In Brazil, the correct measurement of the individual firm energy of a plant is important, since it influences directly the determination of its assured energy which is used to establish contracts between power plants and distribution companies, free consumers, and traders. With increasing technological development and greater reliability in the use of automated techniques for monitoring, the use of the Acoustic Doppler Current Profiler (ADCP), has become a reality in Brazil. The ADCP has many advantages over the traditional techniques used for monitoring flows in gage stations of the national hydrometeorological network. In this context, the purpose of this work is to evaluate the impact of the streamflow rating curve measurement on the evaluation of the firm energy of a hydropower plant. A linear optimization model based on dynamic programming was used to calculate the firm energy and it was considered possible measurement errors in the plant’s inflow values and in the parameters of its polynomials that defines the upward and downward elevation. The results pointed that the two considerations had an impact on the calculated firm energy: the inflow measurements and the streamflow rating curve. Therefore, it is shown the importance of an accurate measurement of inflows for the evaluation of the plant’s firm energy.