An electrical power supply device in an aircraft, the device including an enclosure, the enclosure containing at least: a fuel cell; a dihydrogen generator; a gas feed circuit connecting the dihydrogen generator to the anode of the fuel cell; an oxygen feed device feeding the cathode of the fuel cell; and a cooling circuit of the fuel cell associated with at least one heat exchanger; wherein the enclosure is mounted on an actuator system, the actuator system being configured to move the enclosure from a first position in which the enclosure is housed inside an outer wall of the aircraft, to a second position in which the enclosure projects from the outer wall.