This article intends to spread the results obtained in the degradation analysis of 735 crystalline silicon modules after first operation year. Degradation analysis carried out in this work consists of modules characterization in laboratory before field installation and after one year in operation in real conditions. This characterization includes peak power measurement according to IEC 61215 and additional indoor infrared thermographic inspection. Analyzed modules belong to two large PV plants located in the South of Spain. Analysis shows a general decreasing of electrical parameters values in the range between -1.0% to -3.5% in all modules. Measurements indicate that main power losses are due to short circuit current degradation. Furthermore, no new thermographic defects were detected after first operation year, but higher electrical losses have been observed in defective modules.