[QUOTE=Looigi;15795723]Conservation of energy, it's the law. Energy in, energy out, and energy stored in your body. It's all accountable. You can calculate what you need to eat in order to charge those batteries. If you didn't eat it today, you will eat it tomorrow, or you ate it days ago. It aint free./QUOTE]

No one's trying to break he laws of physics, but we are not dealing with a closed system nor, as you already pointed out, a particularly efficient one. Most of us don't eat the exact amount of food needed to keep us going, and not every bit of exertion on a bike translates directly into forward motion. You can try to calculate the exact amount of food required to generate a specific amount of energy with a dynamo hub, but it will be a number that varies widely from person to person. It will also, I expect, be a very small number if you factor in the assumption that the person would be on the bike, pedaling, in both situations with he only difference being whether or not your using a generator to recapture some of the expended energy.

You're all over the map. Are you talking about physics or economics? From a physics standpoint, the energy requirements to move an SUV a fixed distance far exceed the requirements to move the comparatively much smaller cyclist the same distance. From a money standpoint, the example is very simplistic and ignores many variables while throwing out some arbitrary assumptions on the source/cost of food energy and the energy expended. Yes, you need to eat if you're going to bike, but you need to eat even if you don't bike. Otherwise the drivethru at McDonalds would be tailor made for bikes, getting there Big Macs to refuel. But it turns out that people in SUVs eat Big Macs, too. But the economics of riding a bike vs. driving are not what's at issue here. In fact, being that this is this is the electronics forum, the main thing at issue is how can the OP use a bike to charge stuff. Because the stated reason is environmental, we can certainly hypothesize about whether energy produced on the bike will be more or less environmentally friendly than energy from other sources, but that's not a strict, conservation of energy, physics question, nor is it a straightforward economic one. It's an interesting question, and one I'd love to see answered, but it's complicated. You would need to figure the exact, extra, food requirements to generate that extra energy and the environmental impact of that food compared to the environmental impact of producing that energy at the power plant. Some rather large variables might be he cyclist's fitness and metabolism, the type of food being eaten, and the way the local power company generates energy. It's not a simple physics puzzle. And in real world numbers, I expect it will be very difficult to calculate because I can barely feel a difference in riding when I turn on my generator hub, and I can see no difference in the amount of food I eat.