A new, one-repetitive day simulation method, named MIRA, has been developed for predicting the long-term performance of a wide variety of solar energy systems. Compared to detailed hourly computer simulations, MIRA requires little computational effort and climatic data bases, while offering comparable accuracy. In the MIRA simulation method, the daily solar radiation profile incorporates both random fluctuations and an inherent time-of-day dependence. Satisfactory agreement between the long-term system performance predictions of the new method and corresponding detailed hourly simulations is demonstrated. The range and magnitude of the predictive accuracy of MIRA and other one-repetitive day methods are illustrated. Their usefulness in sensitivity studies and comparative evaluations of different solar system configurations and control strategies is also discussed.