The invention relates to a method for heating water in a domestic water heating system that comprises the steps of: (a) providing a water tank containing mw liters of water; (b) providing at least one temperature sensing unit in said water tank for sensing the temperature of the water in the tank: (c) providing a heating element in said water tank; (d) providing a control unit for activating said heating element, said control unit continuously receiving indication for the water temperature from said temperature sensing unit; (e) providing to said control unit a desired water temperature, and designating a time for using the water at said desired temperature; (f) knowing the current water temperature, the desired water temperature at said designated time, the power of the heating element, and the specific heat of the water, calculating by the control unit the heating period t needed for heating the water in the tank from the current temperature as measured by said temperature sensing unit, to the desired temperature; (g) periodically repeating said calculation and updating said calculated period t according to changes in the sensed water temperature; and (h) when the designated usage time is approaching, activating the heating element a t period before the said designated period.