This presentation was composed when all headlights were incandescent bulbs, which happen to have fairly low overall efficiency. Some modern vehicles now have headlights which are entirely different, like LEDs, where the efficiency is much, much better, and the arguments discussed here are nearly irrelevant for such vehicles! So feel free to have the headlights on at all times for those vehicles. Additionally, feel free to have the "parking lights" on during the daylight. Even though they are possibly still incandescent, they are less bright than headlights so they use much less electricity.
I feel the need to add an unrelated comment here! In the 1960s, I had built a very unique vehicle which resembled the 'cigar-shaped' racecars which had been popular in the Indianapolis Race up to about then. I installed four headlights, as was popular at that time, with standard low-beam headlights on the outside. But instead of using standard high-beam "brights" as the inner headlights, I instead installed the headlights used on General Aviation aircraft, which happened to be of the same size and operated on the same 12 volts. Such headlights were very bright! A Police Officer threatened to give me a Ticket for "illegal headlights", so I had to agree to remove them and replace them with standard brights. The concern he had was that such very bright Brights might blind oncoming drivers (a valid point!) The reason I mention this now is that the Government seems to have altered the attitudes and Laws to allow such very bright headlights today, even for "low beams". I suspect that my aircraft landing lights would now be considered legal! Even though they certainly would have temporarily blinded oncoming drivers. I personally find it offensive (and amazing) that some cars today have ridiculously bright headlights, where even I get temporarily blinded when I encounter such cars on rural two-lane roads at night. Blinding oncoming drivers seems to no longer count! The headlight manufacturers now aggressively promote the fact that their headlights are far brighter than competitors! Apparently, safety is no longer a concern for oncoming vehicles, and only additional sales matters today. Sorry for the Editorial comment!
During the night, and during inclement weather, it is necessary, and even legally required, to have the headlights on. No problem there! But there are millions of drivers who turn their headlights on during daytime driving, too. If that actually represents an additional safety factor, great! Other drivers who may not be paying attention might actually notice the movement of the bright headlights, and thereby avoid an accident. Only you can determine the possible value of this. (My personal solution to this has always been to simply drive really bright red cars, where people tend to notice them during the daytime whether the headlights are on or not!)
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This means that a vehicle that would use 32 horsepower with headlights off might take 34 horsepower when the headlights are on. Essentially, this can result in the gasoline mileage being lowered by nearly 6%. In other words, you may wind up buying 6% more gasoline because of having the headlights on! One aspect of my argument is a little off though, because even when the headlights are not on, the fanbelt is still powered by the engine and the alternator bearings still waste some energy. The DIFFERENTIAL benefit or load regarding just having the headlights off or on, is therefore more minimal than the full two horsepower.
Here are the values for one of my Corvettes, a fairly aerodynamic car. Dynamic pressure (only depends on speed and not the type of vehicle) at 60 mph is around 18.6 pounds per square foot. (at 70 mph 25.3 psf). The frontal area (from GM) is 19.0 square feet, the aerodynamic coefficient of drag is 0.330. The aerodynamic drag coefficient is relatively constant at different speeds. So, for my car at 60 mph, the Aerodynamic drag is 19.0 * .330 * 18.6 or about 116.7 pounds. (at 70 mph it is about 158.9 pounds, and at 40 mph, 51.9 pounds). The Tire Resistance drag is (at 60 mph) around 0.015 of the vehicle weight, and is dependent on the type of tires, their inflation, temperature, speed and other things. Since my Corvette weighs around 3200 pounds, this gives about 48 pounds at 60 mph. This makes the Total Drag as 116.7 + 48 or 164.7 pounds at 60 mph (and 218.9 pounds at 70 mph) (and 51.9 + 32 or 83.9 pounds at 40 mph).
