Abstract A photovoltaic field is classically made of parallel connected strings, each one formed by a number of series connected modules. The total cross tied interconnection, which adds string-to-string wires, allows to increase the harvested energy when non-uniform operating conditions, like partial shadowing, occur. The simulation of mismatched total cross tied photovoltaic fields is mandatory to predict the increase of the power production accurately and for designing the proper power processing stage for the maximum power point tracking function. A fast and accurate simulation is also useful for model-based diagnostic purposes. In this paper an effective simulation model, which profits from a peculiar sparsity pattern of the system of non linear equations for achieving a very short computation time, is presented. It can be implemented in any platform and it has high potentialities also to be ported on embedded systems for real time simulation.