A controller controls a first switch, a second switch, a third switch, a fourth switch, a fifth switch and a sixth switch, so that an alternating current electric power is fed to an alternating current electric grid. Moreover, the controller firstly turns on a first route including the first switch and the second switch, and then the controller turns on a second route including the third switch and the fourth switch if the second route meets a first specific condition, and then the controller turns on a third route including the fifth switch and the sixth switch if the third route meets a second specific condition.