Shade on a PV module can reduce its electrical power output considerably. This is especially the case with silicon crystalline modules, the most common type of modules used.
The solar cells in a PV module are connected in series. Shading one cell is like blocking a stream of water. Power output can be reduced by 80% or more.
Shading an array of PV modules can have a similar effect. The electric current output of the shaded PV module will be that of the entire PV array.
Example of shade on a PV module array. Source: solid gGmbH, Germany.
Shading of PV modules and arrays should be avoided. For systems with only a few modules, this can be achieved easily. However, for larger arrays, or if trees and other buildings are in the vicinity, a proper shading analysis needs to be carried out. Special instruments are available with which a professional shade analysis can be carried out.
PV modules have bypass diodes which mitigate the effect of shade, but electrical power output is still less.