Abstract For practical purposes it can be assumed that the power delivered by a photovoltaic generator that is connected to an MPPT is always maximal. When studying the behaviour of a PV generator working in this way, the most interesting aspect is the evolution of the point of maximum power. So, the analysis of the I-V characteristic must be centred in the area of high voltages. In this situation the five parameter model is very appropriate for characterizing the PV generator. On the other hand, in the case of photovoltaic modules the information currently provided by manufacturers is insufficient to do modelling. Thus, to evaluate the loss resistances it is necessary to use any of the different methods that currently exist. The purpose of this paper is to present a new procedure, based on simplified equations, which allows the estimation of the loss resistances of any PV crystalline silicon module. By simulating the modules with the loss resistances calculated in this way highly accurate results are obtained. Especially in the surrounding of the maximum power point the error is always less than 0.5%.