Solar Panel Tilt
A solar panel facing the sun at noon in mid-summer receives more intensity of the light than when it is at horizontal. However, it can receives more total Solar Energy when it is at horizontal than when it is tilted during a long summer day. Regradless if it is shaded by building or trees. why is that so?
That does seem a little confusing. But is really pretty easy to understand.
When the solar panel is facing the sun at noon it will produce the most power it can be it summer or winter. But there is a however involved here. In the summer the sun comes up in the north east and sets in the north west which would be behind the solar panel in the morning and late after noon. So you lose that part of the day. By laying it back 15 deg more the sun will be able to shine on the face of the solar panel all day long. The panel will not capture as much direct sunlight at noon but the extra gain from the morning and late after noon will more then pay back the loss.
Check out this page. http://www.oynot.com/solar-radiation.html it shows a graph and explains it better.