Solar panels help to keep heat out by day and in at night.

"Rather than the sun beating down onto the roof, which causes heat to be pushed through the roof and inside the ceiling of the building, photovoltaic panels take the solar beating. Then much of the heat is removed by wind blowing between the panels and the roof. The benefits are greater if there is an open gap where air can circulate between the building and the solar panel, so tilted panels provide more cooling."

http://www.jacobsschool.ucsd.edu/news/news_releases/release.sfe?id=1094