Interesting topic....Given that most crystalline-Si module sub-strings (of cells) are vertical in nature, with junction-box bypass diodes protecting each string of cells, could it be considered that mounting in landscape might actually be beneficial where you have shadows rising up the bottom of the module, or down from the top of the module? With a portrait oriented module, such shading will affect all sub-strings of cells and therefore cause more output losses due to all the bypass diodes switching "on". I guess any movement of shading, if present, would typically be longitudinal by nature but does the construction/design of module sub-strings get factored in when considering orientation?
Just curious, ta
Andy