A novel methodology to assess the degradation of a photovoltaic module is presented. The degradation of the module can be represented by a progressive alteration of the equivalent One-Diode circuit model for the device. The involved circuit elements are the series and shunt resistances. It is possible to estimate the alterations for these elements by measuring the divergence between the model and the device behaviour. Two novelties are the key aspects of this work. First, the model is identified through a very accurate procedure. Second, the irradiance is estimated directly by means of the photovoltaic device, without need for additional sensors. The result is a completely on-line estimation of the module degradation that does not require any change in the operating point.