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This paper describes a methodology that could be used by a utility to estimate the actual cost of congestion on its transmission system using limited, non-state estimator data. The assumed problem inputs are a power flow model of an entire interconnected grid (i.e., the eastern interconnect), costs for the utility's generators, and then hourly values of the utility's generation, load and tie-line flows over the study time period. Due to the common lack by most utilities of external measurements, the system is first equivalenced to retain only the utility's own internal buses and a small subset of the external buses. Then, for each hour, the utility's load and generation is set to match their historical values, while the external generation is adjusted to match the tie-line flows. Next, an economic dispatch is performed to determine the unconstrained cost. Finally, a security constrained OPF (SCOPF) is solved to take into account base case and contingent constraints. The methodology uses a complete ac power flow formulation to accurately estimate the impact of voltage constraints and the incremental impact of system losses. The inclusion of hydro generation is also considered. For illustrative purposes only, the methodology demonstrated on the TVA system using publicly available data transmission system data.