As long as you do not need to cool your entire house, or are willing to manage without AC in some parts of the house, room air conditioners can be a cost-effective choice when compared to central AC. They are relatively inexpensive, and have lower running and maintenance costs.
When buying a window AC, choose an efficient unit. It will not cost you significantly more. Also consider features like capacity, noise, ease of installation and size.
Since window air conditioners are designed to cool individual rooms, you have to know the area of the room where the AC is going to be installed. Size depends on the area to be cooled.
The chart below, based on Energy Star data, can help you with the sizing.
Consider also a few adjustments; if the room is very sunny (or shady) consider a unit with 10% more (or less) capacity; if the room has large windows and heat-generating appliances (kitchens, for instance) increase its capacity by 3.000 or 4.000 BTUs; and if the room is higher than 8 feet or under an attic with low insulation levels, you should increase the capacity of the AC by 1.000-2.000 BTUs...
Take also into account your climate. The chart above is based on average summer temperatures around 85º-95º F/30º-32ºC; but if average temperatures in your region exceed 100º F (38ºC) you should buy a unit with a higher capacity.
When buying a window AC choose a high energy efficiency unit (you will not pay much more for it) with less than 75 decibels (Energy Star models). Do not underestimate the issue of noise (room AC are typically noisy) or the importance of efficiency (you will get higher energy savings).
Window AC units are the cheapest type of air conditioners. They can be a good alternative to central AC, if you are willing to adapt your daily routines. They will cost you a small fraction of a central AC and will have a much lower maintenance and running costs. Also buy the most energy-efficient unit you can. The higher the energy efficiency (rated by their EER: Energy Efficiency Ratio) the better.
Room air conditioners with a higher EER are not necessarily more expensive, and can provide significant energy savings.
Window AC units with a rating of 10.7 EER or more are now standard (a model with a 11.7 EER will provide 10% in energy savings when compared with a model with a 9.7 EER).
Also take into account the Energy Star label or other certified units. Pay attention to the yellow Energy Guide label, in the North America. It shows the energy consumption, and provides an estimate of the annual running costs.
Noise and other features
Noise is a common complaint about room air conditioners in many customer reviews, and Energy Star qualified models can provide lower sound levels.
Also consider features such as thermostats, built-in timers, ease of filter removal, or special installation requirements: very large units may require a 220-volt plug, and may not be easy to install.
Prices, models and reviews
As long as you buy the right sized unit, low-consumption window AC units can do a good job. Most reviewers give high ratings to their room AC units.
Prices are are also attractive when compared with those of central AC systems (and lower than those of ductless mini-split systems, or even many portables).
The prices of small 5,000-8,000 BTUs units (for rooms up to 150 sq feet) can be as low as $150-$200, while units with 10,000 BTU or more are often in the range $250-$350. For customer reviews and prices at Amazon.com, see: Window-Room AC.
Window air conditioners are in direct competition with ductless mini-split AC and portables.
Ductless units can have several air-handlers (serving up to four rooms), and are especially designed to be mounted in walls or ceilings. They are a good alternative to window air conditioners if you need to cool more than just one room, or a more air-tight solution, or when the installation of a AC window unit is impractical. See: Ductless AC.
As to portable air conditioners, they aren't a good choice. Their performance is often unsatisfactory, their venting is not easy, and the fact that they are portable – a good idea at first glance – can be rather deceptive. See: Portable AC.