The cost of driving with your headlights on

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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….

Comments

[HID Conversion]

This is a nice study. The study on the effect of the headlights on during daytime. Headlights is one of the tools that avoid accident in the road so it is important that car lights should be of best quality.

[LED HID Head Lights]

Nice Study Indeed!I like the lower wattage bulbs, it does take less energy. I like the LED lighting . I like to feel safe on the road and by using driving lights is a must! Thanks for the good read!

[Denver Garkie]

I'd just like to point out that, through my research, the global average of all internal combustion engines is 13% usable energy taken from whatever fuel is used, may that be gasoline/petrol, diesel, propane, acetylene, hydrogen, or the ever popular "nitro-methane"

[accident council bluffs]

LEDs are awesome. The problem is the average Joe does not have expensive LED bulbs, Audis or BMWs. Remove cool LEDs from your equation and just target Halogen DRLS @ 100 watts per vehicle. I sure could use an extra $60 bucks in my pocket per year after I disable my Halogen DRLS.

[Projector Headlights]

Those are some pretty reasonable numbers regarding the costs. You are right. Turn them on only when needed. You can save money and energy for this.

[Ruzmetova]

morgstickNovember 7, 2011 The chevy volt has a battery of 16 kWh, menniag it takes 16 hrs to charge the battery at a rate of 1000w per sec, or 57,600,000 joules stored in the battery. It would run a lightbulb of 100watts for 576,000 hrsWatt equals joule per second (watt is a rate)1 Joule equals 1 kilogram (2.2 lbs) moving 1 meterAverage car weighs 1500 kg (chevy volt is 1700kg)So it takes 1500 joules to move a car 1 meter2,414,016 joules to move average car 1 milePS chevy volt can only go about 35 miles on the battery alone (it isn't 21 miles because this doesn't take into account friction less than 1 and )

[Audri]

September 3, 2011 The question is wrong but based on the info pdovired the energy consumed per mile will be between 0.2 and 0.4 kW-h per mile. If the car is travelling at 60 miles per hour and has a range of 40 miles, all the energy in the battery will be used up in 40 minutes. The power consumed is a different matter. The rate of energy consumption will be between 12000 and 24000 watts (120 to 240 100-watt light bulbs!) These figures sound more like what you'd expect for a car pulling a trailer than for a small car.

[Djata]

well it depends on the baertty type and the solar panel most have so little watts that they cant produce unless in full light and you can trickle charge which is just below the baertty output voltage the baertty the solar panel voltage will not be the same when it draws power as your baertty will not be same voltage as when its under load the voltage of the baertty is only important to the appliance it is powering so you can attempt to charge it but it will limit the baertty life and will eventually leave a bad memory on the baertty as a project OK but not for everyday use

[John]

I had 2 hours or so (on a 7 hour baterty).Running down baterty to a low value is very bad for it and shortens its life.I wont get sucked into that again, Using it on baterty until 20% then charging then unplug and work off baterty again and repeat this and a few times the baterty life is back? to over 7 hours again.I dont recommend doing this procedure at all.

[Andre]

Washing only a full load in cold water. Saves 220 water heater.Try not to use oven till cold wheetar. 220 runs ac more too. If it is gas, it runs the ac more.Keep the sun out.Caulk around the windows.When not using anything, turn off.Put fridge and hot water heater on save.Keep thermastat over 75 in summer.Under 75 in the winter.Use fan to draw air through the rooms.Use fan in room you hang in. And where you sleep.You can save from $75.00 to $200.00.Watch your out side lights. A timer can be set on those.