This power supply device includes: a power interconnection path connecting a power interconnection port to a power conversion unit via a first switch; a stand-alone power supply path leading from an auxiliary input port via a second switch to an output port to which a load is connected; a storage-battery-side power supply path leading from a storage battery via the power conversion unit to the output port; and a control unit, the control unit causing the power conversion unit to charge/discharge the storage battery when interconnected with a commercial power grid, the control unit supplying power to the load via one of the stand-alone power supply path and the storage-battery-side power supply path when disconnected from the commercial power grid, the control unit switching the power supply path from the one to the other within an extremely short time period such as not to influence the load.

A method for synchronizing sensors. A ratio of a first data rate of the first sensor to the second data rate of the second sensor is 2.sup.n, where n is an element from the set of natural numbers. A central timer is started. A first countdown timer is generated based on the central timer and the first data rate, and a second countdown timer is generated based on the central timer and the second data rate. The first countdown timer and the second countdown timer are started periodically. The measurement by the first sensor begins at the latest when a first latency equals the value of the first countdown timer, and the measurement by the second sensor begins at the latest when the second latency equals the value of the second countdown timer.

A PCS comprises a communication unit that communicates with an external server; and a controller that changes from a grid connected state to a self-sustained state when a communication state between the PCS and the external server is equal to or lower than a predetermined threshold.