I'm using one of these 10-W panels from UL solar in my super-budget system:
It's mounted on the roof.
I am using a charge controller but it's the cheapest one I could find: A Brightwatts from Lowe's special order for around $16 but I can't find it on their page. Maybe they discontinued it.
Here's another option:
I've heard that a charge controller is recommended for panels 10-Watts or higher. Mine's been parked in the sun for a few weeks and the controller has stopped the panel from charging. It's probably not a good idea to over-charge the battery without a controller.