The utility model discloses a power storage system (3) which is provided with a storage battery (30) and a charging and discharging circuit (31) which achieves the charging and discharging of the storage battery (30) through a transformer (40). The charging and discharging circuit (31) is connected to a device which converts a direct current outputted by a direct-current power supply (1) into an alternating current, thereby employing the direct current to charge the storage battery (30). The transformer (40) is provided with a power side winding (41) where the direct current is inputted, and a storage battery side winding (51) connected to the storage battery (30). A turns ratio of the power side winding (41) to the storage battery side winding (51), i.e., the ratio of the turns number of the power side winding (41) to the turns number of the storage battery side winding (51), is configured to be that the turns ratio during the charging of the storage battery (30) is greater than the turns ratio during the discharging of the storage battery (30). Accordingly, an element with low resistance to voltage can be employed for forming the charging and discharging circuit (31), and the power storage system (3), the charging and discharging circuit (31) or a grid-connected device can be constructed at a low cost.