I understand that the A/C compressor runs with the engine belt, so it will always uses some HP when engaged, therefore it will have some effect on fuel consumption.

What I want to know if there are other convenience features that can affect fuel economy. I'm interested particularly in this list of features because this is what my car has:

  • Radio/music
  • heated seats
  • heated wind shields
  • heated mirrors
  • warm air from the vents
  • Cabin fan speed

I think all of this components runs on electricity, and since (I think) the battery is always being recharged every time the engine is running, I don't believe they have an effect on fuel consumption, but I wanted to be sure anyway.


1 Answer 1


Short answer, ALL of the convienences mentioned above reduce the fuel economy of a car, any electircal system you use will affect the fuel economy,to understand why we must know how these systems are powerd exactly, its not as as you think.

The battery operates when you start the car, the moment the car is started the , the power to run the electric appliances and to charge the battery on the car is generated from a device called the "Alternator"

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The black COG is connected to the crankshaft of the engine with a belt just like the compressor of the air conditioner, and runs off of the engine power, think of it as a electric generator for the car.

Now this produces AC current which is sent through a Rectifier to be converted into DC voltage and that is sent to the battery to be recharged, most of the electrical things on your car run of the battery where the alternator provides additional amps to the system while charging.

The the amount of voltage produced by the Alternator is proportional to the engine RPM and the Alternator is at best 50 to 60% efficient meaning it only converts 50% of the mechanical energy into electrical energy.

SO when you are in low speed running all those electrical you are straining the Alternator and making it to rob the engine of additional HP thus ultimately reducing fuel economy, once you reach considerable amount of speed then the effect is reduced.

All said, the fuel economy being affected by these electrical are actually not very noticeable in everyday driving in comparison to say an Air-Con or driving rashly which will produce noticeable drop on economy, the only place where this knowledge will be useful is either if your running a race or about to go empty and searching for a petrol station.

  • 1
    You explanation of a vehicle charging system is not quite correct. The battery supplies the power to the pieces and parts, while the alternator provides the juice to fill the battery back up. They work together to provide the power to the car. If it wasn't this way, your vehicle would immediately die when the alternator dies. Also, because of this, nothing gets separated out and ran differently. The headlights, just like everything else, gets run the same way, from the battery. Your explanation of power consumption and fuel economy are correct, though. Jun 28, 2015 at 20:21
  • At least in my car, I can use everything (including heating) without having to turn on the engine, and that's why I think the run using the battery alone. Therefore, now I don't think they have an effect on fuel-consumption, right? Jun 28, 2015 at 21:57
  • 1
    @Mandx You technically can do that but it will drain the battery faster , mate the F.E is not that much affected due to electrical, as i said if you are about to go empty and need to squeeze most of the mpg from your car you can turn off all the electrical but other than that you can pretty much use em, apart from the Air-Con.
    – Shobin P
    Jun 29, 2015 at 3:27
  • @Paulster2 That is not really the case, in general the battery does not drive anything while the engine is operating. It does provide a little bit of back fill energy if the alternator is not producing enough, like if there is a high electrical load while the engine is at idle. Think about how starting a car with a dead battery feels, if you keep the RPMs up everything works fine. If you let them drop too much before the battery get recharged the engine will die. Or, think about the wiring, the positive terminal connect both the alternator and the rest of the car skipping a battery path.
    – Ukko
    Jun 29, 2015 at 19:32

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