Vanadium redox batteries (VRBs) have proven to be a viable energy storage technology for portable microgrids due to their rechargeability and high energy density. VRBs exhibit parasitic load loss during operation due to pumping of electrolyte across the membrane during charging and discharging cycles, as well as required temperature control in the form of heating, ventilation and air conditioning. This paper focuses on empirically characterizing VRB efficiency based on known climatic operating conditions and load requirements. A model is created to determine system performance based on known climatic and load data collected and analyzed over an extended time period. A case study is performed using known data for a week time period to characterize system performance, which was compared to actual system performance observed during this same time period. This model allows for appropriate sizing of the PV array and discretionary loads based on required energy density of the system.