NOVELTY - The assembly (100) has a cover unit (5) provided with multiple covers and installed in an upper portion of a battery tray (20). A connection flange is extended to an upper side of a side panel (10) along horizontal direction and curved along gravity direction. Another coupling flange is extended to end parts of the battery tray along horizontal direction corresponding to the former connection flange. The former connection flange is arranged in an upper end of the battery tray. The battery tray is fixed in a holding space (21) of a battery pack (1) and connected to the side panel.    USE - Electricity storage system integrated floor panel assembly for an electric vehicle. Uses include but are not limited to plug-in hybrid electric vehicle (PHEV), battery electric vehicle (BEV), fuel cell electric vehicle (FCEV), car and environment-friendly vehicle.    ADVANTAGE - The assembly avoids usage of a separate mounting unit for mounting the battery tray to a center flower and reduces number of component and assembling process for assembling a mounting unit so as to reduce weight of a car body and improve arrangement degree of freedom of a battery pack. The assembly prolongs capacity of the battery pack and improves waterproofness performance of the battery pack. The assembly secures rigidity of the car body and evenly disperses acting of external force on the car body. The assembly minimizes deformation of seat mounting process by collision and improves safety of crash in an assembly part. The assembly improves front side and side collision performance of the battery tray and readily delivers collision energy to a broadside of a lungs sectional part. The assembly easily disperses absorbed collision energy so as to minimize damage of the lungs sectional part by the collision energy.    DESCRIPTION OF DRAWING(S) - The drawing shows a perspective view of an electricity storage system integrated floor panel assembly. '(Drawing includes non-English language text)'    Battery pack (1)    Cover unit (5)    Side panel (10)    Battery tray (20)    Holding space (21)    Electricity storage system integrated floor panel assembly (100)