Abstract: An energy storage device includes at least one battery pack including a plurality of unit batteries, a pack frame accommodating the plurality of unit batteries, and a rack housing accommodating the at least one battery pack with the pack frame, the rack housing including at least one support member supporting a side wall of the pack frame, wherein the plurality of unit batteries are arranged in an overlapping manner in a first direction, the at least one support member extending in a second direction perpendicular to the first direction and traversing the side wall of the pack frame.