A power storage system includes an AC/DC converter, a first control device, a power storage device, and a load. The first control device includes a measuring portion that measures the amount of power consumed by the load, a predicting portion that predicts the demand for power consumed by the load on the basis of the amount of power consumed by the load, and a planning portion that makes a charge and discharge plan of the power storage device on the basis of the demand for power predicted by the predicting portion. The power storage device includes a second control device, a DC/DC converter, a first battery cell group, and a second battery cell group. The power storage device is placed in an underfloor space surrounded by a base and a floor of a building.