Whether replacing an older, less efficient unit, or buying a new unit for a previously un-air conditioned room, you should be impressed with the performance of newer models. The major attributes to consider include:
Cooling Capacity - The ability of a unit to cool an area is measured in BTU/hour. The higher the BTU, the more cooling power the unit has. However, don't chose a model with too much cooling capacity, it will cycle on and off too often, wearing on the machine and not staying on long enough to remove humidity from the air. Make sure you have a rough estimate of the sizes of the rooms you want to cool (in square feet) before you start shopping.
Energy Efficiency - Look for high efficiency units. Efficiency is measured by the EER (energy efficiency rating). All new units sold in the US must clearly display the EER. The higher the EER the more efficient the model and the lower your energy bill. A model with an EER of 10 should use about 20% less energy than one with an EER of 8.
Installation Requirements - Make sure that the unit is the right size for the window you plan to mount it in. Also, make sure you have enough help in installing the unit (most are awkward and very heavy). Some require installation of a metal shell, while other require special electrical wiring (220 volts)
Other attributes to consider are listed below.
Window versus Portable- Window units are mounted in a window. Portable are on wheels with a hose for hot air that must be vented out a window. If you are renting, or only need to cool one room at a time, consider a portable unit.
Wall Units - Wall units are designed to be mounted in holes cut into the side of exterior walls. They don't take up window space and may be more difficult for burglars to use as a means of entry. However, wall units require significantly more installation effort than similar window units.
Thermostat sensitivity - How closely a unit can maintain a given temperature.
Noise Level - Today's units are quieter than older ones. Listen to the unit in the store before you buy.
Air Flow - Adjustable louvers direct the air up, down, right and left. Some models have motorized louvers to blow cold air around the room. Most models have the ability to switch from fresh air to recirculating. Switch to outside air when you want to air out the room, switch to recirculate on the hottest days when the air conditioner is struggling to cool the room.
Low profile models - Some models hang below the window, minimizing the area of sunshine that they block.
Timers - Some units have timers to turn on or off on a schedule. Set the unit to turn on before you come home from work.
Filters - Some units have filters to remove allergens and odors
Remote Controls - Some have remote controls so you can activate from across the room.
Dehumidifying Ability - Part of creating a comfortable environment in the summer is reducing the amount of humidity. All air conditioners provide this benefit. The effectiveness is more dependent on the specific operating situation of the unit that on the unit itself.
How to Buy
For comparisons of different models, see consumer reports.
If possible, buy during the fall or winter to get lower prices. Air conditioners may be out of stock during the peak summer months.