I'm using Sanyo hubs, which have more drag than the others. The difference is even greater without an electrical load. I use this as an excuse to keep my lights on in the daytime. The light might make me safer, and the kinetic cost is negligible. I don't exceed 20 mph often, and when I do, I don't sustain it. I suppose on the very rare occasion I join a bunch of A riders and cruise at 20, I can switch off the light or bring a different bike.

This discussion reinforces my decision to use dynamos. I'm pleased beyond expectation with them.

Does anyone have an estimate of how much old sidewall or bottom bracket dynamos consume? More, of course, but even when I use those, they don't wear my body out. I wouldn't run one in the day without absolutely needing the light. I'd say the noise is more of annoyance than the drag.