The combination of hydroelectric and photovoltaic sources of energy in a generation system may seem unfeasible due to the still quite high costs of the photovoltaic plants. However in the next few decades, significant reductions in those costs are to be expected. Moreover, this combination may reduce the cost of plants implemented in low hydroelectric potential sites for which main system interconnection costs prove prohibitive. The study of hybrid hydro-PV generating plants allows the evaluation of the use of water reservoirs and battery banks as alternatives for energy storage within a system. It also allows the evaluation of the advantages of generation from complementary energy sources. The objectives of this paper is to propose the concept of a theoretical performance limit for the plants and propose a method for the determination of this performance limit using computer simulations with idealized energy availability functions. Using this method, the effect of different degrees of complementarity in time of the energy resources on the performance of hydro PV plants is studied. It has been observed that, as expected, the smallest failure indexes measuring the energy supply to the consumers are associated to the best complementarity in time indexes. The results of the simulations led to the synthesis of failure index curves, as functions of the different degrees of complementarity in time, with data obtained solely with the utilization of idealized functions describing the energy availabilities, as proposed in this work. The results so obtained are a preliminary set treating the influence of the complementarity characteristics on the performance of hybrid plants based on hydro and solar energies and show how the complementarity characteristics may be used to design hybrid power generation systems showing improved efficiency. The subsequent research work shall proceed to at least two different phases. In the first one, the method exposed in the present work shall be applied to real data and compared to the operation of existing hydro PV plants. In the second phase the obtained relations of the performance of the system to the different degrees of complementarity shall be confronted with design parameters of hydroelectric-photovoltaic plants based on complementary resources. (C) 2012 Elsevier Ltd. All rights reserved.