Computer simulation is an increasingly popular tool for determining the most suitable renewable energy system type, design and control for an isolated community or homestead. However for the user without any expertise in system design, the complicated process of system component and control selection using computer simulation takes on a trial and error approach. Our renewable energy system design package, RESSAD, has been developed to simulate a wide range of renewable power supply systems, and to go beyond system simulation, by combining design expertise with the simulation model The knowledge of the system designer is incorporated into the package through a range of analysis tools that assist in the selection process, without removing or restricting individual choices. The system selection process is analysed from the early stages of renewable resource assessment to the final evaluation of the results from a simulation of the chosen system. The approach of the RESSAD package in this selection process is described and its use is illustrated by two case studies in Western Australia.