Car AC, unfortunately, consumes fuel, but how much? Today we will find it out!

*This article will be intended for technical and non-technical audiences. After reading it you will know how much fuel your car AC is consuming in different conditions. I’ll also add some engineering data for those who want to know the science behind all this.*

So, first, let us answer some basic questions about car air conditioning.

### Does Car Air Conditioning Consume Fuel?

**Car air conditioning can consume about 10-20% of your fuel. Under extreme conditions (I’ll define extreme conditions later on) this bracket can quickly expand to 55%. **

### Why Does Car AC Consume Fuel?

Like any other AC, a car air conditioner has four basic components.

*Compressor**Evaporator**Expansion Valve**Condenser*

However, the power-consuming unit is a compressor. It takes power off the engine shaft and compresses the refrigerant. Which is then released from an expansion valve to create a cooling effect. Hence to keep this refrigeration cycle going, the compressor must be provided power.

### Factor Affecting Fuel Consumption Due to Air Conditioning

**Outside temperature and humidity**: This is probably the most important factor that affects fuel consumption due to air conditioning. If the outside temperature is low then obviously the AC compressor has to produce lesser work. If the outside temperature is higher more power will be consumed to achieve the desired temperature.

**Desired temperature and humidity**: Normally with 40% humidity humans tend to feel comfortable at 25 degrees Celsius. But human comfort is ongoing research. In this article, I’ll provide enough data for you to calculate AC fuel consumption at any temperature.

**Cabin space**: Some cars have small cabins and some have large ones. It can also greatly affect the amount of fuel being consumed by AC.

**The efficiency of the Engine:**That’s no secret obviously. If your car engine is efficient, it’ll automatically save your fuel. This means it will produce more power by consuming lesser fuel. Hence, lower fuel consumption for running AC.

**Compressor Power Rating and Type**: That’s another important factor that affects fuel consumption due to air conditioning. Usually, compressors are rated between 3-5kW but depending upon the model of the car it can vary. Latest hybrid cars and air-con systems can further decrease fuel consumption.

*Let’s explore the effect of some of these factors on fuel consumption.*

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### Is it More Expensive to Use Air Conditioning in the Summer or Winter?

The simple answer to this question will be summer. Because in winter outside temperatures are lower already. This means a lesser load on the compressor. Hence the lower consumption of fuel.

*To understand the science behind the fact that fuel consumption due to AC is generally more in summer, take a look at the following set of equations.*

The power consumed by AC referred to as P_{AC} is given by the equation:

P_{AC} = a. T_{OUT} + b

Here T_{OUT }is outside temperature, a and b are liner constants. Their value can be calculated by setting extreme values of temperature. If we consider the highest outside temperature to be 45 degrees and the lowest temperature to be 24 degrees. We get the following result:

P_{AC} = 0.25. T_{OUT} – 6

We chose 24 degrees Celsius as the lower limit because if the outside temperature is 24 and desired temperature is also the same, theoretically power consumed by AC would become zero.

**This equation indicates that as T**_{OUT}** increases, so does the P**_{AC}**. Hence confirmation of the fact that AC will require more power and in turn more fuel in summer.**

Using the above relation, you can calculate the actual power consumption of your car AC according to your temperature.

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### How Much Fuel Does Air Conditioning Consume According to Your Speed?

Well technically speaking fuel consumption due to air conditioning isn’t dependent on the speed of your car. But at higher speeds, the fuel consumption of a car increases. So, the part of the fuel being used to run AC is reduced.

*To understand it, consider the following example. A car consumes 3 liters per hour of fuel at 30 mph. Then AC fuel consumption is about 0.6 liter per hour. And a car consumes 17 liters of fuel per hour at 120 mph. Even then car AC consumes 0.6 liters of fuel per hour.*

Now using the power equation given above if we consider T_{OUT} to be 40 degrees Celsius then P_{AC} becomes 4kW. Since 1 hp is equal to 746 watts, hence 4 kW becomes 5.36 hp.

