This might be a silly question but I was wondering if a car AC unit be run off entirely on battery or alternator?

As this could help in

  1. Preventing power loss during important times such as overtaking and climbing on slopes.
  2. Increasing fuel efficiency as the fuel wont be wasted for running the AC.

Another reason that I could think of is for those people who might want to take a short nap and want to use AC without the car running.

  • 4
    Are you asking if this can be done on a normal car or if there's such a thing as an electric compressor for a car?
    – JPhi1618
    Apr 22, 2016 at 17:20
  • Kind of the latter.I want to know if the compressor load could be taken away from the engine
    – saurabh64
    Apr 22, 2016 at 17:22
  • How would you handle the air flow/circulation? Apr 22, 2016 at 17:30
  • Sure, just find a space to add an electric motor in the engine compartment, re-jigger the belts so that the electric motor drives the compressor, and make it so it starts when the compressor clutch would have engaged. Also beef up the electrical system enough to be able to run said motor. Easy peasy.
    – hobbs
    Apr 22, 2016 at 19:19
  • I know on my car there's a "full-throttle" switch that will disable the A/C compressor when I floor it, it's possible other cars have this too.
    – TMN
    Jul 7, 2016 at 16:58

7 Answers 7


I can't give you numbers or calculations without some work, but I can tell you than energy is never free. Cars have an A/C compressor that is mechanically driven by the engine because this is the easiest way to get the job done in a typical consumer car.

An A/C compressor actually takes a huge amount of energy to operate. In fact a central A/C unit for a house uses more electricity than anything else (in a hot climate). If you want to electrically power the A/C that power needs to come from somewhere.

If you wanted to generate enough power to drive an electric compressor, the required alternator would take even more engine power than the mechanical compressor because of inefficiencies and losses. If you wanted to use battery power, you would need very large batteries and a more powerful charging system.

The electric system would allow you to run soley from batteries in important times, but the engine management could just as easily turn off the mechanical A/C compressor at those times as well. Normally people won't melt if they have to do without it for the 20 seconds it takes to pass someone on the highway.

  • 1
    Currently, you take kinetic energy from the engine, and transfer it to kinetic energy at the compressor. To run it from an electric motor, you would need to do kinetic at the engine, to electric at the alternator, then back to kinetic at the motor for the compressor. Each conversion has losses. This means the alternator would have to work harder to make up for those losses and take more power from the engine.
    – rpmerf
    Apr 22, 2016 at 17:51
  • Interestingly, my power bills tend to be significantly higher in the winter (when I'm running the electric heaters) than in the summer (when I'm running the AC). Apr 22, 2016 at 21:34
  • 1
    @MasonWheeler, ok, yea, electric heat is the biggest power user, but in my mind I lump that in with the HVAC system. Electric heat can use 100A all by itself - more than everything else combined.
    – JPhi1618
    Apr 22, 2016 at 21:39
  • 1
    You are missing one very important factor, anything that is directly driven from the engine needs to have the gearing design so it gets enough power at low engine revs, therefore at normal or high revs the compressor gets more power than is needed. Hence an electric compressor can give better MPG. Likewise with brake pump etc. Apr 23, 2016 at 12:03
  • @IanRingrose Good point there!
    – saurabh64
    Apr 24, 2016 at 11:47

Well, Teslas (and presumably other all-electric cars) have A/C so it's not impossible, but A/C takes a fair amount of power.

On the other hand, Teslas store a lot of energy. The A/C is something like 2.4kW which is about 8,000 BTU/h or about 3HP. So using an electric motor on a conventional car might give you a few more HP briefly, but the alternator has to eventually make up the deficit, and a bit more for its own inefficiencies.

An easier approach would be to drop the A/C clutch out when you tromp on the accelerator. That might allow you to use a smaller engine for the same performance (passing especially) with a simple software change. Probably cars already do that. The A/C would run a bit more after you do that, to keep the cabin air temperature down. No free lunch.

