Let us suppose that the boost battery charger of the previous example is working at the MPP in the kth sampling instant (Fig. 9), at an irradiance level equal to S, and that the sign of the duty-cycle perturbation is negative d d d: consequently, the operating point must move in the direction of higher array voltages from the MPP to point A. If the irradiance level changes between the k-th and the -th sampling instants, for example if it increases (Fig. 9), then the operating point at the -th sampling will be point B instead of point A. In the sequel d is the array output power variation (at a constant irradiance level ) caused by the P&O driven duty cycle variation and s is the array output power variation (at a constant duty cycle d d d)