A point well taken. I am not as concerned about where I mount it as I am on how to mount it. Ideally it would be some way, whether I tour with the trailer or panniers, that would allow reasonably quick and easy access to the bag(s) beneath it, while maximizing its exposure to the sunlight. The unit came with four small carabiners, which proved useless (at least to me).
I thought that mounting it on the trailer would be good in that it provided great exposure, but, being so close to the ground and at the rear of bike, it got covered in dust, mud and road debris - all of which reduced its efficiency. I have yet to use it on a tour with panniers.