The long power production warranties guaranteed by the manufacturers of photovoltaic (PV) modules (25 years) are a vital factor for the economical viability of PV systems. Nevertheless, recent studies have demonstrated that the modules may experience an early degradation, which turns into the reduction of the power production or even the damage of the entire PV module. In this paper, a method aimed at detecting the module degradation is introduced. It detects and quantifies the degradation by means of two indicators, which are aimed at estimating the increment of the modules series resistance, this parameter being one of the most affected by some degradation mechanisms. The increase of the series resistance value is estimated through the evaluation of the error in predicting the position of the maximum power point, which is obtained by comparing the experimental measurements with the single-diode model. The proposed method is experimentally validated, and it is compared with other methods reported in the literature. The results show that the proposed indicators are able to accurately estimate the increments of the series resistance with a low sensitivity to the irradiance and temperature values. This feature improves the accuracy and the reliability of the diagnosis processes and monitoring systems.