In this paper, a study on the mismatch effect due to the use of different photovoltaic (PV) modules classes in large-scale solar parks is presented. For this purpose, a new model for simulating current–voltage and power–voltage characteristics is introduced. The model is then applied for calculating mismatch losses in a number of case studies for a PV plant built in Bari, southern Italy. First, in order to test the effectiveness of the model, this is applied to homogeneous strings and field showing that the mismatch losses are zero. Subsequently, the use of inhomogeneous strings (i.e. made of modules belonging to different power classes) is investigated. Finally, the behaviour of 1 MWp homogeneous and inhomogeneous PV fields is investigated, again with a focus on the mismatch effect. The operational conditions have been introduced starting from the definition of European efficiency. The use of standard test conditions can in fact lead to gross approximations because mismatch losses depend, as well as, on PV module characteristics, electrical connections and electrical architecture, also on the location of the PV system. The results presented in this work can be used both by PV system designers for carrying out yield calculations, and by operation and maintenance personnel for substituting modules during operation without compromising the productivity of the plant. Copyright © 2012 John Wiley & Sons, Ltd.