A power conversion system is coupled between a power grid and a load. The power conversion system includes a battery, a first converter, a second converter and a DC/DC converter. The first converter is configured to selectively convert an input power from the power grid into a first charging power when the power grid is operated normally. The second converter is configured to convert the input power from the power grid into an output power when the power grid is operated normally. The DC/DC converter is electrically connected between the battery and the second converter. The DC/DC converter is configured to convert a storage power in the battery into a backup power when the power grid is not operated normally.