I found a really helpful iPhone app for figuring out when/where the sun will shine on the solar panels at a particular campsite. It's called Sun Seeker 3D.
It works best if you have an iPhone 3GS (because it uses the built-in compass).
You can fire it up, point the phone at the sky above your campsite, and it will show the path the sun will take across the sky (superimposed on the camera view).
I wrote a blog entry about it on our Airstream blog, with some sample screen shots for the place we're currently camped:
Sun Seeker 3D - Riveted