Where the electricity goes
A hot tub spends most of its energy keeping the water hot. Heat leaks through the shell, the cabinet and above all the cover, and the heater runs to replace it. Pumps for filtration and jets add a smaller, steadier load. Soaking with the cover off adds evaporation, which is the fastest way to lose heat.
The model
The calculator estimates:
- Standby loss = heat loss coefficient (W per °C of difference) × (water − outdoor temperature) × 24 h
- Use loss = about 50 W per °C of difference for each hour the cover is off
- Electricity per day = (standby + use) ÷ heater efficiency + pumps and filtration
A well-insulated tub with a good cover loses about 5 W per °C, an average one about 8, and an old or inflatable tub 14 or more. A heat pump heater (COP about 3) uses a third of the electricity of a resistance heater for the same heat.
Example
A 400-gallon tub at 102 °F, an average outdoor temperature of 50 °F, an average cover, three hours of soaking a week and 1 kWh a day for filtration:
- standby: 8 × 28.9 °C × 24 h ≈ 5.5 kWh a day
- use: about 0.6 kWh a day
- total: 7.2 kWh a day, about 2,600 kWh a year
- at $0.17 per kWh, $37 a month or $445 a year
The first fill takes about 46 kWh to heat from 55 °F, or roughly 11.5 hours with a 4 kW heater.
| Climate (average outdoor) | Typical monthly cost at $0.17/kWh |
|---|---|
| Mild, 60 °F | $25 to $35 |
| Temperate, 50 °F | $30 to $45 |
| Cold, 30 °F | $45 to $70 |
How to lower the bill
- Replace a waterlogged cover: a heavy, sagging cover can double heat loss.
- Add a floating thermal blanket under the cover.
- Lower the temperature by a few degrees when you will not use the tub for several days.
- Shelter the tub from the wind and keep the cover clipped.
- Run filtration during off-peak hours if your utility has time-of-use rates.
Cold climates and winter months dominate the annual bill, so run the calculator with your winter average to see the worst month.