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how to size an air conditioner

How to Size an Air Conditioner for Your Home

Let us guarantee your HVAC unit’s optimal efficiency by choosing us for installations and replacements any time of day.

Air conditioners have one job: to cool your residence uniformly. But if you don’t find the appropriate one for your unique household layout, it may struggle to do so. Unfortunately, that means walking into an appliance store and picking up a unit on a whim won’t suffice without a thorough understanding of how to size an air conditioner for your home.

For 15 years, Green Energy Mechanical Inc. has helped residents understand how to measure AC air output levels. However, because we consider these factors when conducting each trusted AC installation in Canton, MA, we can put your mind at ease. Let our five-star team handle all your HVAC needs.

What Happens with Insufficient Sizing

If your AC system is too small, it’ll long cycle, extending the average cooling cycle as it strives to create an indoor environment that matches your thermostat setting. This prolonged cooling pulls more electricity from your energy grid, contributing to higher utility bills and an uncomfortable and sticky environment during our mid-80-degree summer days. 

You may think larger systems work the opposite way, cooling your home and providing relief faster, but larger systems short cycle, starting up and shutting down frequently. This process creates hot and cold spots around your home rather than a uniform, controlled climate.

Long and short cycling causes the unit to work harder, which leads to internal component deterioration and a shorter system lifespan. Instead of regularly undergoing costly repairs or purchasing an untimely pricey replacement, we’ll show you how to size an air conditioner.

Conferring with the Sizing Chart

The first step in determining the appropriate unit size is measuring the area where you want to provide cool air. Consider cooling each room individually with a separate window or wall-mounted units. Otherwise, refresh an entire floor containing a few walls and barriers with one ductless system or add a central unit with ducts for whole-home relief. 

After taking the length and width of the area, multiply the dimensions for the room or rooms. Then, multiply that number by 25 to get the BTU (British Thermal Units) necessary for depicting how much energy it takes to lower the preexisting temperature. Since each cooling unit has a BTU or BTU rating, simply match the number for optimal cooling capacity. 

According to the sizing chart, a home averaging 100 to 150 square footage requires as little as 5,000 BTUs, with about every 50 to 100 additional square feet needing 1,000 more. Some of the largest homes ranging up to 1,400 square feet demand no less than 23,000 BTUs. Since every 12,000 BTUs also equals one ton, you can decipher your new unit’s size in BTU per square foot or tons. 

Other Factors That May Affect the Sizing Chart

Square footage tells you how much space needs cooling, outlining the cooling capacity necessary from your unit so you know how to size an air conditioner. However, other factors may raise the temperature in your home further, requiring more cooling power. 

  • The volume of air in each room needs altering temperature-wise. However, two rooms can have the same square footage but different volumes, depending on ceiling height, so consider your high and low-rising ceiling.
  • Your house’s exterior affects indoor humidity levels since darker-colored or brick sidings absorb heat. Wooden, asphalt, concrete, and tile rooftop shingles also allow UV rays to easily penetrate your attic. If your home gets direct sunlight without a canopy of trees or larger surrounding structures to shade it, even more heat enters your home.
  • Doors and windows don’t have the same level of insulation as solid walls because of the crevices around the openings. These crevices allow outdoor humidity to seep in just as glass surfaces transfer heat, so you may need to upgrade to a larger unit.
  • If you live in a year-round warm climate, invest in a unit with a high SEER (Seasonal Energy Efficiency Ratio) rating. The highest SEER rating of 22 promises optimal efficiency that could earn you energy rebates and incentives.

Go for Green Energy!

Become more energy-efficient by upgrading your existing unit or installing your first with the unit’s BTU and SEER rating in mind. At Green Energy Mechanical Inc., we’ll ensure you get the most from your air conditioner when you call our A+ BBB-accredited, NATE-certified technicians at 781-236-3421. We’ll show you how to size an air conditioner and start reaping the benefits in Canton, MA, today!

Author Bio:

Jonathan Neves

Worked in the HVAC industry since he was 17. He started as an HVAC service technician for commercial buildings and eventually came into the residential market because of his love of working with people. He believes that what really sets Green Energy Mechanical apart is how long they stand behind their work and how compassionate his employees are for their clients.


My furnace was not working this morning. I called Green Energy and within a few hours Terry was here and quickly took care of the problem. Terry has been here before and is very professional and friendly.
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Very professional operation. Great technicians. Follow COVID protocol. Leave your house and property very clear with no trace that work had been done.
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Quinn was really nice, professional, and answered all my questions in a clear and easy manner. The woman I spoke with on the phone was also very kind, so I will definitely use this company again.
Peter T.
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Green Energy Mechanical is a pleasure to work. We had both air conditioning and heating installed. During both jobs, GEM was thorough and professional from start to finish.
Sussane P.
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I could not be more pleased with the service provided. We have upgraded our water heater to a tank less. We are also getting our heat and ac installed by them. They care for your home like you would
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Satisfied Customer

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