An energy storage system has a reduced number of capacitors for storing energy such as renewable energy, thereby reducing cost and improving stability of the system. The energy storage system is configured to store power from a power generating unit, and includes: a storage capacitor having a first end electrically coupled to one end of the power generating unit; a secondary battery having a first terminal electrically coupled to a second end of the storage capacitor, and a second terminal electrically coupled to another end of the power generating unit; and a first converter configured to selectively couple the storage capacitor and the secondary battery to a load.