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Does A Car Defroster Use Gas? (Explained By Mechanic)

Does A Car Defroster Use Gas? (Explained By Mechanic)
Fact Checked and Reviewed by: Kris Jackson, ASE-Certified Mechanic
Kris Jackson has been a mechanic since 2010 after graduating from UTI. He’s worked with several master mechanics and holds several ASE Certifications. You can read more about Kris here.

There are dozens of ways to get the most out of every dollar you put in your car’s tank. From driving significantly slower on the highway (not something I’d recommend) to minimizing aggressive stops and starts, you can end up dramatically improving your mpg. 

But what about being more careful with the defroster? Does your car’s defroster use fuel and is it enough to impact your mpg? 

Both the front and rear defrosters are powered by electricity which is primarily pulled from the alternator. The alternator is powered by the fuel-burning engine, so a car’s defroster system does use fuel but in a slightly roundabout way. If you’re using both front and rear defrosters, you can expect to use an extra 0.345 gallons per hour.

If gas is $3 a gallon, that would be about $1.04 per hour to run both the front and rear defrosters. Considering how important defrosters are for seeing the road, the extra fuel cost is more than worth it!

That’s the quick answer but we’ll dive a bit deeper into how we came up with these numbers and everything else you need to know about your defroster’s impact on mpg. 

Let’s get started!

How Does A Car’s Defroster Work? 

Let’s start by taking a look at exactly how the defroster works so we understand exactly when fuel is used. We’re going to keep this very simple and avoid too much jargon. 

How Your Car’s Front Defroster Works

Most people think of the defroster as just a burst of warm air, but it’s actually doing more than just blowing air. It’s removing humidity from your car too which works several components working together. 

In most vehicles, when you flip the switch to turn your defroster on your car’s A/C also engages and helps dehumidify your car and windshield. Some vehicles let you split these functions, which can change fuel usage, but it’s not standard. Additionally, your car’s heating system engages which utilizes excess heat from your engine. The blower fan pushes this hot air onto the windshield to remove fog, frost, and condensation. 

All of these systems use electricity that’s generated from the alternator. But the alternator is fueled by the engine so fuel is used as part of this process. However, your car’s heating process uses a minimal amount of fuel and it’s the blower fan and A/C system that use up the most fuel. 

How Your Car’s Rear Defroster Works

The way these work is pretty simple. When the defroster is turned on, electricity from the car’s alternator and battery is directed to these elements. As current flows through these resistive elements, they generate heat, which in turn warms the rear window and helps remove ice, fog, and condensation. 

This system is more similar to seat warmers than the more complex system found in the front defrost. Like seat warmers, the rear defrost also has a very low impact on fuel usage and mpg

How Much Does A Car’s Defroster Cost To Run? 

Let’s look at the horsepower impact for each of your car’s defrost systems and how that can translate to fuel costs. 

Cost To Run Your Car’s Front Defroster 

The front defroster operates primarily by using the vehicle’s air conditioning system to dehumidify the air and the heater to warm it. This dual action requires energy. 

We previously found that the typical car A/C uses around 4 horsepower or roughly 3,000 watts of energy. For an engine producing 100 horsepower and consuming 8 gallons of fuel per hour, this is an increase of 4%. If fuel costs $3.00 per gallon, using the front defroster could increase fuel consumption by about 0.32 gallons per hour, costing approximately $0.96 per hour.

The cost of the heating elements is so minimal, it makes almost no impact on the bottom line and the majority of what the defroster uses in fuel and energy comes from your car’s A/C system. 

Cost To Run Your Car’s Rear Defroster

The rear defroster primarily uses an electric grid system consuming about 10 to 20 amps, translating to 120 to 240 watts or 0.16 to 0.32 horsepower. This is a minimal draw compared to the A/C system and the front defrost system takes 12.5 times the energy of the typical real defrost (at least based on horsepower)!

In our 100-horsepower engine scenario, this represents an increase of 0.16% to 0.32%. Based on the same fuel price and consumption rate, using the rear defroster increases fuel consumption by roughly 0.012 to 0.025 gallons per hour, leading to a cost of $0.038 to $0.076 per hour if fuel costs around $3.00 a gallon. 

Okay, Can I Do Anything About How Expense My Defroster Is? 

Your car’s defroster isn’t exactly cheap and even though you usually aren’t running it for hours on end there are a few things you can do to save a little. 

We’ll look at ways you can make your defroster run a bit more efficiently and how you can defrost without a defroster which will obviously save you! 

How To Make Your Defroster Run More Efficiently 

First, let’s look at options to help your defroster function just a little better or reduce the work it has to do. 

Avoid Cranking It All The Way Up

Your defroster uses the most gas when the fan is cranked to its max setting. But this isn’t always required to really get the job done. Instead, leaving your defroster fan on a medium setting, even it takes a little longer, will save on energy. It’s not going to be a lot, but over a long enough time (like the life of your vehicle) it can add up. 

Try Using A Sunshade…In Winter

Sunshades are well known for keeping your car cool during the summer but when used in winter they can actually help heat your windshield and reduce frost. You might get some funny looks, but less frost means less work for your defroster (and more gas savings). 

Turn Off The A/C

Some cars will let you turn off the A/C compressor when your defrost is on and allow you to just run defrost heat. This can have a big impact on fuel efficiency since the A/C compressor is much bigger fuel hog than the heater. 

How To Defrost Your Car Without A Defroster

There are several ways to remove frost without your defroster, but my absolute favorite (and cheapest way) is to use a solution of rubbing alcohol and water. This will reduce the freezing point of water and prevent ice from forming on your windshield. Simply mix two parts rubbing alcohol with one part water in a spray bottle.

In the morning, give your windshield a quick spray and watch the frost melt away. The added benefit is that the rubbing alcohol evaporates quickly, leaving you with a clear view in no time. Not to mention, it’s far more environmentally friendly than many commercial de-icers.

This video shows it in action but just make sure not to get any on your paint: 


Closing Thoughts

Using a defroster can impact fuel economy because it’s mostly the A/C system doing all the work. Consumer Reports and our own math found that the A/C impacts fuel efficiency by around 1 to 4 mpg so we can make the same assumption about the defroster. 

However, the big difference here is that the defroster is much less likely to be running for long periods of time, and most of the time you just need a few minutes to get the job done. That makes the defroster much more cost effective in terms of gas and dollars. Still, it’s going to have more of a negative impact on your fuel efficiency than your car’s headlights or the radio which use very small amounts of fuel to run.   

That’s good because we need a clear windshield to see the road! Ride safe!

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