How Many Degrees Can an AC Cool?

An air conditioner can cool your indoor air from 15 to 35 degrees below the outside temperature. This is an average range. A few systems (if you live in Arizona or southern California) are designed more powerfully, and can cool as much as 40 degrees below the outside temperature – which is what you’d want if it is 115 degrees outside. All well-designed systems should have the capacity to cool the air indoors to 75 degrees under normal weather conditions. 

In a cool climate like Michigan or North Dakota has, that might only be 15 or 20 degrees maximum cooling. In a hot climate like Arizona or Nevada, the unit would need the capacity to cool the inside air up to 35 or 40 degrees below the outside temperature. So, it would have to be bigger for a home of the same size, etc. as one in a cooler area.

And sorry to say it, but if you live in a cool state and a record-breaking heat wave occurs, your AC might not be able to keep up. You’ll have to add fans or spray bottles or a wading pool in your living room : )  Thankfully, these are rare events. 

Pro Note – If you live in a cool climate, and your AC is large enough to cool the indoor air 30 or 35 degrees above the outside air, it is too big! The HVAC system will use more electricity than it should, and it will run short cycles, which can damage the compressor. 

Let’s break this down.

When you have a properly sized air conditioning system installed, it should be able to keep your home comfortable under normal weather conditions for your part of the country. The exact same house needs a bigger AC system in AZ than in MN, right?

As we’ve said, the design temperature for any AC system should be able to keep up with the cooling demand 99% of the time. In a hot climate, that might be 35 degrees below the outdoor temperature. In a cool climate, the difference might be only 15 or 20 degrees. Yes, we’re repeating ourselves, but this is a new concept for many readers.

So, based on your climate, here is what your AC system should be able to do. Remember, the hotter your weather, the bigger the difference needed between indoor and outdoor air. 

  • 70 degrees indoors – from 85 to 110 outside. 
  • 75 degrees indoors – from 90 to 115 outside
  • 80 degrees indoors – from 95 to 120 outside

Where would you be happy with 80 degrees inside? In regions with very low humidity, like much of the Southwest, 80 degrees with very little humidity is quite comfortable.

Misinformation Warning: You’ll find many posts that say that central air conditioning can only cool your home 20 degrees below the outside temperature, maximum, no exceptions. This might be true for a properly sized air conditioner in a cool climate, but in a very hot climate, HVAC systems are designed to keep homes cool – in the 75 to 80 degree range – even in temperatures up to 110 or 115 degrees.

Below we provide you with all of the information, so you can properly size your air conditioner to get the maximum cooling power for your situation.

Can air conditioner cool more than 20 degrees? That’s a common question. And yes, if it is large enough for where it is installed, then it should be able to cool the air more than 20 degrees above the outside temperature. 

What is the Maximum Outside Temperature for an Air Conditioner to Cool

The maximum cooling power of an air conditioner depends on what type of AC it is and whether it is the right size.

As you’ll see in the chart below, most central air conditioning systems can cool from 15 to 35 degrees. Some have the ability to cool up to 40 degrees between indoor and outdoor temperatures. In other words, it can keep your home at 75F even when the outside temperature is about 110F if it is big enough.

If you size your AC unit based on the recommended specs, a window or portable AC usually can cool between 15 and 20 degrees and a mini-split can cool between 20 and 25 degrees. If you live in an area where temperatures can exceed 110 degrees, then you will want a system that can cool the inside air 30 or more degrees below the outside temperature. To do that, you will need to choose an oversized unit to make sure you can get your home down to a comfortable indoor temperature. 

How Many Degrees Can an AC Cool?

This chart will give you the maximum cooling capacity of different types of air conditioners when the unit is properly sized.

how many degrees can central AC cool15-35 degree
how many degrees can window AC or portable AC cool15-20 degree (If your AC is sized based on specs or online store recommendation)
how many degrees can mini split cool20-25 degree (If your AC is sized based on specs or online store recommendation)

Is there a maximum outside temperature air conditioner can cool? Technically, no. But practically speaking, once the outside temperature gets 10+ degrees above the hottest temperature usually experienced in any area, the AC will probably have trouble keeping up

A Properly Sized Air Conditioner

When purchasing an air conditioner, making sure that it’s the right size is crucial to the unit’s ability to keep you comfortable in the summer. Your square footage is part of the equation, but there are other things to consider.

Manual J is a calculation used to determine the amount of cooling and heating a home requires to keep the home cool and dry in summer and warm in winter. Manual J takes into account more than square footage. Below we list some of the factors considered in a Manual J calculation.

Square Footage

An air conditioner’s BTUs indicate how many square feet the unit can cool under normal conditions. For instance, a 9,000 BTU unit should be able to cool a 400 square foot room by 20 degrees.

But if the outdoor temperature is 110F, that’s only cooling to 90F, and that’s not going to keep you comfortable.

