Characterization of the electrical performance of a photovoltaic array can take many forms depending on the end use of the data. Typical uses include buyer-seller negotiations, system performance prediction, and performance measurement. Buyer-seller negotiations may deal with specifying the size (power) of an array to be purchased under some standard reporting conditions, and may treat the warranty conditions governing allowable degradation of this performance with time. System design, on the other hand, requires prediction of performance under varying field conditions, not standard reporting conditions, and must include the non-ideal realities of operating systems: array shadowing, steep angles of incidence, soiling, and array-load energy utilization. Typical uses of predicted array performance include array sizing tradeoffs, tracking-pointing comparisons, load-array interface analyses and system economic evaluations. The third use, performance measurement, refers to the characterization of an as-built array as opposed to prediction of the performance of an array to be built. This may be done to assess actual array performance or to measure performance degradation over time.