On my old bike I had a Shimano dynamo with a Lumotec CYO light. The nice thing was that when the light was off, the wheel spun free, with no added resistance that I could tell. I've still got the hub and I'm thinking about trying it out with a Supernova light. Will I still get the same effect? I don't ride at night all that much, so I'd like to not have the rolling resistance when I don't need it.

EDIT: Nevermind, I think I found my answer. Looks like most dynos have less resistance when there's no power draw. That must have been what I was noticing.

Yep. To tell the truth though, I cannot feel any resistance with the light on. LHT weight fully loaded kinda outweighs everything else.

Tangentially, having owned a Supernova E3 Pro asymmetrical/glarefree, and a Cyo (60-lux longrange version), be aware the 60-lux Cyo reaches much further than the Supernova does. If you want a wide but shallow beam, e.g. for keeping track of the edges of a dark MUP at lower speeds, then the Supernova's good for that. But if you want to see up the highway 2-3 seconds at 20mph+, go with another Cyo 60-lux.

On the original topic, last week I did the same climb on Monday and Friday, a ~5-mile grade, and switched off the Cyo for the Friday run. Over an 18-minute climb, my times were 1 second apart. Other factors were as equal as you're likely to get in the real world (same bike, same equipment) so it's safe to say it doesn't make much difference... random variations in wind speed/direction would dwarf the impact of running with your headlight on.

+1
My commute bike has the Supernova E3 headlight and the Supernova tail light, powered by a Schmidt Dynohub. I can't notice the difference in drag for on/off so I leave the lights on all the time.

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.

+ 2 ... My Surly Long Haul Trucker really over rides any difference

Andrew