energy may be corrected by a correction factor based on the fluctuation between the actually measured mains voltage and the theoretical nominal voltage this factor indicates whether the actual voltage is higher or lower than the nominal voltage for example of 230 v this factor is updated when the resistive elements are switched in so as also to take account of voltage drops in the supply line the regulating means generally comprise a microcontroller some memory and programs for calculating the energy balances and the correction factors to be applied the energy balances the corrections and the switchings of the heating bodies are calculated by the microcontroller at very closely spaced time intervals so as to constantly regulate the amounts of energy supplied to the heating bodies the intervals for calculating the amounts of energy are of the order of a few milliseconds preferably less than 100 ms for example every 30 ms the automatic regulation mode is based on the following principle a measurement of the temperature of the liquid at the inlet to the device is taken by the temperature sensor 40 at the inlet of the device the amount of liquid to be heated is for its part measured by a flow meter 42 on the basis of pulses an intermediate temperature between the first and second heating bodies is also measured by the temperature sensor 48 in an embodiment that comprises no liquid.