The photovoltaic (PV) system design must allow for the variable nature of solar radiation and provide electricity to the load with optimum reliability and minimum cost. There are requirements both to understand the system operation and improve the available techniques. The aim of the current project is to improve the existing PV design concepts; by applying a predicting formalism of solar radiation based on day to-day variations, and examine the possibilities of developing a new sizmg methodology. An alternative methodology is suggested for system sizing, which determines the reliability of supply in terms of how long the system is expected to operate without shedding load. It is based, similarly to several traditional methods, on long-term system modelling. Unlike the traditional approach based on loss-of-load probability the method provides a clear relationship between the required system configuration and the length of the time series used in system sizing. The modelling work points out the importance of suitably designed array-size battery combination in terms of a sizing curve. Much of the modelling work is based on measured time series in the South of UK and Potsdam, Germany. The use of artificially generated solar radiation data from climate experiments for the prediction of the battery state of charge, however, is also examined, opening a new area of research. The model based on the daily energy balance that has been used for this purpose has been validated using a stand-alone system which has been constructed for this project and monitored. Furthermore, battery experiments have been conducted in the laboratory to examine detailed battery operation under conditions close to those encountered in a PV system. This part of the work focuses on loss of battery charge, which is shown to occur close to the full SOc. The electrical characteristics of PV generator are examined through a model developed in Matlab and validated experimentally. Following extensive literature research and modelling work, radiation data have been converted to the inclined plane for use as input in long-term system modelling, by applying the methodology of the European Solar Radiation Atlas.