Maximum power-point tracking for many alternative energy-conversion systems implies the application of control methods, where the operation of the primary-energy-conversion process is optimized through a nonlinear control arrangement. This assumes the presence of constant efficiency values for the subsystems in cascade to the front-end process. In case, efficiencies of the subsequent stages are not constant and are dependent on diverse operating conditions, it becomes important to identify the success of power tracking as seen by the load unit. In this study, hydrokinetic-energy-conversion systems are studied in this regard. A repetitive approach that matches nonlinear efficiency information to the overall performance of the system is presented. With specific focus on "power curve" and "performance/efficiency curve," two figures of merit are introduced to identify issues, such as success of power tracking and divergence from an optimum operating point. A comprehensive simulation study and an experimental test example are also presented. This method can also be used for identifying the effects on efficiency nonlinearity in other alternative energy systems.