Disclosed is a method for electrical supply of an apparatus by a system including an intermittent electrical source, electrical storage unit, a fuel cell, and an electrochemical unit for generating the fuel. The method includes steps of: determining a power balance of the system depending on the power consumed by the apparatus and supplied by the intermittent electrical source; receiving data representative of the stability of the power balance during a safety period; controlling the fuel cell and the electrochemical unit: to activate the electrochemical unit if the power balance is greater than a first threshold and the data are not characteristic of a subsequent decrease in the power balance; activating the fuel cell if the power balance is less than a second threshold, which is less than the first threshold, and the data are not characteristic of a subsequent increase in the power balance.