That calculation is correct if you assume that by "boil water" they mean to bring the water to 100°C (which is what you'd use a kettle for usually). However to

That calculation is correct if you assume that by "boil water" they mean to bring the water to 100°C (which is what you'd use a kettle for usually). However to go from liquid water to water vapor you need to add even more energy [0]. The enthalpy of vaporization for water is 2257 kJ/kg, it takes much more energy to boil water at 100°C than it takes to get there from room temperature. Warming by 75 degrees + boiling = 2570 kJ/kg -> enough to boil 23 liters of water I'm guessing in reality the energy needed to operate the kettles would lie somewhere in between, as a small part of the water will be boiled and most of it kept around 100°C. For 2L kettles the value seems in the right ballpark. [0] https://en.wikipedia.org/wiki/Enthalpy_of_vaporization https://en.wikipedia.org/wiki/Enthalpy_of_vaporization