A knowledge based expert system has been designed in order to imitate the decision-making process used by human experts. This expert system has been developed to carry out a pre-feasibility analysis of an energy storage system in an isolated power system, with both wind and diesel generation. Energy storage can be used to optimize the energy from renewable sources and reduce fuel consumption, offering economic and environmental advantages. The expert system performs a preliminary technical and economical analysis of the system when it determines that there is excess energy that can be stored. The analysis performed helps the user as initial step towards further analysis and design. Results show how the implementation of an ESS would reduce the fuel consumption when compared to the analysis of the system without energy storage, which could financially validate its initial investment.