When we were kids, we loved that "Way Things Work" book. Although we absorbed very little from it, it was always fun looking at the pretty pictures and diagrams. HowStuffWorks.com is the net equivalent of that book, and just like the book, we absorbed very little from the text. Take for example the question posed by the following article on the site:

If daytime running lights were mandatory in the U.S., and all vehicles had them, how much extra gasoline would that use each year?

Except for a couple of major points, the majority of it went in one ear and then out our anus — everything that is, except the following salient details:

A typical headlight bulb uses about 55 watts; sometimes the daytime running lights run at a lower wattage so they use a little less power. Let's say the daytime running lights use 100 watts since there are two bulbs.

To calculate the energy used, we need to figure out how much time people will spend with their lights on. According the to NHTSA, vehicles in the US drove 2,560 billion miles in 1997. We need to make a guess at the average speed people drive including stops in order to figure out how much time people spent driving their cars. Let's guess 30 mph, which means each mile takes two minutes. That makes 5,120 billion minutes or 85.3 billion hours. Now if each car normally drives at night about half the time, that means that the daytime running lights would be on 42.6 billion hours a year. Multiplying by the 100 watts we get 4,260 billion watt-hours or 4.26 billion kilowatt-hours.

To calculate how many gallons of gas this is, you can divide the 4.26 billion kilowatt hours of energy that the daytime running lights consume each year by the 10.5 kilowatt-hours of energy each gallon of gas yields. If daytime running lights were on all the vehicles in the U.S., we would burn an extra 406 million gallons of gas each year.