An energy management system for an electrical system of a motor vehicle subjected to a large number of loads including at least one generator, at least one storage battery, a controller of energy distribution in the system which bases such control at least on efficiency of individual components of the motor vehicle involved in power generation, energy storage and power consumption. In order to determine the overall efficiency of the energy storage unit, which comprises at least one energy store, the state of charge, the temperature of the store and the absolute magnitude of the charging and discharging currents are taken into account. In addition to mechanically driven generators, the system can possess a fuel cell and, in addition to an electrochemical energy store, further physical stores such as double-layer capacitors.