Step-up transformers are exploited to connect large PV plants to the utility network, their sizing being often accomplished only taking into account the PV plant peak power. In the present paper a general design methodology is proposed to optimally select the size of these transformers on the basis of the statistical distribution of the solar irradiation in the selected site and the mathematical model of the plant. It is based on a probabilistic approach based on the evaluation of the so called LPPP (Loss of Produced Power Probability) index. Taking into account full life costs the proposed approach is exploited to find the optimal transformer size for a grid connected 2 MW PV plant located in different geographical sites. Moreover, the effect of the introduction of energy storage systems in the PV plant is also evaluated.