You could always do it with a window air conditioner and a huge inverter, but the efficiency would be pretty bad I bet. The aerodynamics in the below example are also a bit dubious. In a pickup it might actually make sense=- put it in the bed- since a cheap Chinese window A/C is almost free.

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  • If my memory serves correctly.. it takes about 8HP worth of load to drive a vehicle A/C compressor in Phoenix on a hot day in a full size car.
    – zipzit
    Apr 23, 2016 at 6:31
  • 1
    I know the ECU for the early late 80s/90sDodge 2.2/2.5/3.0 will disengage the AC clutch at WOT. Newer vehicles likely use load.
    – rpmerf
    Apr 25, 2016 at 11:17

The short answer to your question is no. As @JPhi1618 noted, the compressor is mechanically driven. Without the compressor you don't have any cooling. The AC runs by changing the state and pressure of a liquid, and compressing the liquid is a big part of that cycle. If you bolted an electric compressor onto the car you would have to find a way to switch between the mechanical and the electric compressor, and that no one makes. You would have to design and machine it yourself.

Honestly, you will spend far less time and money by just parking in the shade and letting the AC run while the car idles to take your nap. (How do I know this, lol) Not a perfect solution, but much cheaper and easier than trying to compress the AC coolant electrically.

Hope that helps!

  • I wonder how AC works in Tesla cars as they are completely electric cars.
    – saurabh64
    Apr 22, 2016 at 17:56
  • 1
    As they're designed for it, I would bet that they have an electric compressor designed for this. Not hard to do, there's one in every residential AC unit there is. Just easier when it's designed for it. And they only have the electrical compressor, not both. Not like an electric car has an accessory belt...
    – cdunn
    Apr 22, 2016 at 18:12
  • 2
    @saurabh64 Tesla, electric cars and many hybrids use, high voltage, electrically driven compressors. Apr 22, 2016 at 19:33
  • 1
    The high voltage part of that makes a lot of sense. A 12V compressor would be unnecessarily large
    – cdunn
    Apr 22, 2016 at 19:47

Yes, this is possible. Several companies (VAG, BMW) have switched to AC units that are driven by electric motors. For BMW, this is part of their Efficient Dynamics strategy, where they'll switch the compressor on/off depending on engine load to optimize fuel consumption.

When the weather's not too hot, you can run the compressor intermittently without the passengers noticing anything, so they run the compressor when you're slowing down and switch it off when you're accelerating. Other companies use similar strategies, BMW is just the first I could find that offered some explanation of why they switched to electric AC.

Electric AC has another advantage. On smaller engines, you can feel when an engine-driven compressor engages. That hiccup in the drivetrain can get annoying. An electric AC is unnoticable.

Running the compressor while the engine is off is another matter. The compressor draws several kW. At 5 kW you'd drain a (really big) 100 Ah battery in 12 minutes.

I don't know how feasible it is to retrofit an existing AC system to electric.

  • Nice explanation. +1 Apr 23, 2016 at 8:33
  • Also an directly drive compressor has to be geared for low engine revs, hence waists power at high revs. An compress driven by an electric motor only takes the power it needs. Apr 23, 2016 at 12:07
  • Also, Toyota hybrids have an electric AC and I suspect it runs on the high-voltage 1-2 kWh battery and not on the low-voltage 12V system. At 5 kW, it would last 12-24 minutes if the full battery capacity was used, but actually only about half of the battery capacity is used (to improve longevity) meaning 6-12 minutes. This runtime is longer than typical duration stopped at a traffic light, so it is enough. Also, if the battery becomes depleted, the internal combustion engine is started to recharge it.
    – juhist
    Mar 18, 2017 at 8:48

There is NO free lunch, you will need many batteries and a much better alternator.

You could in fact attach a window mounted air conditioner like @Spehro Pefhany suggests.


TEC Thermo Electric Cooling

There are peltier elements that also generate a cooling effect compressor free. They are essentially a plate of ceramic postively charged, and one negatively charged. This creates a heat flow where one side is incredible hot and the conversely the other side is cold.

