One critical element to understanding photovoltaic performance is that solar cells are current sources. This means that when you connect them in series, the weakest cell will determine the total current. In many (most?) solar panels, the individual solar cells are connected in series. For example, our 28V panels consist of
60 cells, each producing about 0.48V, connected in series. One shaded cell on a panel will determine the power output of the whole panel.
For example, our 28V, 200W panels produce about 7A per panel under full sun. However, 50% shading of a single cell causes its output to fall to 3.5A, which drags the output of the whole panel down to 3.5A. This is why you often see descriptions of 10% shading costing 50% output. Basically, shading causes the current-voltage (I-V) curve of the solar cell to drop (lower current for same voltage output). This is why all
modern solar cells should have bypass diodes. The resistive nature of the underperforming cell produces a reverse bias on that cell, and the bypass diode conducts, providing a shunt for the excess current produced by the fully performing cells. (That said, although Canadian Solar does say our panels have bypass diodes, they do not say whether each cell has a bypass diode, or if there is a single bypass diode for each panel.) A nice graphic description of how this works can be found on this web site.
Still, partial shading is quite costly. For this reason, there are devices that will adjust the maximum power point (MPP)1 of a panel to up the overall output in the face of partial shading or under-performance. These are called "Power Optimizers". Some examples of these are the SolarMagic, which is produced by National Semiconductor, and SunMizer from Xandex Solar. Unfortunately, these are pretty expensive ($200 or more).
(For those more technically oriented, here is a solar cell equivalent circuit, which I found on Wikipedia.)
It is a good question whether, with bypass diodes, Power Optimizers are helpful. As I mention in the previous paragraph, a bypass diode will not kick in unless the voltage across a cell is reversed. At MPP, a cell is performing optimally for the quantity of sunlight it receives and may still be boosting the system voltage, which removes the diode.
1Maximum Power Point (MPP) is the location on a photocell I-V curve at which maximum power is produced. There is a nice explanation of MPP tracking on the National Instruments website.
Some resources on shade and photovoltaic performance