Originally Posted by slooney
Offroad, as posited by Harlemgirl, the minimum you need is 15 watts? Then what have I been doing for 15 years with far less wattage? Think of all those great rides I would have missed, waiting around until I could afford 15-25 watts? And I never, ever thought "Damn, if only I'd had the 15 watt light, then I could have run this trail." Hell, I've ridden down 6 mile-long canyon access roads with a failing headlamp (Petzl Micro, two double A's) and been fine.
Well, granted I have lousy vision, but I still say most folks I know prefer a minimum of 15 watts. I started out with a 6w Vistalite, so sure, you can do it with less light or even no light, but I found it alot more fun with more light.
It also depends where you're riding, too. Most of my nightriding has been in WA, OR, and BC. No light necessary for climbing up the logging roads, but the singletrack trails in the mostly dense forests are dark in the day time, let alone the night.
Helmet is the way to go offroad. As dumb as it seems, legally in many states the light has to be on the bike. I had a customer who was hit by a car. The driver was cited, but he also got a ticket b/c he had a helmet light instead of a bar light.