The formula
Energy (kWh) = 4.186 × Litres × Temperature rise (°C) ÷ 3,600 ÷ Efficiency
Time (h) = Energy (kWh) × 1,000 ÷ Heater watts
It takes 4.186 kJ to raise 1 litre of water by 1 °C. Boiling a 1 litre kettle from 10 °C takes about 0.1 kWh.
Worked example
Heating 10 L of water from 10 °C to 60 °C with a 1,000 W element at 90% efficiency needs 0.65 kWh and takes about 39 minutes. Via an inverter on a 12 V battery that's around 61 Ah, which is why many van owners heat water with gas, diesel or from engine coolant instead.
Frequently asked questions
How long does it take to heat 10 litres of water?
With a 1 kW element, about 35–40 minutes from 10 °C to 60 °C. A 2 kW element halves that.
Can I boil a kettle on an inverter?
Yes, with an inverter rated above the kettle's power. A 1 L boil uses about 0.1 kWh, roughly 10 Ah at 12 V including losses.