It takes about 4.2 kJ to warm a kilogram of water by one degree, so a litre from 20 to 100 °C takes about 335 kJ, a tenth of a kilowatt-hour.
The specific heat of water is 4,184 J per kilogram per kelvin: that is what a kilocalorie, the food Calorie, is. Multiply the mass, the 4.2 and the temperature rise: 1.7 litres of kettle water from 20 to 100 °C is 1.7 × 4.2 × 80 = 569 kJ, which is 0.158 kWh, or 190 seconds from a 3 kW element if none of it escaped.
Real kettles and pans lose 10 to 20 percent to the container, the air and the steam. The specific heat itself varies by a few percent with temperature (4.22 near freezing, 4.18 near 40 °C), and seawater is 4.0. It is the energy to warm the water only: boiling any of it away takes far more.
Estimate with the rule, then check it against the calculator that models it properly.
Open Specific Heat & Heat Transfer →