An energy storage system of the present disclosure includes: a first battery module in which a plurality of battery cells are disposed; a second battery module in which a plurality of battery cells are disposed, and which is disposed to face the first battery module; a module screw which extends in a front-rear direction in which the first battery module and the second battery module are disposed, and fastens the first battery module and the second battery module; an upper fixing bracket which is disposed in an upper side of the first battery module and the second battery module, and fixes the first battery module and the second battery module; and a lower fixing bracket which is disposed in a lower side of the first battery module and the second battery module, and fixes the first battery module and the second battery module.