George is right, but don't dismiss to quickly the 3 W bulbs found in a generator-based headlight. Battery-driven headlights use a MR-11 spot which throws light everywhere, including in the sky. The Lumotec of Bisi headlights (used with a generator) have a well-focussed beam that goes on the road only. I found that the 3 W Lumotec headlight equals about an 8 W or 9 W MR-11 spot.
Battery headlights are good for 2 hours... but that's in theory. A 2-hour rated battery may last less than 15 minutes at -20 C (been there, done that).
So if you ride on trails (I mean single track, not "civilised" rail-to-trails), then get a powerful battery headlight. But if you ride on the road, then a generator will provide you trouble free service, no battery-charge worries,l and you will get as much light on the road as would offer any battery system, except the HID ones, obviously.
Look at
http://www.bikelist.org and search for Schmidt or dynohub in the i-bob and touring lists.
You'll see many comments!
Michel Gagnon