A power storage system and a method of controlling the same include supplying power to a load by connecting a power generation system and a grid. The system includes a power converting unit that operates in one of at least two control modes including a maximum power point tracking control mode in which the power generation system is controlled to generate maximum power and a voltage control mode in which a boosting ratio is adjusted according to a change in the amount of power consumed by a load. The system also includes a direct current (DC)/DC converter for converting an output voltage of the power converting unit into a DC link voltage, an inverter for converting the DC link voltage into an alternating current (AC) voltage appropriate for the grid, and a central controller for controlling operations of the power converting unit, the DC/DC converter, and the inverter.