# Power use and alternator output

I’m trying to get my head around the issue of “does running my headlights use more gas or cost more money” I believe there are conditions where using my DRL’s is tapping into free energy when the engine is running at a sufficient rate of RPMs and in high gear. I’m trying to figure at what point this happens and how much using other devices such as air conditioner or seat heater impacts the demand on the alternator. My most specific question is how much wattage does using my air condition use (range)? I have a Chevy trailblazer 2007.

1. 1 HP=746watts.

2. A reasonable assumption is that an automotive alternator operates at 50% efficiency.

3. A decent engine can output 15hp, for one hour, on one gallon of fuel.

So, to find the fuel cost of an electrical drain, calculate the wattage, divide by 373, then divide by 15 to get the gallons per hour of use.

An air conditioner compressor is not electric, except for the clutch and fan. It is a piston driven by the engine. I think you should just be happy.

1. The air conditioner compressor uses far more energy (several HP) than the blower that circulates the cool air, so the alternator doesn’t count for much in that situation.

2. There is no “free energy”. Any electrical load causes more drag from the alternator no matter the gear or the speed, but sometimes it can be a tiny compared to other loads on the engine.

3. The drivers right foot has much more effect on MPG than the DRL’s or seat heaters.

@Slangevar I just noticed you’re driving a trailblazer. That’s not a small vehicle. If you’re concerned about fuel economy and energy usage, I believe it makes more sense to get a newer and smaller vehicle. Perhaps a hybrid SUV?

Every single bit of energy you use in your vehicle originates in the gasoline. There is no other source.

The alternator does not generate excess current that gets set aside. The amount it generates depends on the amount of current being demanded by the vehicle. The more current being demanded, the greater the field strength in the windings and the more energy it takes to turn the armature.

Think of it this way. All the stuff you have running in the car is draining electricity from the battery. The alternator is keeping the battery full. The faster you drain electricity from the battery, the harder the alternator has to work to keep it charged. Since all the power to operate the alternator is originating from burning the gasoline, that means more gasoline is used.

There is no free energy, but the power drain on the engine for the lights is minimal. If you turned off the drl, and compared gas mileage if you got 1 of a mile per gallon increase I would be surprised. From Wiki
Fuel consumption reductions of up to 0.5 mpg may be found when comparing a 55 W DRL system to a 200 W DRL system.
You can turn them off if you want to save that .5 possible mpg.
Interesting side note ror my 03 trailblazer, to save some bucks when I needed new tires I went with long trail, bf goodrich I think, from Michelin cross terrain. I had an instant drop in hwy from 23 to 21 mpg, and city 18 to 16. I could have saved money in the long run with the Michelin, an easier way to improve gas mileage.

When the headlights are on, more force is needed to turn the alternator pulley. You’d only get this for “free” if there were some design quirk in the engine where it ran correspondly more efficiently under the somewhat higher load from the extra turning force produced by the alternator when the headlights are on .

If you want to get better gas mileage, fill the tires to about 2 PSI over the door rating when they are cold. Also, keep up with standard maintenance like oil changes, air and fluid filter changes, plugs and wires.

@Barkydog I’ve heard that about Michelins before. They’re more expensive initially, but in the long run they make financial sense.

Pretend there is a light-bulb between your right foot and the gas and brake pedals…That’s where better mileage can be found…

I’m not trying to save money or gas driving my trail blazer, I’m actually just pick out some knowledge to try and convince someone I know to drive with there headlights on so other people can see their asphalt gray car coming in poor visibility. They have Know-it-all syndrome so I’m looking for lots of factoids and trivia about efficient driving practices and to point out he is penny-wise and pound foolish.

@Slangevar I agree that driving with headlights or DRLs on is a good thing.

Back up the clock a couple or three decades and both Federal and state laws came into being requiring motorcycle headlights to remain on all of the time. This was done to make the cyclist more visible in traffic although many bikers may question just how well that works.

My personal experience is it doesn’t work well at all. It only means the lights are burning in a near death situation but that’s the theory behind it anyway…

OK a headlight on is better, sure there are the blue hairs and others that think the light is red, so I have to go faster, there is not a pill to solve ignorance, until then we are all in danger, so look to the left look to the right, repeat then pull forward, unless like me you have to do a wheelie in a triumph 650 that is near impossible because you see a car coming up in your rear view mirror that will make wallpaper paste out of you unless you move fast.

It may be true that all energy not going directly for propulsion still comes from the gas. But, I won’t belittle the efficiency of working through it when mechanical and hydraulic actuated components are turning to electric driven motors. Shifting, steering, etc. are all becoming dependent on the battery/alternator power source. Electric motors are still the most efficient…and I for one will happily trade a couple of miles per gallon for electrically power luxury features instead of much more for other sources.

Pretending there’s a light bulb on the accelerator pedal may help but filling a coffee cup to within 1/4 of full and placing it on the dash to splash out when any sudden change in motion occurs can bring into view all the “errors” made when driving. Hard braking, hard cornering and hard acceleration add up to poor mileage and other expenses also. If a vehicle is in proper order, the driver is the most significant determinant of fuel mileage, brake and tire wear. Watching a fleet of fixed route vans operating for many years with several drivers that remained long enough to have driven different trucks made it clear that the driver’s attitude behind the wheel could make or break the company in many ways, least of which was not the cost of operating the vehicles.

Rod, according to my Grandfather, that is similar to how Roll Royce trained their chauffeurs. They would put wine glass almost full on the dash and the driver in training had to drive around town without spilling any wine.

They were taught to anticipate everything, including the timing of stop lights so that they would vary their speed as little as possible.

I can’t cite any literature on how well lights help you to be seen, but I drive a lot, sometimes in a professional capacity with a 30,000 lb. school bus. One of my MAJOR pet peeves is people driving “asphalt gray” cars on a gray, rainy day with no headlights or DRLs. It really is very difficult to see them coming at times. Despite laws requiring lights when raining, 95% of people can’t seem to be convinced to turn the damn lights on!

I agree, doubleclutch. Lights may use a tad bit of fuel (so small as to be unleasurable), but safety comes first. I’ve often wondered why they don;t just connect the lighting circuit to the ignition key cylinder so the lights are on whenever the car is on.