In this paper is demonstrated optimal sizing of a stand-alone PV/FC/Wind Hybrid System (PFWHS). A PSO algorithm is applied to gain optimal solution. The optimization problem is developed to determine the optimal sizing of PFWHS that can achieve the load required power supply probability (Reliability) with a minimum overall cost of energy (OCE). Also to evaluation the impact of availability of PFWHS components on optimal sizing considering technical and economical indices, two scenarios are considered. As it is clear that considering complete availability of components in first scenario causes better reliability indices compared to second scenario but this is misleading to design the PFWHS, technically and economically. According to obtained results, neglecting the rate of components availability in hybrid power generation systems has unsuitable results on reliability of load supply, so to achieve the actual behavior of PFWHS, the rate of components availability must be considered in suitable insight to designer for reliable and economic designing