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As we all know, in hot climates part of the engine's output goes to power the AC compressor. Now, in such a climate, if your car is painted black, it should absorb more sunlight, and therefore, require more power to run the AC compressor. So, my question is: does a black car have a markedly worse fuel consumption, assuming the car has AC?

A similar effect could in theory be present during the winter: when heating is required, the black car radiates more of its cabin heat outwards, and thus, requires more heating. However, my understanding is that heat is plentiful even given the most efficient engine available, and therefore, this effect of requiring more heat should not cause worse fuel consumption like AC does.

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Let's assume the sun is at the zenith.

car width: 1.8 m
cabin length: 4.0 m
cabin area: 7.2 m2
sunlight specific power: 1000 W / m2
sunlight total power: 7200 W

On one hand, the COP of the compressor is probably around 4, meaning 4 watts of cooling power require 1 watt of mechanical power, but on the other hand, the engine's efficiency is about 25%, meaning 1 watt of mechanical power requires 4 watts of fuel burning. So we are back to square one, and require 7.2 kW of additional thermal power to run the AC.

On a typical mixed city/highway trip, the average speed is 30 km/h, taking into account the time spent at stoplights. At 7 liters per 100 km consumption without AC, this is 2.1 liters per hour. One liter contains about 32 MJ of energy, and therefore, thermal power is 18.7 kW. The AC requires 7.2 kW more. This means 38.5% worse fuel consumption due to AC if the car is totally black, when compared with a totally reflective car with mirror surfaces.

On short trips, the black car can be even more worse, due to the fact that the cabin air is already very hot. However, on long trips, you probably spend more time on the highways, meaning average speed is higher, and thus, the problem of additional fuel consumption caused is lower.

Of course, it may be possible that the car's AC system hasn't been rated to operate in the worst possible condition, sun at the zenith. So, I'm not claiming that when the sun is not in the zenith, the AC would cause 38.5% additional consumption always. And when the sun is at the zenith, if the AC isn't rated for this job, it will be a hot cabin and less consumption increase.

I can only conclude: don't buy a black car!

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    The big thing you have neglected is air convection which will carry away pretty much all of that heat if you are moving at any significant speed. If you are talking about sitting in a parked car with the AC running there may be a difference. – agentp Sep 14 '17 at 20:54
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    Agreed. All of the cooling effects are neglected in this backofenvelope calculation as well as not accounting for the fact that a white painted car would still absorb some of the energy of the sun like the black car will. So the actual % difference in energy would be a lot smaller. – agent provocateur Sep 14 '17 at 22:25
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    A couple other big factors are that the car is insulated so most of the heat radiates out and not into the car. And also most of the heat radiating onto the car is coming in through the windows and heating up internal surfaces. The color of a car has something of course to do with the amount of AC needed, but it is really not much. – Jon Sep 15 '17 at 1:26
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    indeed, the color of the interior is far more significant – agentp Sep 15 '17 at 11:18
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    The only thing I can conclude here is that engine load increases when the air-con is running. How did you determine that car color is to blame? – Zaid Sep 15 '17 at 17:40
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If your car has automatic climate control, then yes. A darker car will use more fuel. Heating is essentially free though because it's just an electric fan blowing over your radiator core.

  • Actually, most cars use a small auxiliary radiator for heating, but yes, it's still essentially free. – Mark Oct 11 '18 at 2:36
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AC compressors are either on or off. The energy draw is exactly the same, regardless of paint color.

Now, a black interior versus a light-colored interior will make a small difference, as the interior of the car will be warmer, and thus cause the AC to run longer. But the difference is so small that you won't notice it.

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    Not all AC compressors are either on or off. Some compressors adjust refrigerant flow depending upon cooling demand, i.e. how hot it's outside and how cold it's inside, and, often, also how much cooling you're actually asking for. – Al_ Oct 3 at 10:14
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    I did not know that. Thanks Al. Is this found more on luxury cars, or is it common on new vehicles? – Spivonious Oct 7 at 17:00
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    It's pretty common on all new vehicles, and almost all luxury cars now have them (one example might be all Porsche Cayennes). My car is a 2003 low end Vauxhall and it has such a compressor (albeit it will just keep the evaporator a little above freezing point and nothing more, since it is a mechanical valve, and not a solenoid valve acted upon by a computer). – Al_ Oct 8 at 11:13

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