On: A $5 Lamp Powered Solely by Gravity

[duskwuff]

That's assuming a perfectly efficient dynamo and no mechanical losses. (Which are both huge assumptions.)

[icoder → duskwuff]

Exactly, and since they state that they think they can make it twice as efficient, that would mean they run at max 50% now.

[duskwuff → icoder]

I suspect it's even less than that. A fancy (= expensive!) bicycle hub dynamo has an efficiency of about 70%, and that's assuming that it's operating at peak efficiency, and that there are no additional mechanical losses in the lamp.