Rating a PV system is complicated by the difficulties of obtaining performance data under the "rating" conditions, the intricate relationships between PV output and prevailing conditions, and the desire for quick results at low cost. Since 1989, PVUSA has been rating PV systems using continuous data collection and a simple regression model. Recently, Sandia National Laboratories has developed an improved IV curve-based method for characterizing PV arrays. Several systems at the PVUSA facility in Davis, CA were subjected to both methods. The results of that work are presented in this paper.