Hot Tub Heating Cost Calculator
Estimate monthly hot tub running cost from gallons, ambient temperature, cover quality, and your kWh rate.
Real numbers, not guesses.
A hot tub left at temperature loses heat all day long. The heater turns on whenever the water drops a degree or so below setpoint, replaces the lost energy, and shuts off again. Almost the entire monthly bill is replacing that ambient heat loss, not heating the tub up from cold.
The dominant variable is the cover. A new four-inch tapered cover with a tight seal cuts standby loss by something like 60 to 70 percent compared to no cover at all. A waterlogged cover from 2018 with a sagging middle is closer to leaving it open. There is no other single change that affects monthly cost as much.
Heat-loss math, simplified. Energy lost per hour roughly scales with the temperature difference between water and air, the surface area of the tub, and a U-value for the cover.
loss_kWh_per_day = U × A × (T_water − T_ambient) × 24 / 1000
For a standard 7-foot square tub with a decent cover, U is around 1.5 W/m²·K. The water is at 40°C (104°F), ambient varies. In a 10°C garage that is a 30°C delta across about 4.5 m² of cover and water surface — roughly 4.9 kWh per day to maintain temperature, or about 24 dollars a month at 16 cents per kWh.
The numbers explode in cold climates. Drop the ambient to −10°C and the same tub uses about 8.2 kWh per day, or 40 dollars a month. That is why people in Minnesota and Quebec who keep their tub running year-round see winter electric bills 60 to 80 dollars higher than summer bills.
Use frequency adds a small amount on top. Each lift of the cover loses around 0.5 to 1 kWh as steam and convection escape. Filling up after a 50-gallon water change in winter means heating that water from 10°C to 40°C, which is about 5.5 kWh of one-time energy. The calculator includes use cycles per week so you can budget that side of it.
A frequently-asked-question that is not really a question. People ask whether they should drop the tub temperature when not in use. The answer for daily users is no, because reheating costs more than you saved standing-by, and the tub is not ready when you want it. For weekly or monthly users in cold climates, dropping to 80°F (26°C) between sessions saves real money. For full shutoff, drain the tub or you risk freeze damage.