The analysis of the degradation of thin-film single junction a-Si photovoltaic (PV) modules and its impact on the output power of a PV array under outdoor long term exposure located in Jaén (Spain), a relatively dry and sunny inland site with a Continental-Mediterranean climate is addressed in this paper. Furthermore, a new procedure of solar cell model parameters extraction experimentally validated is presented. The parameter extraction procedure allows obtaining main model parameters of the solar cells forming the PV array from monitored data of the PV system in real operation of work. Results obtained of the evolution of each one of the solar cell model parameters along the PV system outdoor long term exposure campaign are analysed in order to achieve a better understanding of the performance changes of the PV modules and the behaviour of the output power of the PV array.