Wind diesel hybrid systems can often provide a cost effective solution to electricity supply in many rural and grid remote applications. The potential market for such stand alone systems is vast. The sizing and design of these systems to suite a give application is non-trivial. Design principles for wind diesel systems are presented with stress placed on the role of short term energy storage. It is shown that flywheels are the most appropriate form of energy storage. A user-friendly software package to help engineers design wind diesel systems has been developed over the last three years with support from CEC's JOULE programme. The modelling and software development was undertaken cooperatively by several EEC and EFTA countries. An brief introduction to the software, which models both logistic and dynamic aspects of system operation, is provided and there is a discussion of its validation.