What he said, except a solid-state charger is going to be more like 99% efficient.
Also, whatever you're trying to do, it doesn't sound practical. Assuming you can get 1 A at 6 VDC in direct sunlight, you'd still need 400 hours of direct sunlight to charge the batteries. And 6 W is a big solar panel - roughly the size of a sheet of paper. To charge the batteries in an 8-hour day you'd need an array of panels totaling at least 300 W. Which would probably be on the order of 5 ft on a side.
I'll defer to the experts on the technical side, but I will definitely agree with Sean's assessment on the practicality side.
My driveway is lit by a 20W LED array flood lamp run off a 12V battery "on loan" from my trolling motor. That is charged off a 25W solar panel (it's about 16"x20" in size). In the winter, I am lucky to get about 5-6 hours of light out of it on a good night. In the summer, I might get closer to 7-8 hours.