Hot Tub Cost Calculator (monthly electricity for standby heat, soaking & refills)

Understanding the hot tub running cost calculator — concept and principle

This calculator estimates how much electricity a portable electric hot tub (spa) uses per month and what it costs, split into standby heat loss, soaking sessions and reheating after refills. Use it before buying a spa, to compare models, or to see how much a better cover or a warmer season saves.

Where a hot tub's energy goes: with the default inputs the result is 282 kWh a month (≈ $47.95 a month).

How it is calculated

Volume in US gallons (litres ÷ 3.785).
California's maximum standby power for the size: 3.75 × V^(2/3) + 40 W, tested at a 37 °F (about 21 °C) water-to-air difference.
Standby power at your temperatures = rated (or maximum) standby × (water − outdoor) ÷ 37 °F; × 730 h gives kWh per month.
Soaking = hours per week × extra kW × 52 ÷ 12.
Refills = litres × 4.186 kJ/(kg·K) × (water − tap temperature) ÷ 3,600, spread over the months between refills.
Monthly cost = total kWh × price.

The principle

A covered spa at rest loses heat through its cover, walls and pipes at a rate roughly proportional to the temperature difference between the water and the air. California's appliance rules test spas at a fixed difference and cap their standby power according to size, so scaling that figure to your temperatures gives a good estimate. Using the tub adds heat loss from the open surface and the pumps. Every refill also means heating hundreds of litres of cold water, which takes 4.186 kJ per kilogram for each degree.

Worked example

Default inputs: 1,500 L, water 38 °C, outdoor 10 °C, legal-maximum standby, 3 h a week, 2 kW in use, refill every 3 months, tap 12 °C, $0.17/kWh.

1,500 L = 396 gal; maximum standby = 3.75 × 396^(2/3) + 40 = 242 W.
At a 28 °C (50.4 °F) difference: 242 × 50.4 ÷ 37 = 330 W, so standby = 0.330 × 730 = 241 kWh a month.
Soaking = 3 × 2 × 52 ÷ 12 = 26 kWh.
Refill = 1,500 × 4.186 × 26 ÷ 3,600 ÷ 3 = 15 kWh a month.
Total = 282 kWh = 282 × 0.17 = $47.95 a month, or $575.41 a year.

Each point is the calculator re-run with only “Electricity price per kWh” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Electricity price per kWh	Running cost per month
0.0425	$11.99
0.085	$23.98
0.128	$35.96
0.17	$47.95
0.212	$59.94
0.255	$71.93
0.34	$95.90

How to read the result

Standby is usually the largest part, so a tight, thick cover and a sheltered spot save the most. Many spas are well below the legal maximum: enter the standby wattage from the manufacturer's listing if you have it. Winter months cost more than summer ones because the temperature difference is larger.

Common mistakes

Using a summer outdoor temperature for the whole year.
Forgetting the extra power of jets and heaters while in use.
Leaving the cover off between sessions.
Using a price per kWh that excludes fixed charges and taxes.

Keep in mind: The estimate assumes a portable electric spa with a cover. Results are estimates, not financial advice; electrical installation must follow local codes.

Frequently asked questions

How much does it cost to run a hot tub per month?

At the default settings about 282 kWh, or about $48 at $0.17 per kWh.

What uses the most energy in a hot tub?

Usually standby heat loss through the cover and shell.

Does a better cover save money?

Yes. Reducing standby loss cuts the largest part of the bill.

Sources: California Code of Regulations, Title 20, §1605.3(g)(7), Table G-5 (portable electric spa standby power)

Checked 9 Nov 2026. California Code of Regulations, Title 20, §1605.3(g)(7), Table G-5 (portable electric spas manufactured on or after 1 June 2019: maximum normalized standby power 3.75 V^2/3 + 40 W, V = fill volume in gallons, ΔT_std = 37 °F); specific heat of water 4.186 kJ/(kg·K) (standard value); 730 h = average month. Use hours, extra power and prices are example values.

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