A sodium-sulfur battery belongs to each of a plurality of operation units. A first power consuming body and a second power consuming body are electrically connected to the grid. The first power consuming body belongs to each of the plurality of operation units, and the second power consuming body does not belong to the operation unit. One of the first power consuming body and the second power consuming body may be omitted. Rank is given to each of the plurality of operation units. The ranks become higher as the remaining capacity of the sodium-sulfur batteries belonging to the operation unit becomes greater. When the stand-alone operation is started, one operation unit of the highest rank is selected, and the sodium-sulfur battery belonging to the operated operation unit is paralleled in to the grid.