Air conditioning systems require a great deal of power to cool your car. The higher you have the air conditioning set and the hotter it is outside, the more the system needs to work to provide you with cool air. Since the only place that power can come from is the engine, you may notice a drop in your car's performance as a result of running the air conditioning.
An air conditioner requires differing amounts of horsepower depending on the size of the car and the size of the air conditioner's compressor. However, when the compressor actually is active, it could require as much as 5 or more of the horsepower available from the engine. In a large vehicle that produces hundreds of horsepower, this is unlikely to present a noticeable loss. In smaller cars, the loss of 5 horsepower may be quite noticeable.
In some of the smallest, most compact cars available, the loss of the power required to run an air conditioner is so significant that a larger engine is required to compensate. Early 1990s Suzuki Swifts were available with either a three-cylinder or four-cylinder engine. Air conditioning wasn't available on the three-cylinder engine model because the tiny 53-horsepower engine couldn't spare the power required to run the air conditioner.
To understand exactly why you're losing power at certain times, you need to know how the air conditioner system works. The compressor is the part of the system that has a belt directly connected to the engine. This is the primary pump that drives the movement of the refrigerant when it is in vapor form. Whenever the compressor is on, you're losing power from the engine because energy is being used to drive the belt that powers the compressor.
A large amount of power is needed to run the air conditioning system in your car, so it has an adverse effect on the car's fuel consumption. A report by the National Renewable Energy Laboratory states that it takes as much energy to run an air conditioner with a 400-W load as it does to drive a mid-size car at 35 mph. This means a loss of as much as 1 mpg in fuel economy while the air conditioner is running.
