PURPOSE:To contrive to enhance power-transmitting efficiency, by a method wherein conditions of residual hot water in each of a plurality of hot water storing type electric water heaters are detected, a required power supply period is obtained by a central controller, and the required power supply period is apportioned to the water heaters so that electric power load is averaged. CONSTITUTION:A plurality of hot water storing type electric water heaters 1, 1' are provided with sensors 4, 4' for detecting the amount of residual hot water in a hot water storing tank, sensors 5, 5' for the temperature of residual hot water, and timer devices 6, 6' for setting the power supply periods for heating elements 3, 3' respectively,and the central controller 13 provided with a feed water temperature sensor 8, a calculator 9 and a timer controller 12 is provided. A calorific value to be stored in each of the electric water heaters is calculated by the calculator 9 from the feed water temperature detected by the sensor 8 and the capacity of each of the hot water storing tanks, while the calorific value of residual hot water in each of the water heaters is calculated by a calculator 10 from the values detected by the sensors, and the required power supply period for each of the water heaters is calculated by a calculator 11 from the difference between the calorific values, and the timer devices 6, 6' are so controlled as to appropriately apportion the required power supply period to the water heaters.
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