Im sorry but that simply isn't true any more. Poly tech has advanced. 3 or 4 years ago it may have been true and I have a 220w mono and a 230w poly to prove it. However my last two sets of panels were a 245w mono and a 250w poly and the footprint of the two panels are identical. There is a whole inch of space lengthwise between the cells and frame of the mono panel but it makes no difference in footprint.
There is three types of low light issue to deal with. Overcast sky in which bypass diodes have no effect. My polys reduce their output depending on how thick thick the cloud cover is. My monos dropped like a rock anytime it became more than a thick haze. My current system of three 250w polys will pull 20a in pea soup and while I may have to cut back on the big loads, 20a will run the trailer indefinitely. Three on the monos would be putting out 7.14a at best.
Partial shading in which the bypass diodes are effective as long as you wire in parallel. In series the weakened panel will effect the rest.
Total shade as in under a tree I have a generator for as the output will be minimal.
