This paper presents a planning tool where rigorous plant performance models are used to produce accurate plant performance data reflecting real plant operation under the loads specified by load demand curves. Cumulative figures such as total fuel consumption and production efficiency serve as the basis for a subsequent detailed economic analysis. Basic inputs to the model are the energy demand curves for a specific site on a daily, hourly, or even per minute basis. The plant performance model takes into account the varying load levels, ambient conditions, fuel price, and revenues from electric power delivered off-site. The load/performance simulator calculates plant performance and basic economic parameters at each point in time of the load demand curve to exactly match the given energy demand (Fig. 1). The results are critical economic data like cumulated fuel consumption and expenses, total energy production, and overall averaged plant efficiency. The performance model delivers detailed plant performance data on an overall plant as well as component level. The results reflect real plant operation under all varying conditions, hence produce highly significant performance and economic parameters.