They come rated at different amps. I have friends that have them on thier campers. We use them at work alot. They are 24' X 24' and they keep the batteries charged even when the have something drawing on them 24 hrs aday.
I would bet something like this would work for you. http://www.basspro.com/Brunton-SolarFlat-15-Solar-Panel/product/10210674/-1762222#prodDescription
Just search the net. You can find what ever you want to pay for one.
http://www.solaronline.com.au/55w-solar-e-solar-panel.html
Just make sure you get one the regulates the power so it does not over charge them.