A grid connected power storage system is disclosed. According to some aspects, an integration controller controlling a grid-connected power storage system may include a controller configured to change parallel data to be transmitted to a plurality of external devices to serial bit streams, change serial bit streams received from the plurality of external devices to parallel data, and output a path control signal that instructs a communication path of the serial bit streams. According to some aspects, a communication unit configured to change a communication path and a ground path of the serial bit streams according to the path control signal, and transmit and/or receive the serial bit streams through the changed communication path is provided. A number of isolators and interfaces for data communication with a plurality of external devices is reduced such that the structure of an integration controller may be simplified, and the size and production cost of the integration controller can be reduced.