What Size Air Conditioner Do You Really Need?

There are all kinds of things to consider when choosing a new air conditioning unit or system, but if you’re reading this, chances are you’re keen to get an A/C that’s energy efficient. What does that mean? Well, to get an Energy Star rating, air conditioners must be 10 percent more efficient than similar models. An Energy Star rating isn’t everything though. In fact, the best way to be efficient with energy is to make sure you choose the right size unit for your space and needs (more on this below).

Already know what size air conditioner you’re looking for? Check out our top energy-efficient air conditioner picks here.

Interpreting Energy Star and Consumer Reports ratings

In addition to looking for the Energy Star rating and right size A/C, you may want to account for the likelihood of brownouts (power outages) where you live; some air conditioners work even under brownout conditions.

Consumer Reports test air conditioners in a special climate-controlled chamber by turning up the heat to 90 degrees Fahrenheit and seeing how quickly a unit can drop the temperature by 15 degrees. They also note how well the unit holds to the set temperature, and make sure to adjust the test chamber to properly assess small, large, and medium window A/Cs.

Many folks have harangued Consumer Reports for not testing air conditioners over 12,500 Btus, noting that these units are inadequate to cool a stonkingly hot apartment in many places. While this may be true in some cases, there’s not necessarily an advantage to a bigger A/C and there could be several disadvantages, including the cost of energy.

If you have any control over the enclosure of your building, the best way to ensure energy efficiency in an A/C is to make your home leak-proof. This means adding insulation, caulking, cladding, installing thermal windows, draught excluders, etc. After all, if your home is leaky and letting in hot air, your A/C is going to be working way harder than it needs to.

Fun Fact: Energy Star estimate that if all room air conditioners sold in the U.S. were Energy Star certified, this would prevent more than 6 billion pounds of greenhouse gas emissions annually – the equivalent of the emissions from over 570,000 vehicles (R).

Go big or go… small?

It may be tempting to choose the biggest air conditioner you can afford or accommodate when buying a new A/C. This isn’t always the best option, though, as a unit that is too big for the room will cool down the air too fast, leaving moisture in the air. The result? A cold, clammy space that feels rather unpleasant.

An A/C that is too small, however, will struggle to effectively cool a room and is likely to be less energy efficient overall.

So, how do you figure out what size unit is best for your space? First, you’ll need to know the square footage (approximately) of the room or rooms you want to cool. To do this, multiply the room’s width by its length. If you’re trying to cool a room that adjoins another room without a solid door, you’ll have to add together the square footage of both rooms. Other things to consider include:

• The room’s sun exposure or degree of shade during the day (plus or minus ten percent respectively)
• The number of people using the room – for more than two people, add 600 Btu for each extra person
• Add 4,000 Btu if the unit is intended for use in a kitchen
• Consider the height of your ceilings – low ceilings mean less air volume overall and require less cooling than higher ceilinged rooms
• The size and number of any windows and doors that may affect air flow and sun exposure
• Window insulation – non-thermal windows will mean you need a higher cooling capacity during the day (but heat will escape faster at night)
• Wall, floor, and ceiling insulation
• Other sources of thermal mass (see above)
• Your location – if you live in a high-rise surrounded by other high-rises, you’ll need to account for the heat island effect (especially with everyone else pumping hot air from their own windows!)

Unfortunately, if the last condition applies to you, as it does to many in built-up cities and downtown cores, even the largest of the window and portable air conditioners might not be up to the task of cooling your apartment by itself. In such cases, you may need two separate units beavering away at their highest settings. You can help reduce your use of A/C by keeping shades drawn during the day and installing a ceiling fan to keep cool air circulating.

Energy Star offer the following handy guide to work out the right Btu for a room air conditioner.

Area To Be Cooled (square feet)	Capacity Needed (BTUs per hour)
100 up to 150	5,000
150 up to 250	6,000
250 up to 300	7,000
300 up to 350	8,000
350 up to 400	9,000
400 up to 450	10,000
450 up to 550	12,000
550 up to 700	14,000
700 up to 1,000	18,000
1,000 up to 1,200	21,000
1,200 up to 1,400	23,000
1,400 up to 1,500	24,000
1,500 up to 2,000	30,000
2,000 up to 2,500	34,000

Remember, the square footage above is for a single room, not the whole square footage of your home. Taking some time to measure your space could save you a lot of hassle and expense and is well worth doing some quick math.

Should I put an air conditioner in my bedroom or living room or both?

Only you can decide if it makes sense to have a single portable air conditioner you move from room to room as needed or have an A/C unit in one or more rooms of your apartment or house.

As a general rule, you will get most benefit from an A/C in the bedroom as it’s important to have a cooler space to sleep at night. Your living room or other room of your home where you spend most time is the next-most important area to consider an A/C unit.

The least helpful place to put an A/C is in a kitchen that is closed off from the rest of the apartment or house. If you are cooking regularly, you may be fighting a losing battle to keep temperatures down, and it’s likely you’ll also have windows open or a hood fan on to help dissipate cooking odors, which make A/C much less effective. You may even want to consider a mostly raw food diet in summer, especially if it’s in line with your nutrigenomic profile!

Don’t forget noise pollution

When we think about an eco-friendly home, we sometimes forget to consider noise pollution. If an otherwise eco-friendly appliance runs loud, this in itself can make a living situation unpleasant. This is definitely something to consider if you value a quiet living space and find intermittent or loud noise disturbing while you sleep.

Thankfully, Consumer Reports takes noise output into account in their testing process. They measure how loud each unit is on its lowest and highest settings. Any unit rated Fair for noise is liable to disturb light sleepers even on a low setting and are a real noise nuisance on their highest setting. If noise is a consideration or you’ll need your A/C working overtime where you live, look for those with an Excellent or Very Good rating for noise from Consumer Reports. These units are very quiet and only really produce a low sound from the fan.

Now we’ve gotten all that out of the way, make sure you know the difference between a single speed and variable speed compressor, and whether you want a central air conditioner, window A/C, or a portable A/C.