An energy storage system includes an energy storage device that stores energy generated from a new and renewable energy power plant, a power management device that monitors an amount of power of the new and renewable energy power plant, determines an operation mode of the energy storage device based on a result of monitoring a reference capacity of a line connected to the new and renewable energy plant and the amount of power, and controls a power capacity flowing through the line according to the operation mode, and a power conversion device that receives a command corresponding to the operation mode from the power management device and controls an operation of the energy storage device according to the command.