Originally Posted by
SweetLou
Generating electricity has nothing to do with rolling resistance. So, having the light on or off will not affect the rolling resistance.
"Rolling resistance" may not be the ideal term, since it's usually used to describe the drag caused by the tires rolling on the pavement. But the dynamo does generate drag on the wheel, as you can see by spinning the wheel with the headlight off versus on. With the lights on, the wheel decelerates much faster.