Three key factors dominate efficiency losses: Material Limitations Battery chemistry dictates inherent energy losses. A battery energy storage system (BESS) is an electrochemical device that charges (or collects energy) from the grid or a power plant and then discharges that energy at a later time to. This report describes development of an effort to assess Battery Energy Storage System (BESS) performance that the U. Department of Energy (DOE) Federal Energy Management Program (FEMP) and others can employ to evaluate performance of deployed BESS or solar photovoltaic (PV) +BESS systems. The. When there is less PV power available than is required to power the loads (at night for example), energy stored in the battery will be used to power the loads. This will continue until the battery is depleted (ie. has reached it user-defined minimum % SoC). When mains power is available, any one of. Specific ES devices are limited in their ability to provide this flexibility because of performance constraints on the rate of charge, rate of discharge, total energy they can hold, the efficiency of storage, and their operational cycle life. These performance constraints can be found. Energy storage systems have evolved from simple mechanical solutions to sophisticated electrochemical and electromagnetic technologies over the past century.