[nkurz]

This is a great way of thinking about it, but don't you lose a bit more due to increased reflection from the glass surface at low incident angles? Probably not enough to make a difference a low latitudes in the summer, but at high latitudes in the winter I think it might be a significant difference: https://en.wikipedia.org/wiki/Fresnel_equations#/media/File:...

[nkurz]

Partially answering myself, 'sacred_numbers' posted a link elsewhere in this thread that suggests this effect might be quite small: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6611928/. The paper concludes that for a cell with a good Anti-Reflective Coating (ARC), the reduction in efficiency due to reflection at a 60 deg incidence is only 2%.

On the other hand, the incidence for a flat mounted panel beyond 37 degrees of latitude on the winter solstice is greater than 60 degrees, and it's not clear (to me at least) how well the ARC on an average panel will continue working after years of outdoor usage. My guess is reflection is probably a real issue, but not a stopper unless one is already in a marginal situation.

[fy20]

The biggest issue going north is probably snow.

I'm at 54N and my output has been 0 for the past month as we've had a lot of snowfall. My panels are at 20 degrees, so even in the few sunny days we've had it's not been enough to melt the snow. A steeper angle probably would have cleared it a few times.

Usually the beginning of the year has more sun but it's colder, so I'll see what happens then.