As well known, the market of PV systems is having a great development nowadays. In this process both companies and governments require to evaluate the projects of PV plants both in terms of revenue and quality. Therefore accurate tools for predicting their performances are becoming more and more important. The accuracy of the estimation is key for photovoltaic applications since this technology is characterized by quite low efficiency and high fixed costs. In this scenario, it is extremely important to develop models of each component of the system in order to evaluate the PV plant behavior in any working condition. In this paper a flexible model of PV module suitable for off-line and on-line simulation will be presented and discussed. It will be shown that beside its simplicity, the accuracy is very good. Furthermore its parameters can be easily measured or estimated from the rated values.