tl;dr: Cars get hot in the sun and it takes a lot of energy to cool them off. But most car trips are short, so the cars may not even get cool during that time. If they were parked in the shade, they would cool off faster, saving energy -- but how much?


Cars get very hot in the sun. From the study Evaluating the impact of solar radiation on pediatric heat balance within enclosed, hot vehicles (emphasis added):

In direct solar radiation, a greenhouse situation occurs within the vehicle, trapping longwave radiation and heating the vehicle interior to a steady-state with little-to-no airflow. This situation of radiation trapping highlights the importance of solar exposure in vehicle heating and child vulnerability, even on milder days. In as little as five minutes under an ambient temperature of 30°C [86°F], interior car temperatures can rise to 57–68°C [135-154°F].

Using air conditioning (AC) reduces vehicle efficiency. According to the Department of Energy (emphasis added):

Under very hot conditions, AC use can reduce a conventional vehicle's fuel economy by more than 25%, particularly on short trips. The AC's effect on hybrids, plug-in hybrids, and electric vehicles (EVs) can be even larger on a percentage basis.

Most car trips are short. 85% of all car trips in the U.S. are 15 miles or shorter, according to the Department of Energy:

Share of Vehicle Trips by Trip Distance, 2017


If cars were parked in the shade, how much energy could this save? I'm imagining parking lots at workplaces and shopping centers having canopies installed to shade all vehicles so that they aren't blistering hot at the start of a short trip. If some x% of vehicles in x region were parked in the shade x% of the time, how much gasoline, diesel, or electric energy would this save?

  • 1
    In Australia people are already doing the windscreen shades, I'm not sure I'd say it's rare to see someone not using one but in some places that's definitely true (near beaches especially)
    – Móż
    Commented May 5, 2022 at 1:21
  • 1
    to me the "solar panels over car park" seems easier to get investment for, because there's a definite return to the people doing it. "put up shade for others" ... the carpark is already heavily used, adding shade won't change that. That's a different sense of "not require anything people aren't already doing" though. A quick search says there's multiple businesses in Australia already doing "car park solar farm" so you and me might be a bit slow off the mark on that one.
    – Móż
    Commented May 5, 2022 at 1:23
  • 1
    @Móż I really just want to know what the energy benefits of shading the cars are -- regardless of who or what is providing the shade :)
    – LShaver
    Commented May 5, 2022 at 1:31
  • 1
    Don't do the trip by car in the first place?
    – Erik
    Commented May 6, 2022 at 7:43
  • 1
    I do not know anyone who parks in the sun when shade is available.
    – RedSonja
    Commented May 17, 2022 at 6:32

1 Answer 1


Not very much. The parts of interest to cool are the seats and the air inside the car. If the cabin is three meters long, 1.5 meters wide and 1.5 meters high, then there's 6.75 cubic meters of air. At density 1.2 kg / m3, that's 8.1 kilograms.

Air has 1 kJ/kgC heat capacity. I don't believe that the car gets more than 40 degrees Celsius hotter than the environment. Thus, the cooling needed is 0.324 MJ.

Engines have a marginal efficiency of maybe 35%. Air conditioning compressors generally have an energy efficiency rating of maybe 2.5. Thus, you need 0.324 MJ / 2.5 / 0.35 = 0.370 MJ of fuel to cool down the cabin. Fuel has 34 MJ / liter energy content so that's about 0.01 liters of fuel.

With 0.01 liters of fuel, a car could travel about 160 meters. Compare that to your typical car trip of 10-30 kilometers. That's not even one percent of fuel saving!

Obviously considering the surface layers of the seat insulation too would somewhat increase the mass to be cooled and the needed energy. Note that seats have thermally insulating material so you don't need to cool down the heavy metal seat frames, and not even the whole insulation, but just the very surface of the insulation.

Also consider that if the air is 40 degrees Celsius hotter than outside air, then just blowing in colder outside air without any air conditioning would remove most of the heat. I suspect even when the AC is operating, in the very initial cooling stages, most of the effort is just blowing air in, and not actively cooling it. This would diminish the fuel savings to an even smaller value.

  • 4
    I disagree with this assessment. I get the impression you have never lived in a hot climate. For a car that has been sitting in the sun for several hours with ambient temperatures above 40C every part of the interior will have absorbed heat. Simply cooling the air inside is insufficient. The front of anyone seated may be cooled by AC cooled air from the air vents, but the back, buttocks & thighs will be heated by the heat within the seat. Heat will be radiated from the steering wheel, dash, console, doors & everything else in the car. Opening the windows to let the outside air in to ...
    – Fred
    Commented May 4, 2022 at 0:58
  • ... reduce the temperature inside while driving is no comfort when the ambient temperature is still 40C or above. It's just a hot air blast! Opening all the doors for a prolonged period to remove the ultra hot air inside with ambient air, prior to travel can reduce some of the heat burden, but the interior of the car will be radiating heat for a long time & people's backs will be heated. Most people don't have time to what to replace the interior hot air with ambient outside air prior to travel. In many ways this question is poorly framed. ...
    – Fred
    Commented May 4, 2022 at 1:07
  • ... Driving a car that has ultra hot air in it for a short distance, the AC will do a lot of work, but the occupants of the car may not experience any effective cooling during a short trip. Having left my car out of the sun at shopping centers on blistering hot days, whether it's under an awning/canopy or an underground car park, the difference in temperature of the car interior can be vary significant & it's much more preferable than leaving the car in the sun. Cooling of the car interior is achieved more quickly.
    – Fred
    Commented May 4, 2022 at 1:17

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