A power storage system comprising: a plurality of power storage devices, each of the plurality of power storage devices including a storage battery and a power conditioner configured to convert DC power of the storage battery into two-wire AC power and output the two-wire AC power, each of the plurality of power storage devices being switchable between a power grid interconnected operation and an autonomous operation without being interconnected with the utility power grid; and a transformer including: a primary winding to which a two-wire AC voltage is applied; and a secondary winding connected to a single-phase three-wire power line, a load being connected to the single-phase three-wire power line, wherein the single-phase three-wire power line includes a first voltage line, a second voltage line, and a neutral line, the power conditioner of any one of the plurality of power storage devices is configured to apply a two-wire AC voltage to be output during the autonomous operation to the primary winding, and the power conditioner of the rest (22, 23) of the plurality of power storage devices has an output connected to the first voltage line and the second voltage line during the autonomous operation, if each of the plurality of power storage devices performs the autonomous operation, any one of the plurality of power storage devices is configured to serve as a master device, while the rest of the plurality of power storage devices, other than the one serving as the master device, is configured to serve as at least one slave device (22, 23), the power conditioner of the master device is configured to perform voltage control of controlling an AC voltage output during the autonomous operation toward a target voltage, and the at least one slave device (22, 23) further includes a data acquisition unit configured to acquire measured data of a first current flowing through the first voltage line and a second current flowing through the second voltage line, the power conditioner of the at least one slave device (22, 23) being configured to perform current control of controlling an output current toward a target current during the autonomous operation, the target current having been determined based on the measured data of the first current and the second current. a plurality of power storage devices, each of the plurality of power storage devices including a storage battery and a power conditioner configured to convert DC power of the storage battery into two-wire AC power and output the two-wire AC power, each of the plurality of power storage devices being switchable between a power grid interconnected operation and an autonomous operation without being interconnected with the utility power grid; and a transformer including: a primary winding to which a two-wire AC voltage is applied; and a secondary winding connected to a single-phase three-wire power line, a load being connected to the single-phase three-wire power line, wherein the single-phase three-wire power line includes a first voltage line, a second voltage line, and a neutral line, the power conditioner of any one of the plurality of power storage devices is configured to apply a two-wire AC voltage to be output during the autonomous operation to the primary winding, and the power conditioner of the rest (22, 23) of the plurality of power storage devices has an output connected to the first voltage line and the second voltage line during the autonomous operation, if each of the plurality of power storage devices performs the autonomous operation, any one of the plurality of power storage devices is configured to serve as a master device, while the rest of the plurality of power storage devices, other than the one serving as the master device, is configured to serve as at least one slave device (22, 23), the power conditioner of the master device is configured to perform voltage control of controlling an AC voltage output during the autonomous operation toward a target voltage, and the at least one slave device (22, 23) further includes a data acquisition unit configured to acquire measured data of a first current flowing through the first voltage line and a second current flowing through the second voltage line, the power conditioner of the at least one slave device (22, 23) being configured to perform current control of controlling an output current toward a target current during the autonomous operation, the target current having been determined based on the measured data of the first current and the second current.