At 60 mph, the total required horsepower to overcome this drag and maintain a constant speed is 164.7 * 88 / 550 or 26.4 horsepower. (at 70 mph it is 40.9 a considerably higher drag load!) A horsepower is equivalent to 726 watts so this is 19,700 watts of needed (or usable) output. In one hour of driving at that constant speed, we would therefore ACTUALLY use up an amount of energy equal to 19,700 watt-hours or 19.7 kWh. (at 70 mph, 30.5 kW, and at 40 mph, 6.7 kW). This is the ACTUAL amount of power (expressed in electrical units) that is needed to move my Corvette at a constant speed at those speeds! Sort of amazingly low, as four kitchen toasters use up 6 kW of electrical power!
Included in the example above is the tire resistance, which, at 60 mph, is usually around 1.5% of the vehicle weight. For a modern 3,000 pound automobile, that's around 45 pounds of tire resistance force. The total vehicle drag is the sum of these two (air resistance and tire resistance), which ranges from about 150 to 230 pounds force (again, for standard-shaped cars, and not trucks or SUVs). Wheel bearings and other mechanical frictions also exist, but in a reasonably maintained vehicle, they are very small and will be neglected here.
That was for a mid-sized car. What about a sub-compact car? The frontal area is much smaller, so the air resistance drag is less, and the vehicle weight is less, so the tire resistance is less, too. Such a small car could therefore have a total drag (again, at 60 mph) which is lower than the numbers for my Corvette. (This is why small cars can have less powerful engines! They generally need less horsepower to travel at a constant speed.)
But the headlights are just as bright on a small car, so they use just as much electricity, so they can still need engine power to generate it. In this case, the load on the engine might rise from 17 HP to 19, an increase of around 12%. So, for a sub-compact car, daytime driving with the lights on could increase gasoline usage (and cost) by around 12%, FOR the situation of driving at a constant 60 mph speed. The effect is actually even greater for slower car speeds!
Regarding the headlights, on low-beams, each headlight draws around 4 amperes of electricity, usually at around 14 volts (while the engine is running). That's 55 watts each (at 12 volts) or about 75 watts (at 14 volts) or about 150 watts between the two headlights. There are assorted other exterior vehicle lights that are on when the headlights are turned on, at least two taillights, two front parking lights, four side marker lights, interior lights, etc. These lights add around 70 watts, so the entire lighting electrical load is around 220 watts.
Since one horsepower is equivalent to 746 watts, that is actually around 1/3 horsepower equivalent of electricity. It turns out that there are significant losses in the production of the electricity, primarily in the fan belt drive and in the operation of the electric and magnetic fields inside the alternator itself. The net effect is that between 1 and 2 engine horsepower is used up in creating the electricity for the vehicle lights. On a different subject regarding the production and distribution of electricity, did you realize that only around 13% of the energy in the coal or gas burned in an electric generating plant actually gets to our houses to be usable?
(People have ripped into me for many years regarding this statement, that 60% of the electricity put INTO the power-grid at the power plants, is LOST, and that only 40% of the electricity makes it through the power-grid. So it is refreshing to see that IBM has started running TV commercials in Jan 2009 that start off announcing that "more than half" of electricity is lost in the power-grid! Maybe people will be willing to believe IBM about such statements!)
And that's for "low" beams. High beams are much brighter and take more electricity, so the total amount of electrical load is higher and so a little more horsepower must be taken from the engine.
On a somewhat similar subject, have you ever noticed that, when you turn on the air conditioning when the vehicle is idling, the engine revs up? The air conditioner compressor requires as much as 6 engine horsepower in order to create the air conditioning. If the engine stayed at normal idle, it does not create enough horsepower to provide this and the engine would immediately stall. So all vehicles have a provision for the engine to go to a fast idle, to provide the needed extra power. The automobile headlights do not require so much power to stall the engine, but the general concept is similar. Actually, in some cars, at idle, if the headlights are turned on, especially the brights, you can tell the engine speed slightly changes, as the engine speeds up to create the extra needed power.
