Water Heater Heating Time Calculator
How long it takes to heat a full electric water heater, how many kWh it uses, what it costs and how much shower-hot water you get.
Assumes the whole tank starts at the cold water temperature and ignores standby heat loss, so real heating takes slightly longer. The electricity price should include grid fees and taxes per kWh.
About this tool
Enter the tank size, the cold water temperature, the tank temperature and the power of the heating element. The calculator uses the specific heat of water (4.186 kJ per kg per °C, about 1.163 Wh per litre per °C) to find the energy needed, divides by the element power to get the heating time, and works out the cost and how much water you get when the hot water is mixed down to shower temperature. Metric or US units.
Frequently asked questions
Heating 200 L from 10 °C to 65 °C takes 200 × 4.186 × 55 ÷ 3600 = 12.8 kWh. With a 2 kW element that is 12.8 ÷ 2 = 6.4 hours (6 h 24 min); with 3 kW it is about 4 h 16 min. Standby losses add a little.
About 1.163 Wh per litre per °C, since water’s specific heat is 4.186 kJ/(kg·K) and 1 L weighs about 1 kg. Heating 100 L by 50 °C takes 100 × 50 × 1.163 = 5,815 Wh, or 5.8 kWh.
Keep the stored water at 60 °C (140 °F) or more so Legionella bacteria cannot grow, and use a thermostatic mixing valve so water at the shower and bath is no hotter than about 55 °C to avoid scalds.
Hot water is mixed with cold, so you get more than the tank holds: litres × (tank temp − cold) ÷ (shower temp − cold). A 200 L tank at 65 °C with 10 °C cold water gives about 200 × 55 ÷ 30 = 367 L at 40 °C.
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