This is an expert from an article in June, 2010 issue of Home Power Magazine.
It shows the results of a test in which two identical strings of 11 PV panels each.
The top string was hooked in series to a conventional string inverted which provided MPPT for the whole string.
The bottom string had devices that provided MPPT for each panel individually.
A portion of the right right two panels in each series string was shaded to see how the two approaches would handle the shading.
The partial shading of the two modules in the top series reduces the output of the two shaded PV panels to zero, and the power output of the remaining (unshaded) panels is also reduced.
In all, the MPPT on each panel increases array output by 40% for this shading case.
Gary August 3, 2010