Originally Posted by
cycle_maven
And 10 watts of light is going to use about 20 watts at the wheel- dynamo's aren't very efficient. That's about a fifth of your total output when cruising along at 15 KPH. It's gonna feel like pedaling through mud.
II don't think losses are that bad; maybe 5W under your scenario (~4Ohm resistance in the hub; small voltage drops across various diodes).
Getting 10W out of useful power out of a hub would be real work: 3 Crees in series at 1A -- you're definitely heading down a steep hill.
Overall, at 'typical' cruising speeds, I'm probably putting less than 10W extra into my pedals (2 Crees, Shimano hub) and getting a few hundred lumens. Definitely noticeable difference in power/speed. I could stand to have a little more light, in my opinion, so I supplement with a crappy flashlight (soon to be upgraded) and a battery-powered tail light.