How to Buy an Air Conditioner

Air conditioner sizing

You'll need to buy the right size air conditioner for the size of your room. This chart shows EnergyStar.gov's recommendations; calculate square footage by multiplying the length of your room by its width. Take other factors into account, too: You'll need 10 percent more BTUs if the room is very sunny, or 10 percent less if it's heavily shaded. If more than two people are usually in the room, add 600 BTUs for each extra person. Add 4,000 BTUs if installing the unit in a kitchen. And if your room's ceilings are higher than 8 feet or it's situated directly under the attic, step up one or two BTU classes, advises the ACEEE.

Climate can also make a major difference in air conditioner efficiency, so keep in mind that these standard calculations are based on summer temperatures in the high 80s and low 90s. If you live in a milder climate, you can save money and energy with a less powerful air conditioner. If high temperatures in your area frequently exceed 100 degrees, however, you may need a more powerful unit. This is especially true in areas with high humidity.

For room air conditioners with a capacity of less than 8,000 BTU, the EER must be at least 9.7 -- the higher the number, the better. For a unit to receive an Energy Star rating, it must reach 10.7 EER. You can estimate energy savings by calculating a 10 percent savings for each point difference of EER. For example, a model rated at 10.7 EER is 10 percent more efficient than a 9.7 EER model. By estimating the number of years you expect to use the unit, you can gauge whether a higher initial cost will eventually pay off. All new appliances are required to carry a bright yellow EnergyGuide decal that lists the unit's energy consumption in kilowatts and estimates annual cost of operation.

Air conditioner shopping tips

Here's some other advice to keep in mind when shopping for a room air conditioner:

• Get the right capacity. Air conditioning is half cooling, half moisture removal. A unit that's too small for your room won't cool enough, and one that's too large won't remove humidity effectively. EnergyStar.gov's handy article "Properly Sized Room Air Conditioners" can help you determine how many BTUs of cooling capacity you need for your room.

• Get the right size. Air conditioners aren't a standard physical size. Even those with the same number of BTUs can be several inches wider or taller than another. Measure your window opening to make sure the unit you're considering will fit. If your window opening is taller than it is wide -- if you have sliding or casement windows, for example -- major manufacturers make units designed to fit these, too. If you can't install in a window, some air conditioners can be installed through the wall.

• Consider ease of installation. Even the smallest air conditioners are heavy, and the largest can weigh more than 100 pounds. You'll probably need assistance to install a heavier AC; even with a lighter unit, an extra pair of hands is a good idea. A slide-in chassis makes installation easier because you install the lighter portion first, but this can also take more time.

• Check for special requirements. Some extra-large window air conditioners require a 220-volt plug, and experts recommend that you run an AC on its own circuit to avoid electrical overload. Units in our Best Reviewed section all require a three-prong outlet. Some models have louvers that blow air in multiple directions, so keep that in mind when deciding where to install yours. Because the filters in certain ACs may require additional space on one side, be sure to check out how easy it is to install and remove the filter once the unit's in place. 

• Look for convenience features. Thermostats, digital controls, remote controls and built-in timers are becoming more common, even on low-priced models.

• Know what level of noise you can live with. You might be willing to save some money on a noisier unit if you're not cooling a bedroom, but look for models known for quiet operation if you're a light sleeper.

• Get the highest-efficiency unit you can afford. In general, you'll pay a bit more for a high-efficiency air conditioner, but you'll make up the difference in your electricity bill over the unit's lifetime. Price differences have diminished between high- and low-efficiency ACs, and sometimes more efficient models are no more expensive than others.

Some features might not be worth paying extra for. Sleep mode increases the thermostat's temperature -- usually to 78 degrees -- to save energy costs when you're asleep and may not notice a warmer room. But according to the National Sleep Foundation, sleep researchers have found that temperatures of more than 75 degrees often disrupt sleep. Experts typically recommend keeping your bedroom at about 68 degrees for the most restful sleep, so if you tend to wake easily at night, avoid using the sleep mode setting on your air conditioner.

Energy-saver mode is another feature that isn't always as useful as it sounds. It works by turning off the air conditioner's fan when the room's temperature reaches the desired level and the compressor shuts off. However, leaving the fan on will make the room feel cooler and may allow you to turn the thermostat up a degree or two, saving more energy overall. In addition, the changing noise level might awaken you.