A solar-based power supply system, such as a Photovoltaic-diesel-battery system, is a particularly attractive option for decentralised power supply in southern Africa where solar radiation is ubiquitous in most countries. Such systems can make a positive contribution to the sustainability of rural communities in developing countries that do not have access to electricity grid as they address shortfalls of stand- alone systems. However, a lot more design effort and expertise is required for optimising the sizing and operational strategy of the PV-diesel-battery hybrid system than is required for single-source systems. This book presents the development and application of a simple spreadsheet-based mathematical model for sizing and performance prediction of a PV-diesel-battery autonomous power supply system. The model can be used to compute the energy performance of systems with different combinations of component sizes, for two different diesel generator dispatch strategies. The model could be useful in sizing hybrid systems to supply whole villages, schools and clinics in remotely located rural areas where there is no access to grid electricity.