shading is a problem in PV modules since shading just one cell in the module can reduce the power output to zero
Shading of a Single Cell
The output of a cell declines when shaded by a tree branch, building or module dust. The output declines proportionally to the amount of shading. For completely opaque objects such as a leaf, the decline in current output of the cell is proportional to the amount of the cell that is obscured.
Shading of a Module
The cells in a module are all connected in series. Shading a single cell causes the current in the string of cells to fall to the level of the shaded cell.