So you could mount these either inside your door, or perhaps behind the backseat.

Either way you would need to transfer the resulting heat outside of the cabin. If you don't mind an inferno in your trunk, behind the back seat. You could attach the cold side to the back seat and put giant set of heat sink fins in the trunk, with 12v fan to keep it cool.

If you were mechanically inclined you could also use water cooling. Install a second radiator under the hood and 12v pump and fan. Then the heat would get exhausted that way. If you don't have enough cooling you will break your peltier element.


here is an example: http://www.amazon.com/TEC1-12706-Thermoelectric-Peltier-Cooler-Volt/dp/B002UQQ3Q2

Larger ones exist. However, if you purchased a bunch, say 20 you could have a setup where you control how cool it gets by turning some off. I have no idea how many you would need, but give these are 1.5 inches many would be needed.

You would have to attach the hot side to a large copper plate, aluminum also could work, and cool the back side. The down side to aluminum is it can't store any heat, and having used these to cool CPU on a computer I have come to the conclusion copper is better. On the cold side copper or aluminum would work, but make sure the 2 sides don't touch. The peltiers are less than 4mm in many cases so you will need something thin to keep them apart, depending on the total size of your device. Also a thermal paste will increase conductivity and heat/cold transference.

You will then need a large array of heat sink fins and several 12v fan to cool the back side. Either that or a water cooling loop.

The good news is all of this can be done with 12v-15v. The bad news is even 1 peltier element requires 7 amps at 12v. The fans and pump (if you go that way) will use very little compared to this. You can buy 12v 120mm computer fans cheap that use very little power. Even 10 of these is probably 90A with the pumps and fans. Your battery will die quickly.

In electricity, the smaller the gauge, the thicker the wire.

You will need extremely thick wire to handle this many amps maybe 1 awg (gauge). For simplicity you can connect up to 20A on 12 gauge wire so 3 of the listed peltiers. Then you will have to connect those wire bundles to even thicker wires and so on. Finally to a car battery, hopefully a 2nd car battery. In fact you will need many car batteries to keep this array powered for any length of time.

  • 1
    Peltier elements are really inefficient. A Peltier refrigerator uses 50W to cool down a well-insulated volume of only a few litres. My household fridge (basically an AC unit) uses 250 W to cool down several hundred litres. Extrapolate: a car needs 5 kW of aircon to cool it down, Peltier is 1/100 as efficient as AC, you need hundreds of KW of Peltier elements to keep your car cool.
    – Hobbes
    Jul 7, 2016 at 9:31
  • @Hobbes I agree with you about the inefficiency, but the person didn't say it had to be efficient.
    – cybernard
    Jul 7, 2016 at 11:48
  • yes he did: "avoid wasting fuel". Ever tried running a 500 kW generator? It's not exactly frugal.
    – Hobbes
    Jul 7, 2016 at 12:11

I have seen vehicles with multiple alternators, such as BASS competitions. I can hook up 2 alternators to my car but have seen 10 to 15 alternators hooked up to the subwoofer competition trucks. There would have to be enough power to run a small 5kbtu rv ac unit through your sunroof (with a custom painted fiberglass roof scoop to cover the whole unit ofcourse, allowing adequate air flow).

2 105 amp alternators, big high amp inverter, custom capacitor setup for the surges?

Would be nice for turbo cars that have to run a big front mount intercooler. I would sacrifice 50 to 75 pounds no problem for ice cold ac in Phoenix.


The key here is where does the power come from... If you use the vehicle battery then the charge come from the alternator and therefore the engine - btw the alternator can take approx 10bhp or more if producing full charge.

The possibility of a second battery with a split charge system still requires the energy to come from the alternator and engine.

A second alternator to charge the second battery is still taking power from the engine and, now the engine has more rotating weight to drive.

So, to remove the load of your AC from the engine, either direct or indirect, needs a separate power source - you could consider a bank of batteries that you charge at home and give sufficient power for your trip, but the weight will not improve the fuel consumption and you need to have the space available.

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