Particle Swarm Optimization (PSO) is a highly efficient evolutionary optimization algorithm In this paper a multiobjective optimization algorithm based on PSO applied to the optimal design of photovoltaic grid-connected systems (PVGCSs) is presented The proposed methodology intends to suggest the optimal number of system devices and the optimal PV module installation details such that the economic and environmental benefits achieved during the system's operational lifetime period are both maximized The objective function describing the economic benefit of the proposed optimization process is the lifetime system's total net profit which is calculated according to the method of the Net Present Value (NPV) The second objective function which corresponds to the environmental benefit, equals to the pollutant gas emissions avoided due to the use of the PVGCS The optimization's decision variables are the optimal number of the PV modules, the PV modules optimal tilt angle the optimal placement of the PV modules within the available installation area and the optimal distribution of the PV modules among the DC/AC converters (C) 2010 Elsevier Ltd All rights reserved