In order to estimate the performance of photovoltaic (PV) systems, we develop a real time expert system based on central microcomputer used as microserver and easily consultable with different automatic stations. We present in this paper, the principles and specificities of the measurement system and of the automatic data measuring device and its sensors, and also some aspects of the database and expert system developed for this application. We propose a method based on the determination of the loss-of-load probability (LLP), which allows the estimation of the optimum combination of battery capacity (C-s) and photovoltaic array peak power (P-c). The problem consists of determining the combination which corresponds to a minimum total system cost.