Phrased another way: Does the alternator always have the same amount of mechanical drag on the motor, or does it vary with charging requirements of the battery?

Phrased yet another way: If you go camping and plug a 12 volt cooler, a laptop for watching movies, 5 phones for the whole family, etc into the car's battery, is one consequence of this that the alternator will respond by dragging more on the engine. Or, would it not have any effect on fuel economy?

Actually I Google'd this but only found theoretical answers.

  • 2
    The existing answers are good, so this is only a small comment: there is no such thing as free energy. The energy has to come from somewhere, and in a car, that is the chemical energy in the fuel. There is nowhere else the energy could possibly come from. It must come from the fuel. (This is a simplification which ignores hybrids, solar panels, etc.) Commented Nov 9, 2016 at 12:06
  • Why do you consider theoretical answers not good enough? If you want experimental results, then do the experiment yourself and write the results down.
    – Agent_L
    Commented Nov 9, 2016 at 12:29
  • Throwing in some ballpark figures: 1l of gasoline has about 9kwh of energy. Assuming an efficiency in generating electricity of 25% that'd give 2.25kwh of electric energy. Conveniently assuming your devices combined draw 225W continuously you could run them for 10 hours on 1l of extra fuel.
    – JimmyB
    Commented Nov 9, 2016 at 13:36
  • Or, from another view: The maybe 200W of electrical power are insignificant compared to the dozens of kilo Watts of mechanical power your car needs to keep moving. So the extra fuel consumption due to your electronics may be in the order of 1%.
    – JimmyB
    Commented Nov 9, 2016 at 14:34
  • @Jörg W Mittag That is exactly right. I should have phrased my question differently: "does the alternator always have the same drag on the engine or does it vary with electrical load." Commented Nov 11, 2016 at 5:02

4 Answers 4


The alternator's drag on the engine varies with the electrical load on the alternator. So, yes, as you increase the electrical load, the alternator consumes more power from the engine, and the fuel economy drops.

Except for the cooler, your hypothetical load is pretty small, so you might not notice the loss of fuel economy, but it will be there.

You'll also loose fuel economy if you open the windows at speed, let your tire pressure drop, carry around excess stuff in the car, turn on your lights in the rain, etc.


Quite some people wrongly think that car alternators are just like these old dynamo's that drag on the tyre of your (old)bicycle:
bicycle dynamo
The drag that thing has does not vary if you'd switch on- or off the lamp that's connected to it. In the vision of this common misunderstanding you'd throw away the energy of the alternator if you don't use it. It's not like that in cars, and bicycles for that matter. (sorry i'm dutch so bicycles are love, bicycles are life)

You can see a car's alternator just like the engine and the clutch. At each rpm you can vary the pressure on the clutch to send more or less power to the wheels. It's also like that with a alternator. The stronger you make the 'magnet' in the alternator that is driven by the engine, the more drag the alternator has, and the more energy you generate. You can see for yourself; if you turn over the alternator by hand while it's disconnected, there is virtually no drag beside the friction in the bearings.

So yes, more electrical load consumes more fuel. A fully loaded alternator can consume 100 Ampere(boils down to ~1.3kW) if you go crazy with your radio. That's roughly 10% of the total load on the engine at 100km/h. Most people won't go that far though. Your beams, and charging laptop laptop won't make you notice a difference on fuel consumption.

By the way, installing a bigger battery in your car won't have effect for long. If the alternator can't keep up with your current consumption, you'll slowly drain and deplete the battery and kill it. It's only there for you to be able to start your engine. Theoretically ou could disconnect it as soon as it's started.

  • Alternator provides, radio consumes. And I doubt there are any 1kW radios : ) And dynamo's drag does change with the load, but the tire-dynamo interface is so poor that it loses so much energy that the variance from changing load is unnoticeable.
    – Agent_L
    Commented Nov 9, 2016 at 12:33
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    @Agent_L He meant people putting subwoofers in their vehicles. The amplifiers for large stereo systems could definitely use 1kW. Brackets to mount three alternators on the engine are mass produced these days so people can put even bigger systems in. Commented Nov 9, 2016 at 12:55
  • @Agent_L Correct, incorrect, and incorrect. Like Zach said radios can definitely use power in excess of 1kW. Google something like 'super bass radio' and see for yourself. Second, if you try to turn the bicycle dynamo by hand you'll see that there's little difference with either or not a lamp connected. This is because the lamp is a light load for the dynamo.The poor tyre/dynamo interface comes on top of that.
    – Bart
    Commented Nov 9, 2016 at 15:33
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    @Agent_L The mechanical resistance in this case does not vary noticeably. It may vary a tiny bit, but human perception isn't strong enough to notice. You can try it if you have a bicycle, first turn the wheel with the dynamo's wiring disconnected(no load), then with the dynamo short circuited.(highest load) You won't feel the difference because of the poor tyre/dynamo interface and other resistances. The varying mechanical resistance of the dynamo due to load is way smaller than those resistances and thus out of range. But none of this counts for car dynamo's, but that is a common misconce
    – Bart
    Commented Nov 11, 2016 at 12:12
  • 1
    @Agent_L You are right about the reflection of mechanical load on electrical load though; that's the law of conservation of energy. But as i said difference is so small here you won't feel it. Lastly, the power rating on car radios is always higher than what they'll consume on average. I suppose one reason is they talk about peak power(that's higher, so better for marketing), or they just simply lie to the customer, which is not uncommon.
    – Bart
    Commented Nov 11, 2016 at 12:18

As already said, load on the alternator puts load on the motor, and so costs extra fuel. In my car, I can clearly see a drop of about 50RPM when switching on the headlight, the blower (to max) or the rear window heating when the car is idling in neutral.
The drop is compensated by the motor control system within a second by injecting a little more fuel.

