Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)This paper presents a methodology for obtaining and adjusting of efficiency curves for hydroelectric generating units. It is based on measured data of power, gross head, and water discharge recorded by the company that manages the plant operation. The objective is to determine the actual performance characteristics of the set: turbine, generator, and penstock. In order to obtain the efficiency functions, an iterative calculation is used. Its input data are the functions currently in use of turbine efficiency, generator efficiency, and penstock head losses. For the adjustment of the efficiency functions, the Generalized Reduced Gradient optimization method is employed. A case study was applied to the data from a large Brazilian hydroelectric plant whose operation is under the coordination of the Electric System National Operator. The benefits of the proposed methodology are analyzed using a simulation tool for the hydroelectric operation. The simulator is used to reproduce the past operation of the plant, first with current data and second with adjusted data. The results show that the optimal unit efficiency functions significantly contribute to bring the real and simulated operation closer.14018691Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)