That $199 special may not be a bargain

Energy saving window AC units will cost you far less to operate than the $199 special you pick up at your local building center at the start of the summer cooling season.

Don’t be tricked by super-low prices into buying a cheap window air conditioner – it’ll cost you far more in combined up-front and energy costs, than if you buy the most energy saving window unit you can find.

And make sure you buy the right-sized unit; an efficient window air conditioner can become very inefficient if its capacity is much bigger or smaller than the space you cool with it.

How you install a window AC unit will have a big impact on its efficiency, and how you use it will too. And of course, don’t forget to dispose of your old air conditioner legally and safely. All this is covered in detail below.

Buy ENERGY STAR window AC Units

I have featured a few high-efficiency window air conditioners on this page, but have a more extensive set in the Room air conditioners page of my online store. I only feature ENERGY STAR rated room air conditioners and I list the EER ratings of all units featured.

The “+xx%” shown in products featured below is the percentage more efficient each unit is compared to the mandated standard; ENERGY STAR air conditioners must exceed the standard by at least 10%.

Disposing of your old window AC unit

Don’t just throw your old window AC unit in the trash. Don’t give or sell it to a scrap dealer. Why? First, you may be breaking the law. Second, you may be helping destroy the ozone layer.

You may have heard of chlorofluorocarbons or CFCs, chemicals that are harmless at ground level but that have been destroying the ozone layer. CFCs have been phased out in fridges and AC units since 2010, but some older used models may still contain them. Models you buy new will contain more benign HCFCs, but even these are not 100% ozone friendly, and both CFCs and HCFCs are potent greenhouse gases (although HCFCs are not quite as bad as the original CFCs). Make sure you dispose of your old AC unit through an outfit authorized to reprocess the CFCs so that they don’t escape into the environment.

Old inefficient window AC unit

An old, inefficient window AC unit
Wait – is that a Bible!?
Photo by Jason Eppink, via FLICKR

Contact a local refrigeration service center, HVAC dealer, or your local waste or environment department for information on safe disposal.

If you want to upgrade to a more energy efficient window AC unit, don’t give away or sell your old clunker to a junk store. You’ll just be passing on your problem of an energy wasting window AC unit (and high operating costs). From an environmental perspective, doing that doesn’t really help at all.

Buy the right-sized window AC unit


Don’t buy too big a unit. You might think a larger window AC unit with the same energy efficiency rating (EER) as a smaller one will do a better job. But if it’s too big for the room, it will cycle on and off frequently, which is both annoying and inefficient. It also wears the unit out faster.

An undersized air conditioner will be running a high percentage of the time, which shortens its operating life, and it may overdry the room.

You need to figure out the surface area of the room and buy an air conditioner whose BTU (British Thermal Unit) capacity matches the room size. To figure out the surface area, for a simple rectangular room, multiply the width by the length, in feet or meters, to get square feet or square meters.

Don’t try to overestimate size and think you’re being conservative – again, you’ll wind up with too big a unit and you won’t get optimum efficiency. Once you know the surface area of the room, use this chart to select the right capacity – and adjust as indicated below. Note that room air conditioners typically come in sizes from 5,000 to 14,000 BTU per hour.


Square feet   Square meters
From To BTU From To
100 150 5000 9 14
150 250 6000 14 23
250 300 7000 23 28
300 350 8000 28 33
350 400 9000 33 37
400 450 10000 37 42
450 550 12000 42 51
550 700 14000 51 65
700 1000 18000 65 93
1000 1200 21000 93 116
1200 1400 23000 116 130
1400 1500 24000 130 139
1500 2000 30000 139 186
2000 2500 34000 186 232

These estimates assume two people regularly occupy the room. Increase the capacity by 600 BTUs for each additional occupant over the first two. Also add 4,000 BTUs if the unit is in a kitchen, as a kitchen produces considerably more heat. Finally, go to the next highest BTU level if your room has high ceilings (over 8 feet), has heavy exposure to the sun through windows facing south or west (in the northern hemisphere), or where the ceiling borders a hot attic, particularly if the attic is poorly insulated or vented.


