I use a 55 watt automotive type halogen light. You can get them at the auto dept at wal-mart for about $20/pair. I power them with two 10 amp/hr sla batteries. One would be enough, really. But with two, mounted on either side of the wheel, it gives better balance plus 3 hours of run time. That's more than I ever use so the batteries are never drained very far.
I mounted the light down low in order to avoid blinding oncoming cars. I also run three standard bicycle LEDs up near handlebar level so that oncoming cars aren't fooled by the low light into thinking that I'm a motorcycle much further away than I really am.
It seems that I've uploaded a photo of this setup in another thread. If I've figured out how to link properly, then this should get you there;