That’s really a matter of personal preference. Technically speaking, most runners find that anything over 100 lumens gives them what they need, but if you aren’t certain, you can always go to the store and test out some of these running lights.
You may always be wondering why everyone measures lights in lumens now, instead of watts. Wattage is based on energy output, while lumens are based on the actual brightness of the light. With all the technological advances of solar power and LED features, you can get a much more accurate idea of how luminous the light you are buying will be when measured in lumens. However, if you still want an idea of watts vs lumens, 100 watts is about equal to 1,600 lumens. Comparatively, 40 watts is about 450 lumens.