In this paper, the results of the analysis of the degradation of a set of single-crystalline silicon modules after 21 years are presented. The comparison of the main electrical parameters and the series and of the shunt resistances measured in 1996 and 2017 is performed, so that the annual degradation rate is evaluated. In addition, a detailed analysis of the parameter uncertainties has been performed in order to determine its impact on the results. A visual inspection of the modules has also been carried out in order to show the correlation with the variation of the electrical performance.Finally, the temperature coefficients of the degraded modules have been estimated and compared with the nominal ones. The results shown in the paper reveal that the mean annual degradation rate in terms of power is close to 0.9%. KEYWORDSI-V curve correction, outdoor measurement, photovoltaic degradation, temperature coefficient, uncertainty analysis, visual defect main electrical parameters and the increase in their dispersion have to be considered. As each module tends to degrade in a different way, the behavior of the modules will be more and more different as time goes by, thus affecting negatively the global performance of the plant.According to Köntges et al. 2 a module defect is an irreversible damage that causes either a degradation in terms of power or a security risk. These defects can be classified in different categories wileyonlinelibrary.com/journal/pip

The temperature effect on photovoltaic modules is usually quantified by means of some coefficients relating the variations of the open‐circuit voltage, of the short‐circuit current, and of the maximum power to temperature changes. In this paper, comprehensive experimental guidelines to estimate the temperature coefficients when measurements are performed outdoors are given. In addition, a correction procedure is applied to the short‐circuit current to cancel out the influence of the fluctuations in the irradiance level. The experimental results shown in this paper refer to several single‐crystalline silicon modules that have been operating for 21 years. Consequently, the estimated values of the temperature coefficients also account for module ageing processes. The coefficients of determination achieved in the case of the temperature coefficients of the open‐circuit voltage and the maximum power are very close to one. Results concerning the temperature coefficients of the short‐circuit current are also suitable if they are previously corrected.