The warmer temperatures have arrived. You have probably already installed those room air conditioners, which is a good way to cool frequently used rooms that can be closed off from the rest of the house.

We often get questions on the best approach to sizing a room air conditioner. In some cases, several small air conditioners may be better than one larger, central unit, because they can be turned off individually when not needed.

When looking for a new air conditioner, window units or through-the-wall units, usually have capacities of one-third ton to two tons, meaning they are able to move 4,000 to 24,000 BTUs per hour (one ton is 12,000 BTU per hour).

The most important number is the total square feet of the room you want to cool. For example, a room that measures 10 feet by 10 would need a 5,000 BTU unit. For square feet 100 up to 150, 5,000 BTU; 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.

Energy Star's website has some good information on sizing a room air conditioner. You may need to upsize it if it is a very sunny room or more than three people are always in the room.

A word of caution in the case of an air conditioner, bigger isn't always better. There is a tendency to want to buy the larger unit since it has more cooling power. However, the downside is that while it may "cool" the room down quicker, it doesn't run long enough to remove the humidity, which is the primary function of an air conditioner.


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When purchasing a new unit, look for its energy-efficiency label and EER rating.

A unit with a high EER usually costs more to purchase, but less to operate. Unless they are rarely used, over its 10- to 15-year "life-span," high EER units are extremely cost-effective and provide quick paybacks.

As a general rule, small units and portable air conditioners are considered "energy efficient" if they have an EER of 8 or above, although most later models are higher than that. A large window or through-the-wall air conditioner is highly efficient only if the EER is 10 or higher.

(This is a monthly column by Unitil on using energy more efficiently. For more about its energy efficiency programs, please visit unitil.com.)