18 watts and 1200 milamps (1.2 amps per hour?) is about what you'd want to
maintain a battery.
Look at it this way, if you've got a Group 27 battery, that's rated at about 100 amps (or 50 useable amps) your looking at recharging about 25 amps if you've used a quarter of it. Thus if you've got a six hour
solar window, you are needing a 4-5 amp system (about 85 watts) if my math and expectations are sesible.
I did my own folding 60 watt system, based on Bruce's advice:
I think I've got about $125 into the panels (2, 30 watt panels that I hinged), controller, and cord. I scrounged the panels off of eBay. Ask me how it fits my needs in September!
Tony