sep 24, 2026)
- U.S. Department of Energy: Estimating Appliance and Home Electronic Energy Use, formulas and usage monitors (archived copy, checked Sep 30, 2026)
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