While visiting in Europe, I am borrowing a bicycle with a Shimano hub dynamo that is rated 6 volt 2.4 watts. It powers headlight only and seems to do a decent job for lower end hub. Bottle dynamos and some other hubs I have seen generate 3.0 watts. What determines the watt ratings? A local shop has a similar 6 volt 2.4 watt Shimano dynamo hub at a reasonable price. I was thinking that it might be useful on my utility bike. Is there a compelling reason to avoid a 2.4 watt generator? I would only power a head lamp.