hilly, I had a 10 watt panel hooked up to both batteries at my old place and it worked a treat, as others have mentioned make sure you do have a voltage regulator (these can cost as much as the panels sometimes) as even when the sun was near the horizon the damn thing was still putting out a charge they are that efficient these days.

I really need to get mine back on the roof of the new carport, sounds like I found a job for tomorrow.