A cheap LED dynamo light draws 2W at just about any speed. On the one hand, this results in less light than the 7W you mention, on the other hand it is less work for the rider.andrew_s said:A decent bottle dynamo is 40% efficient; a good hub dynamo is 60% efficient. So for a 3W bulb light, a bottle will make you put in an extra 7.5W of pedalling power and a hub will cost an extra 5W.
Modern LED lights will draw more power than this (about 7W from a hub dynamo at 30kph).
Lights off drag from a good hub is about 0.5W at 20kph, 1W at 30kph. An ordinary front hub will give about 0.25W drag at 30kph