Desalination process is an essential demand to overcome the lack of drinking water in remote areas in Egypt. Hybrid energy system drives desalination techniques to provide potable water for islands and coastal zones where there is no electrical grid. This paper analyzes the technical configuration, sizing and economic optimization of off-grid hybrid energy system powered reverse osmosis plant. The hybrid system consists of wind turbines, photovoltaic panels, converters, storage batteries, and a diesel generator. The objective of this study is to satisfy the electrical energy demand of the reverse osmosis (RO) plant with a freshwater capacity of 100 m3/day for use at Nakhl, North Sinai, Egypt. This simulation studied eleven different configurations of energy sources to select the optimal case at the proposed location. Homer package is used to select the cost-effective, sustainable and socially accepted system. The simulation is based on calculating the net present cost (NPC), excess system electricity, cost of energy (COE) and carbon dioxide emissions (CO2). Through the study, the optimal configuration consists of 160 kW photo-voltaic (PV) panels, 19 strings of lead-acid batteries and a 50 kW diesel generator system. The optimal case achieves the lowest cost of energy ($0.107/kWh) and net present cost ($502,662).