I've been riding with my Sanyo dynamo hubs for a while now, and as far as I can tell, I don't notice the drag at all. I notice the vibration, but that's different. Nominal output is 3W of electrical power, so let's say it's 50% efficient. So it's drawing 6 watts of my kinetic power. While I tool around gently on my mostly flat streets, how much does that add up to? Is there a way to put this in perspective? I can lift the bike up and spin the wheel, and I notice it stops before a regular wheel would, but it's not hard to spin the wheel. This leads me to believe that the cost of running these is vanishingly close to zero.

Thoughts?