A new performance index for PV system analysis

Abstract

This paper describes a term called the performance index for analyzing photovoltaic (PV) system performance. The PI is the dimensionless ratio of actual to expected generation over any time interval. PV system performance is often described by applying terms widely used for conventional power plants. However, there are drawbacks to using some of the most common quantities, such as capacity factor, efficiency, and availability. For PV systems, these quantities exhibit wider seasonal variations and smaller magnitudes than for conventional power plants. Consequently, PV systems are often judged to be operating poorly, even in the absence of failures. At PVUSA, the PI is now being used for both real-time power plant monitoring and for longer-term analyses. A description of the calculation procedure and sample results are included.