An auxiliary power unit (APU) starting system for an aircraft comprises an APU comprising a starter motor and a generator, a DC power network, an AC power network, an electric connection line, and a control unit for controlling the APU starting and generating phase. The APU starting system additionally comprises switches for alternatively connecting through the electric connection line the DC power network with the APU starter motor, and the AC power network with the APU generator, wherein the control unit is further configured to operate the switches to connect the DC power network with the APU starter motor during the APU starting phase, and the AC power network with the APU generator during the APU starting phase.