Window A/Cs: Inexpensive, Efficient

Mar 20, 2001 (Updated Apr 15, 2001)

The Bottom Line Window Air Conditioners today are exceptionally efficient, they are quiet, and they cost far less to buy and operate than central systems.

While it's true that the "best" way to cool an entire house -- all rooms and areas -- is to install a central air conditioning system, this energy-hungry solution is going to cost a minimum of $2,000 and possibly over $10,000. Moreover, it's not even an option for those living in apartments or rental homes. Proper selection and use of window air conditioners can keep you as cool -- or cooler -- than a central system at a small fraction of the cost.


You may not need to cool your entire living area, or you may want to. Sizing of air conditioning systems is a science unto itself, and proper calculation of a building's cooling load requires knowledge of the construction type, number and type of windows, square footage, ceiling height, occupancy patterns, and still other factors. But because you live in your house and know the temperature characteristics of certain rooms -- as well as how cool you want to be -- you actually have some advantages over a professional who comes in and has to figure it out with a calculator. Using the categories below and your own common sense, you can choose the right size unit:

Measure the rooms you want to cool and calculate total square footage. Unless you are cooling one room only, don't forget hallways or any other areas that can't be closed off from the area you want to cool. If the area you want cool includes a kitchen, add 4,000 btu/h (British thermal units per hour) to the recommended btu/h to arrive at a figure that allows for the heat generated through cooking, heat thrown off from the refrigerator, etc. (This is an EXTRA 4,000 btu/h over and above what the square footage of the kitchen calls for.)

With the square footage in mind, consider the following categories:

-100-150 square feet: This will almost always be a small bedroom, and 5,000 btu/h is the perfect size for this size of room, no matter where you live. The only exception would be a VERY hot room which has many windows or is located right under a hot roof. In this case, 6,000 btu/h would be preferable, unless you plan to use the unit only at night, in which case 5,000 will still do the job.

-150-200 square feet. This will almost always be a single room application. If the room is very well insulated, or if you plan to use the unit mainly at night, you can still buy a 5,000 btu/h unit. For most average rooms, 6,000 btu/h will be suitable for 24 hour operation, while in very hot rooms you might want to go with 7,000 btu/h.

200-350 square feet. This is a perfect size for an 8,000 btu/h unit in an average home. Again, if you know that this is a hot area, or you like it really cool, you could bump this up to 10,000 btu/h.

350-500 square feet. This normally calls for 10,000 btu/h, although in areas of this size air circulation concerns can begin to arise. For instance, if you are trying to cool two bedrooms and you put a 10,000 btu/h unit in one of them, will you have to get that bedroom too cold so that the other one is not too hot? These problems can be alleviated by using small portable fans in doorways to move air from one room to another, or by purchasing two smaller units instead of one larger one. While two small units will use more total wattage than one bigger one, they may not use more total energy because you won't have to overcool one area just to keep the adjacent area bearable. Plus, you can operate one without the other if cooling is only desired in one of the rooms.

500-700 square feet. In a normally insulated home, this is an application for a 12,000 btu/h unit. If you are near the top of this square footage category, live in a hot area, and don't have much insulation, move up to the next category.

700-950 square feet. This area will need at least 14,000 btu/h, and that's normally about as big a unit as you can get that still only uses a standard 115v outlet.

950+ square feet. Units of 20,000 btu/h and above are available (all require 240v circuits), but are problematical in the sense that they create a lot of very cold air right where they are located, and because of the large volume of air they have to move, they tend to be relatively noisy. Unless you have an area that is very open and where really good air circulation from room to room occurs, it's probably better to use two smaller units for areas of 1,000 square feet or larger.

Consider your own goals: If you're mainly wanting to take the edge off of really hot days so your house isn't sweltering, any level of cooling is going to help, especially if you supplement the air conditioner with fans. On the other hand, if you're going to insist that your house remain at 72 degrees regardless of how hot it is outside, make sure you use the above data get the size unit that will keep pace with the heat, remembering that the categories don't take into account ALL of the variables and using your common sense to bump up the size a little or down a little based on what you know of your home.

Don't forget that a unit which is oversized can be worse than one that's a little too small. Not only will it needlessly consume more energy than a smaller model, but it will also cycle off (stop cooling) before the area becomes properly dehumidified, reducing comfort. A smaller unit running constantly or nearly so generates the most comfortable environment.

Having arrived at a size, you're ready to look at specific units:


While you're going to pay from $200 to $700 or so for an air conditioner in one of these size categories, the real cost of air conditioning is in the energy it consumes, and this is especially true if you live in the Northeast or one of the other areas where electricity is expensive, or in the Southern states where summers are long and hot.

This is where today's window units really shine. By looking for the U.S. government's Energy Star label, you can pick an air conditioner that consumes less than half of the energy that a comparable model would have used just a few years back.

