Is provided a power supply system optimizing a power conversion efficiency of its entire system, including at least one main power supply unit converting an input power into an output power supplied to a load; an auxiliary power supply unit including a power storage unit supplying an output power by the power storage unit to the load; a power measurement unit for measuring an output power from the main and auxiliary power supply units; and a power supply control unit for calculating an optimal output power in the main power supply unit maximizing its power conversion efficiency according to the output power measured by the power measurement unit and an optimal output power or charge power in the auxiliary power supply unit, controlling the main power supply unit based on the optimal output power, and controlling the auxiliary power supply unit based on the optimal output power or charge power.