Even under ideal operating conditions, existing methods for characterizing photovoltaic (PV) system performance are subject to substantial uncertainty and present difficulty for predictive modeling. An overview of current performance tests and their advantages and drawbacks is provided, and the concepts of a plant performance curve and weighted performance ratio are introduced. In addition, several performance test methods are compared using data collected from three adjacent PV systems comprised of different module technologies. These results illuminate the shortcomings of current performance testing and point to the urgent need in the industry for improved modeling capabilities to predict PV performance accurately.