A power source system for supplying DC (direct current) power to a load, including first and second power source apparatuses and a control apparatus. The first power source apparatus is connected to an alternating current (AC) power source, and configured to convert AC power received from the AC power source to first DC power. The second power source apparatus is connected to a battery, and configured to convert power of the battery to second DC power. The control apparatus is connected to the first and second power source apparatuses, and configured to so control the first and second power source apparatuses that the DC power supplied to the load is the first DC power in a normal mode, the second DC power in a back-up mode, and a combination of the first and second DC power in an assist mode.