Of course, if you are actively trying to reduce your cooling costs, for example by following my Summer energy saving tips, you may want to adjust the BTU requirement downward one category. For example, if you’ve thoroughly insulated your house, ventilated your attic, and keep windows covered during daylight to cut your solar heat gain, have changed most or all of your heat-producing lighting such as incandescent and halogen lights to compact fluorescents or LEDs, and are willing to live with a slightly warmer, but still air conditioned home in exchange for greater energy savings, you will wind up with a system whose capacity is too high if you just use the BTU table above.

Once you’ve selected the BTU level you need, shop for the unit close to that BTU setting that has the best energy efficiency score. The EER (Energy Efficiency Rating) or SEER (Seasonal Energy Efficiency Rating) number from the US Department of Energy provides a good indication of efficiency. Buy the highest EER or SEER you can find. To convert between SEER and EER, see my conversion formulas at Air conditioner ratings.

Remember to check the amperage, and ensure it won’t blow the electrical circuit you plan to plug it into. For units of 7.5 amps or less, they can be plugged into any 15- or 20-amp circuit. Higher-current units will need their own dedicated circuit. (Central AC units require a dedicated 230 volt circuit).

Also look for a filter that slides out easily for cleaning, as an energy saving air conditioner can soon become very inefficient if the filter clogs up with dust. Other useful features are a unit that can thrust the air further into the room (if cooling a long, narrow room), a digital temperature readout, and a built-in timer. Take a look at the controls for the unit and make sure they are simple to use. An energy saving air conditioner whose controls are incomprehensible could lead you to overcooling the room, which could eat up any savings you might have obtained from buying the unit.

Install the unit in a shade-facing window

The more sunlight strikes the outside of your window air conditioner, the hotter the sun will make it. Since the job of the outside of the air conditioner is to get rid of excess heat extracted from the room, the unit will have to work harder to get rid of this heat if heat from sunlight is thrown in on top. So where you have a choice, put the unit in a shade-facing window, or a north- or east-facing window (assuming you’re north of the equator).

South-facing windows are the worst as the hottest sun of the day comes from the south; west-facing exposure is worse than east-facing because the setting sun in the west throws more heat (and at a hotter time of day) than the east-facing sun of dawn. (If you’re south of the equator, the sun still rises in the east but will probably be north of you not south, so reverse the north/south guidance and keep the east/west guidance!)

If there are trees or other buildings shading one window all day, choose that window – as long as the shading objects aren’t so close to the unit that they impede air exchange. A neighbor’s wall two feet from your window is not a good place to be blowing hot air onto.

When you install a window AC unit (or a through-the-wall AC unit), make sure the unit is perfectly level so that condensate drains out of it properly. Never install a unit sideways. And seal the unit into its window frame or opening so that there are no air leaks around it, as these will allow cool air to escape or hot air to enter.

Don’t use your window AC unit’s outside air setting all the time


Your window AC unit will operate more efficiently if, for the most part, you keep cooling down air inside the room rather than bringing in hotter outside air (at least, during times when the room air is cooler than the outside air).

Although the hot air may become cooler than the inside air as it passes into the room, it takes energy to extract the heat from that hot air to make it cooler, and you’ve just blown out the nice cool air that was already inside.

Use the low fan setting when dealing with high humidity

When the room being cooled is very humid, use the low setting on your window AC unit fan. This will do a better job of drawing out humidity than the high setting. Although it may not cool the room down as fast or as far, it will feel cooler. A humid 78F feels warmer than a dry 83F.

Check the window unit’s seal

At the start of each cooling season, check your window AC unit to ensure that the seal between the unit and the window is airtight. Moisture from indoor air in the winter, or from rainfall on the outside of the unit, can warm window frames or cause corrosion in the metal frame of the window AC unit, both of which can damage the seal. A window air conditioner with a poor seal will be less efficient. Hot, humid outdoor air may also get inside.

Cover the outdoor portion of your window unit in winter

Once the air conditioning season is over, cover the outdoor part of your window AC unit, or remove and store the unit indoors. This will reduce corrosion or dust build-up on the exterior fins, and will cut heat conduction from inside, through the unit, to the outdoors.

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