The invention concerns a portable device for electric power supply comprising: connection means (8) for connecting an appliance (1) to be powered; and a power supply circuit (6; 16) supplying in output a supply voltage for the appliance when the circuit is selectively connected to a mains supply or to a low-voltage electric power source. It may also comprise a battery (10). In that case, the circuit (6, 16) can charge the power storage means when the circuit is connected to an appropriate electric power source, the connection means ensuring power supply to the appliance (1) from the power storage means (10).