Well.. actually First law of physics.."Energy cannot be created or destroyed, it can only be transferred from one form to the other, the lowest being heat"'
Thus you ride with your headlight on you are burning energy, where does it come from? Yep, the fuel tank. The guy is correct.. however.. It uses half of sod all on a stormy night energy, there you go..Amount of energy used by the generator.. The "generator" works harder when it is putting juice into the battery. When the zener diode which, allows electrical flow into the battery until a certain voltage then allows the voltage from the battery to flow back. So when the voltage output of the zener diode and the battery are equal there is no POTENTIAL DIFFERENCE in energy of the two systems. The potential difference is called Voltage by the way. SOOOOOOOOOOOO with no current flow because of the Potential energy difference there is no net transfer of energy. So.. turn on the light and there is an increase in consumption of energy and blah blah blah blah blah..
still stuff all on a stormy night though...