Hot Tub Heating Cost Calculator

Estimate the cost to heat a hot tub's water.

Water volume

gal

Temperature rise

°F

Electricity rate

$/kWh

Heater efficiency(optional)

0-1

Default 1.0

Results update as you type.

Energy needed

—

Heating cost—

Heat energy—

How it works

Heating the water is the main energy cost of owning a hot tub, and this calculator estimates the energy and money needed to bring your tub up to temperature. You enter the water volume in gallons, the temperature rise you need, your electricity rate, and optionally the heater efficiency, and it returns the energy in kilowatt-hours, the cost, and the heat in BTU. The physics is the specific heat of water: raising one pound one degree Fahrenheit takes one BTU, and since a gallon weighs 8.34 pounds, heating the whole tub takes gallons times 8.34 times the degrees of rise. Converting BTU to kilowatt-hours and multiplying by your rate gives the cost of a single heat-up from the starting temperature to your target. This is the energy to heat the water once; the bigger ongoing cost is maintaining temperature, since the tub constantly loses heat to the air, especially without a good cover. A well-insulated tub with a tight cover holds heat far better and costs much less to run. Electric resistance heaters are close to 100 percent efficient at converting electricity to heat, while heat pumps can deliver several times more heat per kilowatt-hour. Enter your tub volume, temperature rise, and rate to estimate heating cost.

Formula

BTU = gallons × 8.34 × °F rise; kWh = BTU ÷ 3412 ÷ efficiency

It takes one BTU to raise one pound of water one degree Fahrenheit, and a gallon weighs 8.34 pounds, so the heat needed is gallons times 8.34 times the temperature rise. Dividing BTU by 3412 converts to kilowatt-hours, and dividing by efficiency accounts for heater losses; multiplying by the rate gives cost.

Source:Specific heat of water

We review all our tools from time to time to ensure accuracy.

Worked example

Water volume:400Temperature rise:40Electricity rate:0.15Heater efficiency:1

400 × 8.34 × 40 = 133,440 BTU ≈ 39.1 kWh; at $0.15 ≈ $5.87 to heat up.

How to use this calculator

1
Enter your values
Enter water volume, temperature rise and electricity rate.
2
Adjust the options
Adjust heater efficiency if they apply — sensible defaults are already set.
3
Read your result
Energy needed updates instantly as you type — no button to press.

Frequently asked questions

How much does it cost to heat a hot tub?

To heat the water once, multiply gallons by 8.34 by the temperature rise for BTU, divide by 3412 for kilowatt-hours, and multiply by your electricity rate. Heating 400 gallons by 40 degrees costs about $5.87 at 15 cents per kilowatt-hour. Maintaining that temperature day to day adds more, depending on insulation and cover quality.

Is heat-up cost the same as running cost?

No. This calculator gives the cost to heat the water from its starting temperature to your target once. The larger ongoing expense is maintaining temperature, because the tub continuously loses heat to the surroundings and the heater must replace it. Monthly running cost depends heavily on insulation, cover quality, climate, and how often you use the tub.

How can I reduce hot tub heating costs?

Use a well-fitting, high-quality insulated cover, since most heat escapes from the water surface. Keep the tub well insulated, lower the temperature or use an economy mode when not in use, and shelter it from wind. A heat pump instead of a resistance heater can cut heating energy substantially. These steps together dramatically reduce running costs.

What is heater efficiency?

Efficiency is how much of the input energy becomes useful heat in the water. Electric resistance heaters are nearly 100 percent efficient, so use 1.0. Heat pumps effectively exceed 100 percent by moving ambient heat, delivering two to five times the heat per kilowatt-hour, so you would model them with a higher effective value or lower cost per unit heat.

How long does a hot tub take to heat up?

It depends on the heater's power and the water volume and temperature rise. A typical residential electric hot tub heater raises the water only a few degrees per hour, so heating from cold can take many hours. Using the BTU figure and your heater's wattage, you can estimate the time, which is why many owners keep the tub warm rather than reheating from cold.