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 a total amount of power produced by at least one new and renewable energy power plant in a plurality of regions, determines an operation mode of the energy storage device based on a result of monitoring a reference capacity of an integrated line through which power of the plurality of regions is transmitted and a result of monitoring the total power amount, and controls a power capacity flowing through the integrated 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.