Shading in PV Panels
If PV panels are shaded, not so bad: 3 bypass diodes typically bypass the problem.
For a high voltage string - say 10 panels, 3 bypass diodes each - 30 'strings' total - each string is 3.3% of the voltage. So you get a power loss just for that string reducing voltage by that string's voltage.
Since strings run the long way on a panel - panels should be mounted horizontally to facilitate shading losses under the assumption that shade would happen at the bottom of a panel. Ie, shading will affect a large portion of one of 3 strings in a panel, as opposed to a small section of multiple strings, bypassing all strings and making the entire panel inoperative.