Partial shading of solar panels causes the output power of solar panels to be drop more than one would expect. Even waiving a hand in front of a panel shows up in the output voltage of a 6'x4' panel. The reason for larger than expected loss is that a panel is made up of multiple electrical circuits, which in turn is made of multiple small solar cells. Removing sunlight from even 1 cell in the circuit causes the entire sub circuit to switch off. This is similar to Christmas lights that are connected in series. 1 bulb goes out and entire string is switched off. Note that even though 1 sub circuit goes off, others on the panels are still generating power.