The trip average was usually very close to 25 MPG. I came to be attentive to it because I figured that if it would be noticeably lower it would possibly mean that the engine needed tuning! I usually made that trip in the morning and early afternoon, during the daylight. But one trip, I drove that section during the night, and the trip average was only slightly over 23 MPG. My first assumption was that the engine needed maintenance. However, in the return trip, it was back at 25 MPG. For the next trip, I intentionally drove through at night, and again the average was only a little above 23 MPG. And I decided to make the return trip at night, and again it was around 23 MPG.
Now, THIS was getting interesting! Why was the gas mileage so much lower at night? I thought there were only four possibilities: (1) different weather conditions; (2) cooler temperature at night; (3) added air resistance (drag) due to the Corvette headlights rising up when the headlights were on; or (4) the added electrical load of this discussion. I eliminated (1) because I made so many such trips and didn't see significant changes due to seasons, and that the weather in Georgia seems to be consistently good. In order to examine (2) I made the trip both ways, with the headlights on, during the daytime. The average reading was slightly above 23 MPG. That seemed to eliminate any effect if cooler air at night or any other nighttime effect. (I had seriously doubted if that was possible anyway, but I felt it necessary to eliminate it.)
That seemed to imply that the headlights were the direct cause. In order to check (3), I turned the headlights on once in the morning, so they popped up, and then I disconnected the motor connector so they could not go back down, and turned the headlights back off. Now I had a situation exactly like normal except that the added air resistance of the headlights sticking up was there. I made four trips (two each way) like that, and found that the average reading was around 24.5 MPG. This seemed to indicate that around 0.5 MPG of the fuel consumption was due to the added turbulence/drag of the headlights sticking up, but not using electricity.
It would have been possible for me to drive that trip during the day, with the headlights on and with the door motors disabled DOWN, where the headlights would have shown down under the car toward the ground. When I first tried that, it looked truly goofy, and I was not willing to drive for six hours like that (in a Corvette!)
So, my personal experiments were such that I found around 1.8 MPG lower average highway gas mileage in that one Corvette when the headlights were on and up, of which around 0.5 MPG appeared to be due to the added air resistance. Therefore, I concluded that the remaining 1.3 MPG reduction must have been due to the added electrical loading on the alternator of having the headlights on. This was based on around 40 trips each direction, 80 trips total, so I felt there was sufficient data for statistical validity.
A Corvette has a more aerodynamic shape than most cars so its frontal drag is less. However, it has extremely wide tires, so the tire resistance drag is higher. The net is that the total vehicle drag is not too different from most passenger cars. So the discussion above regarding about 30 horsepower being necessary for constant highway speed is about right. In my experiments, the 1.3 MPG that I attributed to electrical consumption was 1.3/25, or 5.2%. Five point two percent of 30 horsepower is around 1.5 horsepower. That is in general agreement with the arguments and conclusions above. I feel that it represents decent experimental confirmation of this significant effect.
It is certainly true that my experiments were NOT in a strictly scientific environment, but the fact that around 80 separate trips were involved (over around 8 years) adds some statistical validity. IT is possible that some unknown variable affected the results, for example, maybe an unusual strong headwind existed only on the days of the headlights-on runs. If there was only one or two such runs, that might be a serious concern, but there were so many runs that such a possibility seems unlikely.
Now, this becomes even MORE interesting when we consider the same car going at a constant 40 mph! The power actually needed to overcome total drag (calculated above) is around 6700 watts, while we would still need that 1 horsepower (746 watts) (or more) to create all the electricity for all the vehicle lights. This is now an 11% increase in the engine loading, just to have the lights on! At a constant 40 mph, this suggests that having the lights on reduces gas mileage by about 1/9, or maybe 3 or 4 mpg less! It seems like an argument to NOT have the headlights on when driving during the daytime AT SLOW SPEEDS.
Also consider the other end of the spectrum, the highway semi-trucks. Their engines are constantly providing at least 100 horsepower to maintain their 70 mph highway speed. When THEY have their headlights on during the daytime (a REALLY GOOD IDEA!), the added one or two horsepower hardly has any effect at all on their gas (diesel) mileage, just a small fraction of a mile per gallon.
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C Johnson, Theoretical Physicist, Physics Degree from Univ of Chicago