A knowledgeable salesperson familiar with Manual J calculations or an HVAC tech can help you decide on the type of air conditioner and how many BTUs you will need to stay cool.

Total Air Space of the Room

If your ceilings are over 8 feet tall, you will have more cubic feet of air to cool.  For a room with high ceilings, you’ll need to add 2,000 to 5,000 BTUs to the AC size.

Insulation and Windows

How well your home is insulated and how energy efficient your windows are make a big difference in keeping your home cool. If warm air is entering your home because it’s poorly insulated, you will need more BTUs of cooling power to keep your home at 75F on a hot day.

Sun Exposure

If the space you’re cooling faces south or west and/or has no shade from trees, you will also need a more powerful unit to cool the room or rooms.

Number of People Living in the Home

A household with many people, pets, and appliances will affect how well your AC cools the space. If the doors to the outside are opened frequently or you cook often, you will need more BTUs to keep your space comfortable.

The Design Temperature in Your Area

The Design Temperature is based on a 30-year average of the highest temperature a specific area gets 99% of the hours of the year.

This means that the outdoor temperature will rise above the Design Temperature only 1% of the hours each year, or about 88 hours.

If the Design Temperature in your area is 95F and you want your room or home at 75F, that’s a 20-degree difference.  A correctly sized air conditioner should be able to keep you cool.

If you live in an area where the design temperature is over 95F, like Arizona, California, or Kansas, you will need an air conditioner that has more BTUs per square feet in your home.

How Many Degrees Can A Portable AC and Window AC Cool?

Room air conditioners, which is the broader name for window and portable ACs, should cool a room 15 to 20 degrees when you buy one sized for the space.

Portable ACs: Keep in mind that portable air conditioners don’t cool as effectively as window units. Why?

When a portable AC exhausts warm air from a room, it also pulls some warm air back into the room from outside or from other rooms. Plus, since the unit sits entirely inside, some of the heat captured by the refrigerant leaks back into your living space before it can be exhausted.

That’s why it is important to use the Department of Energy (DOE) SACC ratings for BTUs when considering a portable air conditioner. The SACC ratings are 25% to 45% lower than standard ratings.

If your situation includes some of the issues listed above, like high ceilings or poor insulation, choose a unit with about 20% to 25% more BTUs than the SACC ratings recommend for your square footage.

Window ACs: A window unit will also cool about 15 to 20 degrees if it is the right size for the room where it is installed. If the room is extra-hot because it faces west and gets a lot of sun, it might not be as effective. Consider buying an AC with 2,000 to 5,000 extra BTUs of cooling power.

How Many Degrees Can a Mini Split Cool?

A mini split system can cool between 20 and 25 degrees when you buy a system sized by space alone. However, that isn’t enough cooling power for homes in areas with summer temperatures well above 100 degrees. 

The solution is to determine the right size mini split system based on Manual J calculations – something an HVAC technician should recommend doing before selecting the right system for your home. The Manual J takes outdoor temperatures into consideration. For example, you will need more cooling BTUs for a home in Arizona than you would for the exact same house in Minnesota.

If you have issues that might make your home more difficult to cool, consider a larger unit with a variable speed/inverter compressor. Most mini split systems use inverter compressors, so this shouldn’t be a problem. The key point is that these units can be oversized at 1.3 times the calculated size, which will help ensure that your home gets the cooling power you want.

When Should I Choose an Oversized Room AC?

It makes sense to buy a window AC or portable AC rated for more square feet than you have if:

  • The room has ceilings higher than 8 feet
  • It will be used in the kitchen when the stove or range is in use
  • The room is often occupied by 3 or more people (or pets)
  • The room gets a lot of sunshine
  • You plan to use the AC in a garage or other space that isn’t well-insulated
  • You like the air really cool

These tips apply to a mini split system too.

Cooling tip: It is easier to keep a room cool than it is to cool off a room that is really hot. If you want the space comfortable, turn the AC on before the room gets unbearably warm. Not only will you be more comfortable in the room, the AC won’t have to work as hard to cool it off. And that might add years to its life.

Is There a Coldest Temperature for an Air Conditioner?

Asking if there is a coldest temperature air conditioner systems can produce is a hypothetical question with no perfect answer, but our experts say, no, there is not a coldest temperature that an AC can cool air to. If you keep taking heat away, which is what air conditioning does, the air will continue cool.

In practical terms, how cool your room or house can get depends on the outdoor temperature, how well insulated the room or home is, the sun vs shade factor, the air conditioner’s cooling capacity and the other factors discussed above.

It also depends on the temperature of the room when you turn on the AC. For instance if you’ve gone to work in the morning and left the AC off on a very hot day, you can expect the room to be quite hot in the late afternoon.

Written by

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree from Lone Star College and EPA & R-410A Certifications.

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