Buying a room air conditioner isn't as simple as looking for the biggest one that fits your window and your budget.

Certainly, cooling capacity is important, but so is energy efficiency. How well the air conditioner dehumidifies is also important, as are a host of other factors.

An air conditioner with too little capacity won't cool adequately. Less obviously, one with too much cooling power may not dehumidify properly because its compressor will cycle off too often. The smallest window-mounted air conditioners have a cooling capacity of about 5,000 British thermal units per hour. We tested air conditioners with capacities ranging from 5,000 to 8,000 Btu/hr. These should be large enough to cool a fair-sized room throughout much of the country.

A tag lists each new unit's energy-efficiency rating (EER) -- its capacity in Btu/hr. divided by power consumption in watts. The EER of models we tested range from 8.0 to 10.8. (A model whose EER is 10 should use about 20 percent less energy than one whose EER is 8, other factors being equal.) You may have to pay extra for a high EER model, but you should recoup that expense -- especially if you use the air conditioner often -- with lower utility bills.

Dehumidifying the air is especially important in muggy weather -- and running an air conditioner at low speed is the most effective way to dehumidify.

Other factors to consider include:

Airflow. Most air conditioners have adjustable vertical and horizontal louvers to divert airflow. And many offer a fresh-air-intake or exhaust setting for some ventilation. An energy-saver setting on some units stops the fan when the compressor cycles off.

Installation. The air conditioners we tested weigh from 44 to 87 pounds; installation could well be a two-person job.

Maintenance. Most manufacturers recommend cleaning the air filter every few weeks. Most filters in the models we tested slid out easily.

Reliability. From our knowledge of the industry and what our readers have told us of their experiences, we believe that few 1- to 6-year-old air conditioners need repair. In the unlikely event of a breakdown, most models are covered by a one-year parts-and-labor warranty, and a five-year warranty on the sealed refrigeration system.

From our tests, we recommend the following models:

In the 5,000- to 5,800-Btu/hr. group (which can cool a 100- to 350-square-foot room), the $300 Sharp AF-R609X was top-rated. The Panasonic CW-606TU scored close behind, but it costs $70 more.

In the 6,000- to 6,600-Btu/hr. range (suitable for a 175- to 450-square-foot-room), the Amana Quiet Zone 7M11TA ($400), the Kenmore 78079 ($400) and the Fedders A3Q06F2BG (one of our Best Buys at $270) are closely matched.

In the 7,000- to 8,000-Btu/hr. range (suitable for a 200- to 550-square-foot room), the Sharp AF-M808X ($395) and the Panasonic CW-806TU ($430) are tops. But you can save about $130 to $160 with the very good Goldstar LW-B0714CL ($265).

Regional retail chains such as American Appliance, Appliance Depot, Campo, Hechinger and Home Quarters generally offer the lowest prices. Among national retail chains, check out Circuit City, Kmart and Lowe's.