This work aims to study the impact of different models for the evaluation of the efficiency of a double axis PV tracking system on the monthly probability distribution function of the AC power. Two components of the global efficiency are analysed, that is: the effect of PV cells temperature on the module efficiency and the DC/AC converter efficiency. In particular, the temperature efficiency model combines basic parameters characterizing the array, with the local monthly average temperature and the monthly clearness index to yield a monthly average efficiency. The simulation results are compared with experimental data related to a 9.6kWp PV plant installed in ENEA research centre located in Portici, Naples (Italy). The tuning of the model is performed by both system measurements and environmental data.