Regardless of whether a unit is Energy Star rated or not, you can tell how much electricity it will use by looking at the EER: Energy Efficiency Ratio. The EER is calculated by dividing the unit's btu/h capacity by the wattage it uses. An 8,000 btu/h unit that uses 1000 watts would have an EER of 8 -- a very average showing. On the other hand, an 8,000 btu/h unit that uses 800 watts would have an EER of 10, which is a good showing. The federal EnergyGuide label on the appliance has a chart which indicates how well the model you are considering stacks up next to other similar models in terms of energy efficiency.

Using the above example, how much money will you save with an efficient unit versus a less efficient one? An 8,000 btu/h air conditioner with an EER of 8 will use one extra kilowatt/hour of electricity for every five hours it operates, compared to a unit with an EER of 10. If you use the unit 300 hours per month (10 hours a day), it will cost $5.40 a month less on your electric bill to run the more efficient unit, if you pay the national average electric rate of 9 cents per kilowatt/hour. Obviously, the more hours you use the unit and the more you pay for power, the more you save with a more efficient unit.


Units are available these days that have many advanced features such as automatic timers that will start and stop the unit at certain times, and digital thermostats that will help to maintain a precise temperature. These can be useful -- just consider whether you actually have a use for them and if so whether you're willing to pay the extra cost they add to a unit. Timers in particular can pay for themselves because they can help you control energy usage by allowing you to set the air conditioner to start up, say, in the afternoon before you get home from work so the house is cool when you get home. This saves a lot of power over leaving the unit running all day long.

Digital thermostats look high-tech and give the sense that temperature control is precise, but remember that they're sensing the temperature right there at the unit, which may be different from the temperature elsewhere in the area you're cooling. Standard rotating knob thermostats are quite accurate if you use them properly:

The first time you use the air conditioner, turn the thermostat all the way down to the coldest setting. When the room reaches a comfortable temperature, slowly rotate the thermostat knob back toward "warmer" readings. Stop turning the thermostat when you hear a slight click from the thermostat and hear the compressor cycle off. Now you have pre-set the unit to maintain that temperature. There is no need to turn the thermostat down low again each time you turn the unit on -- the compressor will automatically run until that pre-set temperature is reached.

Some units now even have remote controls. Again, these are a cool and high-tech type of feature, but for most people they really do not have much of a practical purpose: Turning the temperature up and down all the time is hard on the unit and increases energy use. If you find a comfortable temperature and leave it set there, there's little need for a remote control. Of course, there are a few exceptions: They can be handy for elderly folks who have trouble getting up and getting to the unit, and they are also useful if a unit is placed in a window that's inaccessible due to furniture placement or the fact that it's up high on a wall.


By far, the most important feature of a window a/c -- the one you'll remember every hour you use it as long as you own it -- is noise. Picking a quiet unit will ensure that you can use it comfortably in a bedroom as you sleep, or if it's in a living room, that you can watch TV without having to crank the volume up every time the unit is turned on.

In appliance stores they typically operate the fan only when the unit is in "demo" mode because they understandably don't want to use all of that power to run all of the compressors, and because they don't want the air conditioners dripping condensation all over everywhere. However, if you are seriously considering a unit, have the salesperson set it so the compressor will come on, at least for a minute or two. Remember that it's going to sound louder in the store than it will at home because, installed in your window, the compressor is going to be outside and not right there in the room where you can hear it so easily. Still, this gives you a way to compare the noise of the fans and compressors of different models. Many very quiet units are available today, and air conditioners in general are much quieter than in years past.


Most models have louvers which allow you to direct the air up or down, and some also let you direct it from side to side. This feature is not a really big deal unless you want to locate the air conditioner somewhere such as in a corner or near a doorway and want to send some or all of the air a certain way.


Most air conditioners come with multi-year warranties on the compressor, which is the most expensive part to repair. Whether or not to buy an extended warranty is a matter of personal choice, but as a general rule air conditioners are reliable these days, and with proper care will last many years. Personally I do not purchase an extended warranty with a window a/c unit.

Use caution carrying and installing an air conditioner. Except for the smallest bedroom units (5,000 to 6,000 btu/h), most weigh enough that two people are needed to carry and install them safely.

If possible, choose a window which does not receive direct sunlight all day (the North side of a house is best if possible, and windows which get some tree shade are also good). Units operate more efficiently if they're shaded some, but make sure that obstructions aren't within three feet or so of the unit, because they need adequate air circulation around them. This also applies in the room: Don't install the unit in a place where the air which blows out of it will be obstructed by a nearby wall or furniture. This will cause the compressor to "short cycle," resulting in unsatisfactory cooling and drastically increased wear to the air conditioner.

When operating the unit, use a high fan speed at first to lower the temperature of the area, and then reduce the fan speed a little to cut down on noise and reduce annoying drafts.

Window air conditioners are an efficient and effective cooling option. The best units exceed the efficiency of central air conditioning systems when the ductwork losses of central systems are factored in. With a little time and planning, you can stay cool and still stay within a budget.

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