The present invention relates to an energy management device (40) and a method of operating the same. The energy management device (40) comprising a buffer controller (41) and an energy buffer unit (45) wherein the buffer controller is configured to connect the energy buffer unit (45) to operate a load device upon detecting that the load device is in an accepting operational state and the voltage provided by a power supply device is between the first and second threshold, during a power negotiation period between the power supply device (10) and the load device (20).