A method and system are provided for calculating a marginal cost curve for a plant at a plant load range comprising a plurality of specified plant loads. The method includes providing a model used for controlling the plant. The model relates plant load and one or more given cost contributing factors. The marginal value for each of the one or more given cost contributing factors at each of the plurality of specified plant loads is determined using the model. A variable cost at each of the plurality of specified plant loads is determined by: (i) multiplying the marginal value of each of the one or more given cost contributing factors at each of the plurality of specified plant loads by a respective unit cost of each of the one or more given cost contributing factors, and (ii) if there are a plurality of given cost contributing factors, summing the results of (i) at each of the plurality of specified plant loads. A marginal variable cost is determined at each of the plurality of specified plant loads by computing a derivative of the variable cost at each of the plurality of specified plant loads. A collection of marginal variable costs at the plurality of specified plant loads defines the marginal cost curve for the plant at the plant load range.
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