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How much engine power a car’s aircon uses

A car’s air conditioner (AC) adds load to the engine when it is turned on because it operates through the alternator and a compressor that both draw power from the motor, consequently, it also uses fuel.

Therefore, the cooling system consumes anywhere between 1-3kW of engine power when it is turned on, depending on the type of car, according to SVI Engineering’s Nicol Louw.

This is one of the reasons why you can feel a more tangible dip in the performance of low-output vehicles in comparison to high-output vehicles when you press the AC button.

Aircon vs Open Window

Many motorists believe that an open window will always be more efficient than an AC; while this is true in some cases, it is not as black-and-white as one might think.

Depending on the speed you drive, the aerodynamic drag created by a window can easily put more pressure on the engine and in turn burn more fuel than what an AC would have at the same speed.

In a CapeTalk interview, Louw explained that drag is a quadratic equation and therefore increases exponentially the faster you are travelling.

“If you are driving with a standard type of vehicle at 100km/h, to overcome the aerodynamic drag at that point on a level road, you are looking at about 20-30kW of power that you need from the engine to overcome drag,” said Louw.

While the exact number is determined by various factors – such as vehicle’s size and shape, its age, ambient temperatures, the desired cabin temperature, the incline of the road – it’s clear that the maximum of 3kW an AC needs is far lower than the minimum of 20kW for an open window at 100km/h.

It works the other way around, too.

If you are driving slowly in stop-start traffic or on suburban streets, there can be little to no adverse impact on the powerplant if the window is down, which would make it the more frugal choice over an AC.

As a general rule of thumb, though, an AC will be more economical than an open window above approximately 60km/h for the majority of vehicles.

Fuel savings aren’t the only benefits, noted Louw, as an AC also dehumidifies the air inside the cabin which allows your body to cool down faster than it would with humid air flowing through the side openings.


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