A multi-junction concentrator system computer model is presented, that aims to accurately m odel the power generated based on environmental data input, such as the irradiance, air temperature, humidity and pressure. The device model is discussed, illustrating the importance of statistical variation of module components when simulating a multi-cell module. This comprehensive concentrator computer model will be released for general use, aiming to become a useful resource for those designing and testing multi-junction based concentrator systems.