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I have read in many sites that a car with lights on will consume more fuel that with lights off. The (hand-waving) justifications that I see tend to mention that "energy has to come from somewhere" and so they deduce that more fuel should be used.

But the way I see it the engine is doing some work on moving the alternator, whether there is an energy need or not. And the mechanical friction caused by the alternator does not depend on whether electricity is being needed or not. In other words, when energy is not needed it is wasted; it is not that more energy is generated when needed.

So, do lights-on increase fuel consumption?

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    Got any references for that claim? My hand-wavy justification is that while yes, more power is being used, the difference in fuel economy should be immeasurably small, and swamped by things like going up a slight incline on part of your drive. – Michael Kohne Apr 26 '14 at 18:09
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    We want to focus our attention on doubtful claims that are widely held or are made by notable people. Please provide some examples of places where this claim is being made. – Jamiec Apr 26 '14 at 18:52
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    I'm seeing phrases like this repeated on many websites NHTSA estimates that only a fraction of a mile per gallon will be lost, depending on the type of system used. General Motors estimates the cost to be about $3 per year for the average driver. (Talking about the impact of Daytime Running Lights) - a good answer to this question will find the original source, not just every website repeating the claim! – Jamiec Apr 26 '14 at 18:59
  • There's an experiment you can do for yourself to demonstrate that you do need to do more mechanical work (and therefore spend more energy) to generate enough electric to power a given load. Hook a hand-cranked dynamo or similar up in series with an incandescent light bulb and feel how much effort is required to turn the crank fast enough to light the bulb. Repeat with 10 bulbs, or 20, or however many you can get. You will find it takes more effort to light 10 bulbs with a hand cranked generator than it does to light 1. – GordonM Jul 9 '18 at 13:49
  • Not flagging to close. Even though this isn't up to current standards for Skeptics.SE, it was asked in 2014, back when our standards were different. Only reason this got bumped back to the top was because of a bad answer recently provided. – DenisS Jul 11 '18 at 14:16
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And the mechanical friction caused by the alternator does not depend on whether electricity is being needed or not.

Correct.

In other words, when energy is not needed it is wasted

Wrong. The electrical energy produced by the alternator is not the result of mechanical friction but of magnetic force on top of the (very small amount of) friction, which the motor has to overcome.

Inside the alternator, wire coils are made to move through magnetic fields. the force they have to overcome is proportional to the current that goes through the wire. If there is no current (i.e. no electrical energy is being used), there is no magnetic force. The higher the current, the more force the motor has to overcome.

As for how much fuel this uses: the California Energy Commission cites a number of studies that claim it's somewhere between 0.5% and 1% - but that seems to to be the increase expected when mandating lights to be on during daytime, which would be on top of existing nighttime use.

The National Highway Traffic Safety Administration (NHTSA) reports that extra fuel consumed with the use of DRLs is “a fraction of a mile per gallon,” while the Insurance Institute for Highway Safety cites a range of $3 to more than $40 per year in extra fuel cost (a consolidation of costs reported by GM and Transport Canada). The Institute for Road Safety Research of Netherlands (SWOV) reports 0.9 percent additional fuel use for DRLs based on European programs that have been in effect for several years. A Swedish study completed in 2002 estimated a fuel economy penalty range of 0.5 to 1.5 percent for various approaches among member nations of the European Union in implementing DRL programs. A recent study in Switzerland under “real world” Swiss driving conditions in European design gasoline passenger cars indicated about 0.8 percent fuel use when adjusting reported results to DRL use conditions.

DRL in the above quote stands for Daytime Running Lamp. There are different types of DREs, with different lighting intensities (and therefore energy use). Assuming that DREs burn with a lower intensity than headlamps (many do), then this sets a lower bound on the cost of running headlights.

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    A reference about the principles of operation of an alternator would improve this answer. (For the paragraph "Inside the alternator...") – user5582 Apr 27 '14 at 1:44
  • @Articuno: I omitted that because the information I found was not very clear, partiallly because there seem to be a number of differen alternator designs in use, and I'm not sure modern ones still use permanent magnets, which the basic explanations alle refer to. – Michael Borgwardt Apr 27 '14 at 13:34

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