Power-supply units, each of which has an AC-to-DC conversion circuit and a DC-to-DC conversion circuit, and battery units, each of which has a battery and a DC-to-DC conversion circuit, are connected in parallel to a shared DC busbar that connects to both of said units, and a load is connected to said DC busbar. In a normal mode, power is supplied to the load by the power-supply units. If an AC power supply fails, the power-supply system transitions to a backup mode in which power is supplied by the battery units. Also, the power-supply system transitions to an assist mode if, for example: the load exceeds the total rating of the power-supply units; the input voltage drops, without constituting a power failure, such that sufficient power cannot be supplied; or a subset of the power-supply units stop due to a malfunction, maintenance, or the like. In said assist mode, if the supply of power from the power-supply units is insufficient, the battery units cover the shortfall.