In this work, the optimal design of a hybrid electric power generation system for isolated zones, using Particle Swarm Optimization (PSO) technique, is presented. A study is carried out taking as case of study, the city of Cujubim, in the state of Rondonia-Brazil, consisting of photovoltaic panels, a diesel generator, batteries and an organic Rankine Cycle, which is used for the heat recovery of the exhaust gases from the diesel generator sets, what is the novelty of this study. Decision variables, involved in the optimal design problem of the hybrid system, are the number of photovoltaic panels, the number of batteries and the nominal power of the diesel generator. Directly connected to the size of the diesel generator, is the nominal power of ORC system, not being a decision variable. All components are modeled and the objective function is the cost of power generation, with system reliability, results of the simulation show that the hybrid system is profitable to supply electric power demand of a selected city. (C) 2017 Elsevier Ltd. All rights reserved.