How much energy does hot water take?
Heating water takes a fixed amount of energy: 4.186 kJ per liter per degree Celsius, or about 1.163 Wh per liter per °C (2.44 Wh per gallon per °F). From there, the time depends on the heating power and the cost on your electricity rate.
- Energy (kWh) = liters × 4.186 × temperature rise (°C) ÷ 3,600 ÷ efficiency
- Time (hours) = energy (kWh) ÷ power (kW)
- Cost = energy (kWh) × price per kWh
The efficiency covers heat lost through the tank walls and pipes: about 95% for an electric tank heater, 80 to 90% for a kettle, much less for a pot on a stove.
Worked example: a 50 gallon electric water heater
- Volume: 50 gal = 189 L. Temperature rise: from 50 °F to 120 °F, which is 70 °F or 38.9 °C.
- Energy: 189 × 4.186 × 38.9 ÷ 3,600 ÷ 0.95 = 9.01 kWh.
- With a 4,500 W element: 9.01 ÷ 4.5 = 2 hours to heat a full tank.
- At $0.17 per kWh, that costs $1.53, or about $560 a year if you reheat a full tank every day.
Other volumes (50 → 120 °F, 4,500 W)
| Volume | Energy | Time | Cost at $0.17 |
|---|---|---|---|
| 1 gal | 0.18 kWh | 2 min 24 s | $0.03 |
| 10 gal | 1.80 kWh | 24 min | $0.31 |
| 30 gal | 5.41 kWh | 1 h 12 min | $0.92 |
| 50 gal | 9.01 kWh | 2 h 0 min | $1.53 |
| 80 gal | 14.41 kWh | 3 h 12 min | $2.45 |
A kettle for comparison
Bringing 1.7 L (a full kettle) from 59 °F to 212 °F needs 0.17 kWh before losses. A 1,500 W US kettle does it in about 7 minutes; a 3,000 W European kettle on 230 V, in half that time.
Ways to save
- Lower the thermostat: 120 °F (49 °C) is the setting recommended by the Department of Energy. Each 10 °F reduction saves 3 to 5% on water heating.
- Heat pump water heaters move heat instead of making it and use a half to a third of the electricity of a standard tank.
- Insulate the first few feet of hot water pipe and, on older tanks, the tank itself.
- Use less hot water: a low-flow showerhead at 1.8 gal/min instead of 2.5 cuts shower energy by about 28%.
- Heat only what you need in a kettle: boiling a full kettle for one cup wastes most of the energy.
Limits
The result assumes the whole volume is heated at once. Real tanks reheat partially as hot water is drawn, and standby losses add roughly 1 to 2 kWh a day for an electric tank, even when no hot water is used.