Faults and performance deterioration issues related to increases of the series resistance in PV modules or arrays are one of the most common causes to decrease the energy yield of photovoltaic installations. Therefore, the early detection of such failure types is very important in order to minimize the time period with reduced power production. This paper focuses on the estimation of series resistance of flat silicone PV panels or arrays during operation, in the vicinity of their MPP. The method presented in this paper helps to detect failures by monitoring <br/>changes in the panel’s or array’s series resistance, compared to a reference value that can be determined either by a measurement in controlled conditions, or using datasheet parameters. The method is based on the measurement of the slope of the I-V curve at a fixed distance from the short-circuit current.