PURPOSE: To prevent a water quantity from heating up more than it is required, by a method wherein a necessary electrification time is reckoned to a hot water quantity preset for the following day's use by referring a detected value of a calorific value for residual hot water in a hot water reservoir, and electrification is started so as to enable to consume the above electrification time at a completion of a midnight electrification.

CONSTITUTION: A hot water quantity for the following day's use is applied to a hot water quantity setting device 5, correction temperatures, such as a fixed temperature for supply water immediately after an inrush into an electrification time and the lowest annual water temperature for various places is set beforehand and a calorific value K1 which is to be stored within a hot water reservoir by a completion time of the electrification time is calculated by an arithmetic unit A 6. A necessary electrification time is calculated by an arithmetic unit B 8 through a net calorific value to be applied, which is obtained by deducting a calorific value corresponding to residual hot water based on a calorific value K1 calculated by the arithmetic unit A 6 and a calorific value K2 for the residual hot water detected by a calorific value detecting device 7 for the residual hot water. Electrification to a heating unit 10 is started by a timer 9 from the midway of a midnight electrification time by a timer 9 so that the necessary electrification time calculated by the above arithmetic unit B 8 is obtained at a completion time of midnight electrification.