Summer heat and humidity will soon set in, and here are some tips if you're looking to cool off with an air conditioner.

Need to know: The size of the room you want to cool, as well as how big the windows are, the height of the ceiling, whether the space is insulated, and whether it is fully exposed to the sun in the afternoon - the hottest time of the day. All these factor into the air-conditioner capacity you'll need.

Most capacity is rated in BTUs per hour. The people at Fedders, the air-conditioner manufacturer, recommend a 7,000-BTU unit for a 250-square-foot room; add 4,000 BTUs if the room is a kitchen, to compensate for the heat from ovens and dishwashers. Brick houses and rooms with high ceilings also require larger-capacity units.

Common mistake: Don't buy an oversize unit - it won't cool the space better. In fact, it will cost more to operate and will be less effective on hot, humid days because it will remove heat quickly but will remove only some of the humidity. The result: a cool but clammy room. For tips on calculating which size unit is right for you, visit www.energystar.gov.

Be sure to ask: Whether the unit has motorized louvers, which distribute cool-air flow more evenly; some units have louvers that must be adjusted manually. Also ask whether the unit can be switched from outside air to recirculating. Set the dial to outside air when you want to refresh the air in the room. Switch to recirculate on the hottest days, so the air conditioner's job is easier - keeping cool air cool.

Good advice: You cannot safely plug an air conditioner into an extension cord, so make sure there is an appropriate outlet nearby. Units that draw less than 7.5 amps of electricity can be plugged directly into any 15- or 20-amp household circuit not shared with another major appliance. A larger air conditioner requires its own 230-volt circuit.

Bad advice: "The unit will fit any window." That's true only if your house is new or if you've replaced your windows recently. If you own an older house, you might need to use cardboard or rolled-up plastic to fill in gaps and get the unit to fit properly.

If the air conditioner isn't tight in the opening, it will be noisier, cause the window to vibrate, and allow cool air to escape. Which means it will take longer and cost more to make the room comfortable.

Don't forget: An air conditioner can be heavy, up to 90 pounds. Get help lifting it into and out of the window.

What it will cost: The typical window air conditioner runs between $250 and $800. The more energy efficient the unit, the more it tends to cost.