Let's have a look at your devices:

  • Cooler - Depending on model, it permanently consumes 5A, maybe 10A@12V, i.e. 60-120W
  • Laptop - maybe 20-25W for watching a video and not charging, but can increase to 100W and more under heavy load plus charging
  • Phone - takes up to 2A@5V, i.e. 10W, but only until the battery is full. Charging takes longer when the phone takes less current, and when it's full, it pulls almost no current.

So let's assume a load of about 200W.

The power needed to maintain speed depends a lot on the car, especially its size and aerodynamics. Just to give some numbers: I found 5'000W to maintain 50km/h (31mph) and 16'000W for 100km/h (61mph) for a streamlined sports car. I guess a reasonable car for camping needs twice the power. For this car, your electronic devices add 4% at 50km/h or 1.25% at 100km/h to the load of the motor, which is not so much.

Or a different approach: If the fully loaded car weights 2'000kg, the energy needed to accelerate it to 50km/h could also power your devices for 16 minutes. And instead of accelerating from 0 to 100km/h, you could power the devices for 64 minutes.

This is only a rough estimate, I didn't put in any losses (the drive train steals up to 20% of the motor power!) or any fuel economy.

tl;dr: Running all this devices will put additional load on the motor and increase fuel consumption, but compared to the power needed to drive the car, this will be hardly noticeable.

  • Not that I disagree with the overall sentiment, but ~100W for a running a car AC unit sounds quite low; do you have a citation for that?
    – user
    Commented Nov 9, 2016 at 11:57
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    @MichaelKjörling "Cooler" means "portable fridge", not car AC. Car AC usually is non-electric load anyway.
    – Agent_L
    Commented Nov 9, 2016 at 12:31
  • 1
    @MichaelKjörling I'm not speaking about a car AC. That takes some 1000W. But "plugging in a 12V cooler" sounds more like a cooling box, and they draw about 5-10A.
    – sweber
    Commented Nov 9, 2016 at 12:31
  • Ah, well, that makes more sense, then. // cc @Agent_L
    – user
    Commented Nov 9, 2016 at 12:35
  • Sweber, that's a great response. I found these kinds of answers through Google - the only thing was missing from this theoretical analysis was how the alternator interacts with the engine, which you addressed for a moment when noting increased electrical loads drops engine RPM by 50 for a moment. Commented Nov 11, 2016 at 5:50

Does the alternator always have the same amount of mechanical drag on the motor, or does it vary with charging requirements of the battery?

Varies. Whenever it's an alternator, a hand-wound flashlight or the megawatts-sized generator they have in power plants, the amount of electric energy drawn from the output is taken from the input shaft (actually about 2-10% more, to cover the loses of conversion). When you disconnect every load from an alternator, the amount of mechanical energy drawn from the engine drops to nearly 0 (nearly, because some loses are constant, like those of the driving belt, bearings, etc).

If you go camping and plug a 12 volt cooler, a laptop for watching movies, 5 phones for the whole family, etc into the car's battery, is one consequence of this that the alternator will respond by dragging more on the engine.

Yes, exactly. Discarding the loses, when you connect 100W electric cooler, that's 100W less at the wheels. (If we assume that efficiency of the alternator is same as efficiency of the entire power train.)

Or, would it not have any effect on fuel economy?

It will have effect on fuel economy, but it can be easily hidden. Even load as large as 100W is not very noticeable when compared to even moderate engine that can pump out 40-80kW. 100W taken from the lighter socket that's roughly 100W less at the wheels. When you're driving, your speed will drop slightly. Now 2 things can happen: either you won't notice, so the total fuel economy won't be changed at the cost of slower driving. Or you (or cruise control) will notice, and will press the accelerator harder so the speed will be maintained at the cost of increased fuel consumption. And when you consider that your car AC is another mechanical load on the engine, that consumes power in range of several kW, few hundred watts can easily pass unnoticed.

It's a really simple math: as the energy conservation principle says: energy can't be either created nor destroyed. Both sides must be equal, so EnginePower = DrivingPower + AlternatorPower (+ AirConditioningPower).

Now, could an alternator that always draws same power even exist? Sure, it could. But that energy, if not taken away in form of electricity would have to go somewhere, which usually means wasted as heat. So our hypothetical device would heat up like hell without load and run considerably cooler when loaded to the max. A 1kW alternator doesn't heat up with the power of a 1kW space heater when idling, which shows that it doesn't waste that much power. And when the energy is neither transmitted away (eg. in form of electricity) nor wasted in the form of heat it can only mean that it was never there in the first place.

  • But it spins at the speed of the engine anyway. Regardless of whether its under load or not. So it uses more energy, but only at low revs via the engine management system? At high revs the management system wouldn't need to increase revs. Therefore its not strictly energy for energy, Because of the constant alternator drag. electricity generated at higher revs does not increase fuel consumption. unless i'm missing something?
    – Mark
    Commented Jun 13, 2022 at 22:41
  • Yes, you are missing something. Extra current = extra load = extra resistance to rotation of the armature = extra fuel.
    – Chenmunka
    Commented Jun 14, 2022 at 8:32
  • @Mark The alternator drag is never constant, it varies according to the electrical load. And the speed of the engine doesn't matter, 100W @ 1000rpm is still 100W @ 5000rpm.
    – Agent_L
    Commented Jun 14, 2022 at 11:06

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