I was having one of “those” conversations the other day. You know the ones that sound silly at the time, but make you think about it later. This one was about driving with your headlights on during daytime. A number of people had reasons they refused to turn on their headlights until it was too dark to see. I argued then that the reasons were unfounded, but I decided I should prove so. This is only “sort-of” about energy, but…
Before I begin – let me say this is a safety issue. You turn your headlights on in inclement weather, or during the daytime, in order to better be seen by other drivers. And it works. Any number of studies have shown that Daytime running lights significantly reduce accidents – especially the nasty ones where someone turns in front of someone. That is why they are required on motorcycles, and why many, if not most, other countries mandate daytime running lights. OK, Enough with the safety lecture.
One argument was that it “wore out the battery”. No. As long as the engine is running, your alternator is supplying power to all the electrical accessories. The battery is only for starting (and running the radio at drive-ins – you remember those). With the engine running, the battery plays no part in supplying electricty. In fact, once the engine is running you can completely remove the battery without any problems. Now I DO NOT recommend that in a modern "electronic – mobile" for other, unrelated reasons, but on an older car, it is perfectly fine.
The other argument was that it reduced gas mileage. And I agreed, but only by an extremely tiny, insignificant amount. Since as you know by now, I like all these numbers, I decided to figure out how much.
Well, the headlights do take power, energy. And we know that energy HAS to come from the gas tank since we do not have warp plasma reactors in our cars. In a typical sealed beam setup, the headlights draw 55 watts each on low beam (Halogens are similar). This means both headlights draw 110 watts (about the same as your table lamp). The taillights are also on, and draw 7 watts each for 14 watts total.
So, when we turn our headlights on, we have increased the electrical demand by about 125 watts (BTW, that is 10 amps at 12 volts). This would be equivalent to setting your heater blower on medium, but, I digress.
Since there are way too many variables to come up with a general number for all driving conditions in such a basic discussion, I will concentrate on one condition. We will say you are driving down the road at a steady 60 MPH with your headlights on. You take a trip of two hours (120 miles) and you get 30 mpg during the trip. So, in that 2 hour trip:
You have burned four gallons of gas. That is 496,000 BTUs (A measure of energy content). Your headlights have consumed .250 (point 250) Kilowatt-hours of electricity. At 3,414 BTUs per KWh, that is 853 BTUs. Now, that electricity was made with an engine that is 25% efficient, so we actually used 4 x 853 = 3,414 BTUs of our gasoline’s energy content to run the headlights.
.68 percent of the total energy in that four gallons of gas was used to run the headlights. Which is, in essence, how much your mileage would have increased if you had turned them off. Point 68 percent, which is .2 MPG at 30 mpg. At $2.50 a gallon, out of ten bucks worth of gas, you spent 3 and a half cents to turn on your headlights.This is not absolutely, incredibly, accurate (the many variables thing), but close enough for our purposes. It surprised me a little bit, the .2mpg thing, but I still say it is well worth it.Now, you know….