Optimal Solar Panel Angle

Here is a paper on Optimum Orientation of Solar Panels. However:

• anything less steep collects snow and dust. If you lose your first three hours of sunlight because you've got snow on the panels, it doesn't help your efficiency.
• The owner of this offgrid house claims that the totally vertical panels are best because they don't collect dust; the panels can be washed regularly but if you don't do that, their efficiency goes down.