IV. MODELS
Predicting the performance of PV systems under various scenarios requires models for their outputs, based on insolation models. A large number of insolation models have been developed by researchers around the world. They can be grouped into four generic types: 1) spectral models; 2) cloud models; 3) time-series models; and 4) probabilistic models. By combining a PV array model and an insolation model, outputs of PV systems can be predicted. PV array performance models become more involved when additional factors such as the influence of cell operating temperature, variation of efficiency with insolation, and other phenomena are included. In many cases, such sophisticated models are not necessary. A simple model based on a beta-distributed cloud cover has proven to be useful in performance assessment of PV systems. The time value of PV output can be included in the evaluations by using a simple hourly load model that assumes the load to be uniformly distributed between a maximum and a minimum value that depend on the hour of day and time of year [7],[8]. A number of models are available commercially and from federal laboratories to predict the performance of PV systems [9],[10].