A validated predictive model of a-Si:H solar cell arrays was developed. The performance of a-Si:H solar cells was modeled by predicting the performance before degradation first, and then modifying it with terms that account for degradation and recovery effects. A unique approach for the determination of the fundamental rate controlling parameters for the degradation and recovery process was carried out by observing the variation of the short-circuit current. The experimental annealing of a-Si:H silicon samples showed that the percent recovery from the degraded state to the as-grown state by annealing was virtually independent of the initial state at the start of the annealing process. This allowed the recovery parameters to be determined independently of the prior degradation process. An extremely simple and fast running algorithm for the long-term performance was developed in terms of the incident solar radiation, the panel temperature, and the total radiation exposed. Also it was found that the entire process of the Staebler-Wronski effect could be adequately represented by a correlation in which the degradation and recovery processes are solely a function of the total radiation exposure of the panel at ambient conditions.