It happens every year. After a mellow start, the summer kicks into high gear, and suddenly the air in your house is so sticky you can't sleep-you can't even think. You go down to the basement to dust off the air conditioners, haul them up, and install them in their usual windows. It's instant relief-until the electric bill arrives.

A room air conditioner can be your best friend when it comes to chilling the air and cutting through the humidity, especially if you live in a climate with a short cooling season, or in an apartment or a house that isn't large enough to make central AC practical. But it can also eat up alarming quantities of energy, especially if you've been using the same old machine for years on end. A 10-year-old room air conditioner uses 35 percent more energy than a new model approved by Energy Star, the government-sponsored program that promotes efficient appliances.

What's more, the folks at Energy Star report that many consumers own air-conditioning units that are too large, as it's a common misconception that bigger is better. In fact, a too-big model will operate inefficiently, resulting in wasted power and less comfort for you. So if that machine you're putting into service each year is a giant, it's definitely time to start thinking about a replacement. (And don't just put the old one out with the trash.)

Here are some things to consider when you shop.

Window or wall: installation options
A room air conditioner can be installed in one of two ways: It can be placed in the window, with extensions on the sides to block any open space between the unit and the window frame, or it can be mounted in an opening made in the wall. Each configuration has its benefits: Window units can be easily installed, then removed for storage at the end of the season, while through-the-wall units are typically permanent and require you to knock a hole into a perfectly good wall. On the other hand, window units can block a nice view and might not be as visually pleasing as you'd like, whereas through-the-wall models can be less obtrusive. Whichever kind of installation you choose, look for a model made for that use.

Degrees of comfort: the capacity question
How much power do you need? Maybe not as much as you think. Yes, you want something that can do the job of cooling you off fast. But an air conditioner that cools too fast is actually not a good thing. Here's why: If the machine cools the space quickly, it will cycle off and back on again more frequently than a smaller unit that runs more consistently. Those short bursts of cooling aren't adequate to dehumidify the air, so you can end up with a room that feels cool but damp, which is neither pleasant nor a good use of your power.

The cooling capacity of a room AC unit is measured in British thermal units per hour (Btu/h). Most appliance stores will provide you with a chart to help you find the best size for the square footage of your room; you can also find one on the Energy Star website. As an example, for a room that's 280 square feet in size, you'll want a 7,000 Btu/h model. If the room is heavily shaded (by thick draperies or by trees and other landscaping elements outside), you can go about 10 percent smaller; if the room is very sunny, you'll probably want to go about 10 percent larger. You'll also need to consider where you'll use the air conditioner. If it's destined for your kitchen, the standard recommendation is to add 4,000 Btu/h to the capacity, to offset all those heat-generating appliances.

Do you live in a climate known for hot, wet summers? "You want to balance the actual cooling effectiveness with dehumidification effectiveness," says Alex Wilson, co-author of Consumer Guide to Home Energy Savings, published by the American Council for an Energy Efficient Economy (ACEEE). "Removing moisture is an important part of the cooling process, as it deals with the latent heat in moist air." Fighting that moisture is part of the job of a good air conditioner, but some do it better than others. Choosing the correct capacity will help. Beyond that, you'll want to look at the available features. Some very efficient machines with high fan speeds cool quickly, leaving dampness behind. It can be useful to choose an air conditioner with an optional low fan speed, as well; it will cool less quickly but will fight moisture better on those really humid days.

EXTRA, EXTRA: FEATURES WORTH A LOOK

Timer: Do you dread arriving home at the end of a long day, knowing the house will be stuffy and hot? The alternative-running the AC units during your entire absence-can waste a huge amount of energy. A timer lets you avoid both the guilt and the unpleasant wait for the house to cool down. Set it for a short time before you get home, and it will turn on the machine and start the cooling process for you.

Variable fan speeds: As described above, this option lets you choose between the speediest cooling and a slower, steadier fight against humidity, depending on the weather. This will keep you comfortable and save energy.

Control panel with thermostat settings: Another energy saver, this feature lets you set the machine for the precise temperature you want-say, 76 degrees. The machine will cycle off when it reaches that point, saving power.

Easy-to-remove filter: Keeping the filter clean means better operating efficiency, so look for a model that makes removing it a quick and simple task.

AC LINGO: WHAT THE LABELS MEAN

Energy-efficiency ratio (EER) This acronym will pop up on air-conditioner labels and in manufacturers' literature. The term represents the cooling output (in Btus) of a room AC unit divided by the power it consumes (in watts). The higher the EER, the more efficient the machine.

Energy Star This program is sponsored jointly by the Environmental Protection Agency and the Department of Energy (DOE). For a room air conditioner to earn the Energy Star rating, it must use at least 10 percent less energy than the federal standard. When you see that familiar blue star, you know the model is among the better performers. There is a wide range of efficiency in AC models, however, and some do quite a bit better than this minimum.

EnergyGuide This yellow sticker-created by the Federal Trade Commission-is required to appear on all room air conditioners and on numerous other household appliances in retail stores. It tells you the EER of the air conditioner you're considering, and compares it to the EER of similarly sized and configured models. This sticker also gives an estimate of annual operating costs. An 8,000 Btu/h unit, for instance, might have an EER of 10 and cost around $80 a year to operate, assuming average use.

One last check: side-by-side comparisons
Once you have a general sense of the style, size, and features you're looking for in a new room air conditioner, you can do one last bit of homework before making a purchase: Visit the Energy Star website's room air conditioner page and take a look at the product list. You'll see every Energy Star-approved model on the market, and can compare them by energy consumption and the percentage by which each exceeds the federal standard. Armed with that information, you can decide what's best for you.

Whatever you ultimately choose, there are some simple steps you can take to make your new purchase even more effective. "With any heating or air-conditioning system, the first priority is to look at demand and ask yourself, 'How can I reduce the amount of air-conditioning I need?'" says Wilson. "There are very effective strategies to reduce your cooling load." These include blocking some of the incoming sunlight through thoughtful placement of trees, shrubs, and other landscaping elements; using draperies and blinds indoors; and employing ceiling fans to keep the air moving. "Fans don't cool the air in a room," Wilson explains. "But they increase the velocity of airflow, which causes moisture to evaporate from your skin, so you feel cooler at a warmer temperature." Increasing ventilation throughout the house at night by opening windows and using a whole-house fan, then closing up again and pulling blinds in sunny spots during the day, can also do a lot to maintain a comfortable temperature.

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