Originally Posted by
datlas
It's really a personal decision.
When I go out at night, I have a really bright headlight (it's a magicshine clone and plenty bright for my needs). It's good for 2+ hours. But I also keep a reasonably bright (200 Lumens?) LED flashlight on the bars as a backup in case, for some reason, my main light dies. I am cool with that. But if you are doing longer night rides or Randonneur rides, you almost have to get a dynamo.
I have a Niterider Lumina 500, which advertises 5.5 hours on low, 3 hours on medium, and 1.5 hours on high. I feel like I can probably count on half of that on a full charge to give me some insurance, but at the medium setting that only gives me 1.5 hours of time I feel like I could count on. The low setting would give me more, but I'm always wanting more light than that setting provides.