Let us calculate the percentage of fuel consumed due to car air conditioning at different speeds. But before that let’s fix the total power of the engine to 2000 CC or 134 hp.

Factor | UOM | Speeds | |||

Vehicle Speed | Miles per hour | 30 | 60 | 90 | 120 |

Car Mileage | Miles per Liter | 10 | 12 | 9 | 7 |

Total Fuel Consumed | Liter per Hour | 3 | 5 | 10 | 17 |

Engine Shaft Speed | RPM | 1913 | 2510 | 3000 | 3278 |

Engine Power | HP | 48 | 84 | 96 | 107 |

AC Fuel Consumption | Liter per Hour | 0.6 | 0.6 | 0.6 | 0.6 |

AC Power Consumption | HP | 5.36 | 5.36 | 5.36 | 5.36 |

% of total power consumed by AC | %age | 11.2% | 6.4% | 5.6% | 5% |

% of total fuel consumed by AC | %age | 20% | 12% | 6% | 4% |

Hence, you can see from the table that as speed goes up the percentage of fuel consumed for running AC is reduced accordingly.

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### Can You Save Fuel by Turning Off AC on Highway?

People tend to neglect an important factor relating to car speed and AC fuel consumption, while they’re on highways.

As we know that, a car traveling on a road has to fight specific pressure of air. This air pressure against a car is also known as air drag.

*At speeds higher than 60 mph, air drag becomes very high. In this scenario, open windows of the car can increase this air drag even more.*

This air drag increases the load on the engine. As a result, it consumes more fuel. This amount of fuel is more than a car AC would consume.

**Hence, if you’re planning on saving fuel by turning off AC and opening windows on the highway. It is a really bad idea. Because you would end up losing more instead of saving.**

### How Much Does It Cost You to Use Car Air Conditioning?

We must decide on certain variables if we want to calculate the running cost of car AC in a year. We consider that the average temperature in 6 months of summer is 35 degrees Celsius and during 6 months of winter it is 10 degrees.

So, you don’t need AC in 6 months of winter. During the rest of the year at 35 degrees, the average power required to run AC will be 2.75 hp. The amount of fuel consumed will be 0.35 liter per hour at this power.

**Now let’s get to some calculations:**

*Average distance traveled in a month: 2,000 miles*

*Average speed: 45 mph*

*Total hours: 45 hours*

*Total fuel consumed by AC: 0.35*45 = 15.75 liters*

*Total fuel consumed in 6 months of summer: 94.5 liters or 26 gallons*

*Price of one gallon: $ 3.6 (You can insert recent price here)*

*The total cost of fuel consumed by car AC in a year: $ 91*

**Average cost per month: $ 7.6**

Hence, your car AC costs you $ 7.6 for one month and $ 91 for a whole year.

### In Which Moment Does Air Conditioning Take the Most Fuel?

While your engine is running idle, car AC consumes the highest amount of fuel.

AC in a still car as opposed to when it’s traveling can consume about twice the amount of fuel. Since starting motors of a car also needs power. They can take 5-10 horsepower in a car.

*Hence in addition to 5-10 horsepower required to run AC, an extra 5-10 hp is consumed to keep motors running. This results in higher fuel consumption.*

You must always avoid turning on AC, while the engine is idle.

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### How to Save Fuel with Air Conditioning on?

You can save a great deal of fuel with air conditioning by following some simple rules:

1. **Replace Cabin Filter:** A clogged cabin filter can increase strain on your AC compressor. It’ll consume more fuel and produce less cold air. So, get your dirty cabin filter replaced to save fuel.

2. **Keep AC Speed Low:** Don’t switch to the max speed of AC as soon as you sit in the car. Because in the start AC compressor is running cold and it consumes a lot of power. Always start at a lower AC speed, then gradually you can increase it.

3. **Adjust Speed Once:** Don’t give mixed signals again and again. AC compressors aren’t smart devices, multiple inputs can adversely affect their efficiency. Hence, adjust the speed of AC only once or twice during your travel.

I hope this article was helpful and you got all information related to the car air conditioning system and its fuel consumption.

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