This report describes the development of a method to assess battery energy storage system (BESS) performance that the Federal Energy Management Program (FEMP) and others can use to evaluate performance of deployed BESS or solar photovoltaic (PV) plus BESS systems. The proposed method is based on actual battery charge and discharge metered data to be collected from BESS systems provided by federal agencies participating in FEMP's performance assessment initiatives. Long-term (e.g., at least 1 year) time series (e.g., hourly) charge and discharge data are analyzed to provide approximate estimates of key performance indicators (KPIs).