A simple deterministic model for the optimum program of the generation expansion and operation of electric systems was tested against a sophisticated probabilistic model for 5-, 10-, and 25-yr time horizons and different levels of aggregation of plants in the sophisticated model The results indicate that the sophisticated model, in its least aggregated form, is best suited for small system operation or short-and medium-term investment planning. The simple model proves very useful for large system operation or long-term investment planning.