Originally Posted by folderfan550
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.
A lot of people with 3W hubs split 2.4W to the head lamp and 0.6W to the tail lamp, so I guess that's the rationale behind the 2.4W head-lamp only version. Obviously, 3W would be brighter. Personally, I would like more than 3W, but no one makes one (on account of German standards, I think).
As for the rating, it all has to do with the design: magnets, coils of wire, other things that I vaguely understand. There might be less inductance in the 2.4W version, so it might be marginally more efficient, but I think you're pretty much 'stuck' at 80% of the power at any given speed.
Incidentally, I have measured the inductance, resistance, and voltage production of my 3W hub: if anyone has the numbers for a 2.4W hub, let's compare notes.