The size of your home’s AC unit is based on a Manual J calculation, which measures square footage, the number of windows, insulation, and other factors.
Manufacturers measure AC units by BTUs or tonnage.
To cool your home, you can choose between window units or central air.
While buying a new AC unit for your home may seem pretty straightforward, there are many factors that you need to consider. During your search, you may find yourself wondering, “What size AC unit do I need?” Unfortunately, the answer isn’t so cut and dry. The most basic details you’ll need for an exact calculation include the size of your home, what shape your insulation is in, and the climate where you live. With our tips and advice below, you can be confident when choosing the size of your next AC unit.
Why Does HVAC Size Matter?
Think about this question: “What is HVAC?” Beyond its basic definition of heating, ventilation, and air conditioning, HVAC is a complex web of systems that all work together to make your life indoors as comfortable as possible. If parts of the system aren’t the right size to fit your needs, the whole unit will fail. A unit’s size is an important part of the entire system’s operation that can make or break its efficiency and have a huge impact on your home and your bank account.
Because of the price tag that’s attached to HVAC systems and components, some people are tempted to save money by opting for a smaller model, but doing that would definitely be a mistake. If your AC is too small, it will struggle to cool your home. This won’t just make you sweat; your air conditioner will have to work overtime, leading to premature wear and tear of many different system components and higher energy costs.
An AC that’s too large is also an issue. It cools too quickly, meaning it will turn on and off in short, repeated cycles. In other words, it’s still working overtime (but in a different way). This wears down your AC, raises your utility bills, and causes discomfort. You’ll have bursts of frigid air with periods of heat—not ideal.
A well-maintained unit that’s the right size will efficiently and evenly cool your home. It shouldn’t prematurely break down, and your home will be a lot more comfortable. You also won’t be wasting energy, which will reflect on your utility bill (and your wallet).
Understanding Air Conditioner Ratings
There are a few different ratings that measure how your HVAC unit operates. The two most important measurable components of how well any given system works are the BTU rating, which measures the cooling capacity, and the SEER, which measures energy efficiency.
The Importance of BTUs
What is a BTU? In short, it’s a measure of the amount of heat needed to raise the temperature by 1 degree Fahrenheit. BTU stands for British thermal unit, and there are 12,000 BTUs per one ton of AC capacity.
Air conditioning units remove BTUs from the atmosphere. For example, if you see an air conditioning unit rated for two tons, it can remove 24,000 BTUs from the air. More tonnage equals a higher rating, which indicates a larger cooling capacity.
What Does SEER Stand For?
SEER stands for Seasonal Energy Efficiency Ratio (though you may also see the term Seasonal Energy Efficiency Rating). This rating measures how efficiently your AC unit cools your home by weighing the cooling capacity with the total amount of electricity used. Higher ratings mean higher efficiency (and lower electric bills).
At minimum, you’ll want a unit with a SEER rating between 13 and 14. Experts consider ratings between 15 and 20 to be efficient, and any rating over 21 is above average. Keep in mind that SEERs represent a unit’s efficiency at maximum capacity. You’ll want to look at the average performance to get an idea of how the unit will realistically function.
How Do I Calculate the Size of My Ideal AC Unit?
The U.S. Department of Energy recommends that everyone looking to install a new AC unit insist that their professional conduct a Manual J calculation to determine the size of the unit necessary to cool their home. While it’s a complicated calculation, Manual J takes a few different factors into account. Square footage is one of the most important.
Your home’s square footage is one of the primary factors you should consider when thinking about the size of your AC unit. If your home has higher square footage, it will need a higher-tonnage AC unit to remove more BTUs. The Manual J calculation also requires you to provide the number of heat-generating appliances in the home, which tends to increase with square footage.
Here are some of the most common AC unit sizes (in both BTUs and tonnage) people buy based on the square footage of their homes:
|Home Sq. Ft.||AC Tonnage||BTUs|
|400 – 500||1||10,000 – 12,000|
|500 – 1,000||1.5||14,000 – 18,000|
|1,000 – 1,500||2||21,000 – 24,000|
|1,500 – 2,000||2.5||30,000|
|2,000 – 2,500||3||34,000|
|2,500 – 2,700||4||48,000|
|2,700 – 3,300||5||60,000|
Additional Factors That Affect the Air Conditioner Size You Need
While square footage is a great place to start, there are other factors that come into play. For example, if you have a large home with a lot of windows, you may need a larger air conditioning unit to make up for the heat generated by the sun. Here are a few things to consider.
According to the Office of Energy Efficiency & Renewable Energy, the United States has five climate zones defined by the temperature and humidity of each climate. The hotter and more humid your location’s climate, the higher the tonnage needed to power an air conditioner and cool your home.
The Manual J calculation takes the age and condition of your ductwork into account. If your ducts are older or are in poor condition, they won’t work as efficiently as newer ones in better condition.
The size and efficiency of your windows matter when figuring out what size AC unit you’ll need. If your windows don’t do a good job of preventing air leaks, that will put a higher strain on your HVAC unit. You can expect HVAC professionals to ask you to provide the number of windows in your home and whether they’re newer double-paned or older single-paned windows.
Where your home sits in relation to the sun affects how hard your AC unit has to work to cool the house. The more shade you have over your home, the easier it is for your HVAC system to keep your house from heating to uncomfortable temperatures.
Insulation is a big factor in determining how efficiently your AC unit runs and the cost of your utility bills. If you run your air conditioner but your home isn’t well-insulated, the cold air will escape much faster, causing your system to work harder and driving up your energy bills.
Number of People in the Home
Yes, body heat plays a role in the Manual J calculation. Humans produce a lot of heat, and the more people you have moving around your home, the harder your system has to work to cool it down.
Homes with higher ceilings are more difficult to cool because there’s more area for the air to circulate through. The higher your ceilings are, the more your AC has to work to cool all that extra space.
Believe it or not, the color of your home can actually make a huge difference when determining the ideal size of your air conditioner. Both color and material will impact how much heat is absorbed versus how much is reflected.
For example, dark colors and materials like brick will absorb heat. In this case, you may need a larger AC unit. Light colors will reflect heat, and materials like vinyl siding provide good insulation. In this case, you may be able to get away with a smaller unit.
Rooms with heat-generating appliances, like kitchens with stoves and refrigerators, will take more energy to cool down. If you’re looking for a window unit for a laundry room or kitchen, err on the side of a slightly larger one.
Window Unit vs. Central AC Unit
If you’re only looking to cool one or two rooms, then window AC units or mini-split systems will suffice. If you need to cool the whole home, a central AC system will be a better investment in the long run, even though it has higher upfront costs. Installing a new central AC unit costs between $3,800 and $7,500 on average, but the maintenance costs and utility bills will be lower over the unit’s lifetime versus buying several window air conditioners.
By now, you can probably answer the question, “What size AC unit do I need?” on your own with relative accuracy. However, it’s always safest to let a professional determine what size unit will suit you and your house the best. Call an air conditioner installer near you to have them visit your home and help you make the best decision possible.
Amy Pawlukiewicz contributed to this piece.