Abstract: An uninterruptible power supply (UPS) system includes a plurality of UPS devices configured to provide a single-phase AC input voltage to a load and a battery when an operating mode is a normal mode and configured to convert power of the battery into a single-phase AC output voltage to provide the single-phase AC output voltage to the load when the operation mode is a blackout mode. Each of the plurality of the UPS devices commonly receives the single-phase AC input voltage to commonly output the single-phase AC output voltage when a configuration mode is a single-phase parallel mode. At least three UPS devices independently receive each of a three-phase AC input voltage to output each of a three-phase AC output voltage when the configuration mode is a three-phase mode.