How much gasoline does headlights use (hours/gal)?

Question

Headlights obviously must use some power from the engine to keep the battery charged. What is the cost? What about large SUV types vs small economy types, assuming the same lights?

Headlights use 75-100 watts at about 15 volts from a running alternator. That power is not free - otherwise you might be able to power the whole city with one auto.

The easiest way to note the extra drag on the engine is to "jump" a friends dead battery.

Answer 1

I agree with arbiter's approach and, mostly, his answer. But I'd use an alternator efficiency more like 50% and therefore get about 75 ml/hour of gasoline. (0.02 US gallons/hour).

For my Prius cruising at 65 mph, getting 48 mpg (1.35 gallons per hour), that would reduce gas mileage by 1.5%. Or from 48.0 mpg to 47.3 mpg. Maybe a touch less because the gasoline engine only comes on during more efficient loads (i.e. it never idles or operates at 5%) so it does a bit better than 30% efficiency.

I know a really meticulous German guy who approaches 70 mpg in a diesel VW by using all the tricks - no lights, no fan use, keeping the junk out of the trunk, shutting down as he approaches stops, high-pressure tires, slow starts, etc. But, man!, it is nerve-racking to drive with someone like that.

For a Hummer getting 14 mpg on the highway (with a tailwind!), it would drop to 13.94 mpg. Pretty dang hard to notice.

Over 100,000 (daytime) miles at an average speed of 30 mph, 0.02 gallons per hour adds up to 67 gallons. For either car.

Of course, one head-on collision by someone who didn't notice your car, and you'd wish you'd spent the $200 on the extra gasoline.

Interestingly, if the Hummer averages 12 mpg over 100,000 miles, and gas is $3/gallon, it will cost $25,000 to keep it fueled up. With which, one could have bought a Prius. Or two Echos (which would seat 10!). Hmmmmm.

Answer 2

You might think it does take more juice to power those headlights, wouldn't you? But it doesn't.

When you drive, the engine is constantly driving the belt to your alternator, whether your headlights are on or not. The alternator has to be going to keep spark to the plugs and keep the battery charged.

There might be some REALLY miniscule reduction from the added draw, but it would be so completely small (maybe like .0000001% or so) that it wouldn't matter.

If you're worried about the cost of using that vehicle, my advice is to drive in the maximum-mileage range (50-55 is considered best on most models of car) and KEEP THOSE TIRES properly inflated. That alone would save a bundle in gasoline costs!

Answer 3

This may help:

"For several years now Canada has required all new cars sold to have daytime running lights. Any time the car is running the headlights are on, but the taillights and other lights are off. You have to turn on these other lights from the dashboard at night. Studies seem to indicate that having the headlights on during daylight hours reduces the number of multiple vehicle accidents (although there has been some controversy about people forgetting to turn on their other lights at night -- a mistake that causes extra accidents, and a good example of the "law of unintended consequences"!).
The US has not adopted this law, but if it did they would definitely consume gasoline. Headlights require power, and a car's engine produces power using gasoline. If you make a few assumptions, it is possible to estimate how much gas the law would consume.

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. The U.S. uses about that much electricity nationwide in 12 hours.

Now we need to figure out how much electrical energy we can get out of a gallon of gas. A gallon of gas contains about 60 kilowatt-hours of chemical energy, but this energy has to go through two conversion processes before we can use it in a light bulb. First the chemical energy must be turned into mechanical power by the engine of the car. Car engines don't do this very efficiently -- only about 25% of the chemical energy can be turned into mechanical power, and the rest is wasted as heat. After the engine gets done with our gallon of gas we have 15 kilowatt-hours left.

Now the alternator on the car has to turn the mechanical power from the engine into electrical power. The alternator does this a lot better than the engine, but it is still only about 70% efficient. In the end we get about 10.5 kilowatt-hours of electrical energy out of a gallon of gas.

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. That's only a couple gallons for each vehicle, but in total it is more than all of the vehicles in the country burn in a day. At $1.50 a gallon, that's $600 million per year. Looking at it another way, an extra 8 billion pounds of Carbon Dioxide would be added to the atmosphere by this law.

It's an interesting question because it shows how a simple idea like, "let's have everyone turn on their headlights all the time" can have a real cost when you try to implement it! Whether the benefit is worth the cost is an important question in almost any public policy decision.

taken from:

http://auto.howstuffworks.com/question424.htm

Answer 4

All of the electrical in a car runs off of the battery when the car is off and the alternator when the car is running. The alternator is alway running when your car is running regardless if your lights are on or off. Headlight do not use any gas. Your aircondition on the other hand is a seperate unit and when you turn it on it takes away power from you engine and causes you to use more gas.

Answer 5

Great question!

Let's say that it's 100 W. Then each hr, you use 100 W*hr. Gasoline's energy density is about 8900 W*hr/liter. If we assume that the engine is 30% efficient and that the alternator is likewise 30% efficient, the net electrial efficiency is 9%. So the gas consumption is then:

100W*hr/((8900W*hr/liter)/9%) ~ 120 mL of gas/hr.

The efficiency is indeed a function of the MPG rating of the vehicle, which is a summary of the size of the engine, the RPM, the transmission, etc.

Answer 6

Are you saying you want to know how much gasoline it takes to keep the lights on?
Or are you saying that you want to know by using the engine to charge the battery then the lights how much gasoline does it take?

Answer 7

Its the friction of the car wheels moving forward that charges the battery.All energy to maintain the lights comes from the battery. Have you ever left the lights on and the battery died. When the battery is drained there is not enough electricity to start the motor. Turning off your lights wont save gas.

Answer 8

Hope you're not planning on a career in engineering or auto mechanics. That has got to be one of the not so bright questions I've seen on here.

Answer 9

HEADLIGHTS HAVE NOTHING TO DO WITH YOUR GASOLINE INTAKE. YOUR HEADLIGHTS WORK FROM THE JUICE OF YOUR BATTERY AND YOUR ALTERNATOR RUNS WITH YOUR BELTS ASSISTANCE WHICH KEEPS YOUR BATTERY CHARGED.

Answer 10

Depends on many factors, are they driving, sitting still, 4 banger compared to a V8. Wild Q, GL with it