I've used this set up for quite a while. The lights are old Union 6V. They are a little big for my taste, but what the heck. They were cheap. One side is 3W, good enough for road riding. The other side is 6W.

The battery is 6V 5AHr. With the 3W (drawing 1/2amp/hr) it technically should get me 10 hours of light. Of course it won't. I can rely on 6-7 hours. Of course, the 3W isn't really very bright. It's adequate. The additional 6W helps in really dark areas.