Car lights; air conditioning etc

[Segilla]

I'm told that the more these are used, the more fuel the engine uses. Can someone please explain this.

[homer75]

When you use air con your engine has to drive a compressor pump which means more load on the engine - with lights etc more effort is required to spin the alternator which supplies the electrical power for these things. More load = more fuel.
Hope this helps.

[Toureman]

As homer explains so well, nothing in this life is free!

[tali122]

air con will use significantly far more fuel than any other of the loads

[ugly_bob]

although, since the alternator is constantly charging the battery anyway, using your lights/other electrics won't make a difference. The air-con, however, will.

[Loosehead]

Lights do make a difference, unless you've found a way of getting light energy for nothing Bob.

Even the makeup mirror can be converted to a fuel cost segilla, so assuming you are a woman, stop doing your face at traffic lights! and if you are a bloke..... err same!

[ugly_bob]

what i'm saying loosehead is that the alternator is constantly charging the battery anyway. Using lights uses some of the energy from the battery that is replaced by the alternator. Usually this energy is just wasted.

[Jay70]

Ugly Bob - I think you'll find the lights or other equipment will get power from the altenator primarily when the engine is running. This is why if you only do short trips in the winter with lights, fan, heated rear window on the altenator can't put back into the battery the charge taken out of it that was required for starting the car. If these short trips are frequent it's not long before you get a flat battery even with a good battery.

[Toureman]

It takes several miles to replace the charge in the battery that is used by starter motor just to start the car.

The alternater does not charge the battery constantly, only when needed, the energy is not wasted.

[Melliexr]

The alternator is constantly under load and producing its maximum amount of current at anything above about 2000rpm. What the regulator/rectifier does with the charge is pure electronics and has no effect on the strain on the engine. No amount of 100W spotlights will have any effect on your fuel consumption.
Air-con will affect your mpg, but with modern systems it's not much. Driving with windows open will actually affect your mpg more than air-con, due to less efficient aerodynamics.

[Catso]

Melliexr, you're wrong! Once the battery is charged, and with no electrics turned on, the alternator is effectively doing nothing. To demonstrate: after a run (to ensure battery is charged) leave car on tickover, listen to engine, turn heated rear window on and you'll hear the change in the engine note as the alternator takes up the load. Can in my car, anyway.

So half-a-dozen 100w spots will certainly increase your fuel consumption.

[Jay70]

Melliexr - the output side of an altenator i.e the rectification circuit is pure electronics but the input to the rectification circuit involves the conversion of mechanical energy to electrical energy via a magnetic field inducing current in a conductor. If more current is draned from this side of the circuit then magnetic fields increase which gives an increase in rotational resistance withing the alternator. This increase in resistance has to be overcome by the car's engine which means more load which equals higher fuel consumption. This may not be as high as the load produced by having air con operating but it is indeed present and does increase with increasing electrical load such as lights / heated rear window etc.