Conventional power generators are fueled by natural gas, steam, or water flow. These generators can respond to load changes by varying the fuel input by a valve control. Renewable power generators such as wind or solar, however, are not controllable since their fuel sources are intermittent in nature. This creates difficulties for planning generation systems having renewable energy sources. A methodology is needed to evaluate the generation system reliability and estimate the extent to which plants with intermittent sources can substitute for conventional sources. These calculations are important in generation system planning with intermittent sources.