Back in 2003, the use of air conditioning put such a major demand on the nation's electrical power grid, that many local governments asked people to keep their air conditioning off.

The Association of Home Appliance Manufacturers now offers a worksheet that can help you estimate how powerful an air conditioner you need. People who use this worksheet can get the most out of their air conditioning. The worksheet guides you through the measurements needed to calculate the size of the air conditioner. The answers are automatically calculated.

The principal behind air conditioning equipment, is that they are designed to reduce the humidity of the air that they process. The relatively cold evaporator coil on the unit will condense water vapor from the processed air, like a cold drink will condense water on the outside of a glass. The condensed vapor becomes water, which is then sent to a drain which removes water vapor from the cooled space resulting in a lower relative humidity.

Believe it or not, a larger air conditioning unit may not always supply more cooling. An air conditioner needs to be sized for the room to be its most efficient. If properly sized, it should operate long enough to get rid of moisture from the air. When moisture is extracted from the room, it will actually make the room feel cooler. Unfortunately, many people make the same mistake that I used to. In their rush to purchase a window air conditioner, they buy a model thats too large.

The fact is, many people dont realize that too big of a unit can actually provide less cooling than a smaller, properly sized unit. This is because an oversized air conditioner wont run long enough to cool. Consequently, the short run time results in a poor job of cooling, because it is oversized for the room.

Air conditioners are rated in btu's. British thermal units are a measurement of how much warmth is being transferred from the room being cooled to where the heat is being dumped out doors. This is why the cooling coil inside the room is cool to the touch, while the part that hangs outside is super hot.

A window air conditioner used in the average household room can be as low as 4000 british thermal unit. While 5000, 6000, and 8000 are the most common sizes bought.

So the question remains, how do I determine what size a/c do I need?

Below you will find a standard sizing chart for calculating the proper window air conditioner for a room. You can calculate the size of the room by multiplying the width of the room by the length of the room. Use your room size to determine what btu's are required to cool the particular room size.

Room in sq ft-----room in sq m-----a/c size in btu's

100 - 250............9 - 23............5000 - 6000

250 - 400...........23 - 37............6000 - 8500

400 - 550...........37 - 51............8000 - 11000

550 - 875...........51 - 81...........11000 - 15000

The information above is based on a room occupied by two adults, with average insulation, number of windows and average sun exposure.

In addition to the size of the unit, its also important to put the air conditioner in the best room possible. Wherever it gives the most comfort is where it should be installed. If you only have one air conditioning unit, then I suggest the bedroom. This is because most people find that if they can sleep comfortably during a hot summer night then their next days work will be a lot easier. On the other hand, some people do not like a cool or cold sleeping environment, so they prefer to place it in a living or family room. Some people will place it in the family room since the whole family can sleep there. If you have younger kids, you could break out the sleeping bags and turn it into an adventure for everyone. Think of it this way, sleeping on the floor in sleeping bags may be better than waking up to find your pet filled with the family pets, kids and you.

Some people like the family room because they say that window air conditioners are noisy. This is in contrast to those who find the drone of the fan actually promotes good sleep. These people actually say that the air conditioner works as well as a sleeping pill.

Whichever room you pick don't expect the impossible from your a/c. Window units are not typically meant to cool more than one room or one open area. This is because the blower in a window air conditioning appliance is not extremely powerful. It is sized to cool one open area, not a number of interconnected rooms. If you have multiple rooms that you expect to be cooled, a couple of smaller units will do a much better job, compared to one large unit.

Lastly, remember to properly secure the air conditioner in the window. This is important for two reasons: 1) you dont want the unit falling out of the window, and 2) you dont want someone to be able to break into your house. It may take some time to get your unit properly installed. Ive found that many units dont come with sufficient brackets and hardware to prevent them from falling out of the window. So you might need to make a few trips to the garage to cobble up some additional parts to ensure your security (this is what always happens to me). To ensure nobody can get in your house through the same window as the a/c, I use a trick I learned a long time ago. Install a piece of wood to stop the window from being lifted. If you have wooden sills this isnt necessary, because a wood screw at the top of the raised window will also prevent the window from being opened by an intruder. Believe it or not, there are people who actively look for window air conditioning units to see if they can gain entry by lifting the window above the unit.