Room air conditioners are a good way to cool frequently-used rooms that can be closed off from the rest of the house (i.e. studies and bedrooms). A typical unit can easily provide cooling to one or several small rooms.
Room air conditioners can be mounted in windows or directly into walls where they are known as through-the-wall units. Because they are sized to cool just one room, a number of them may be required for an entire house.
In some cases, several small air conditioners may be better than one larger, central unit, because they can be turned "off" individually when not needed. In areas where the cooling season is not too severe, a small unit can cool one or two rooms, which can be used if the rest of the home becomes "uncomfortable".
Window units or through-the-wall units usually have capacities of one-third-to-two tons meaning they are able to move 4,000 to 24,000 BTUs per hour. (One ton is 12,000 Btu/hr)
To get the best energy performance from a room unit, operate it on "re-circulate" rather than on "outside air". Keep the outdoor condenser coil "shaded" and the air flowing across it unrestricted by trees, bushes, fences or buildings. Indoor air filters should be checked regularly and cleaned (or changed) when they become dirty. If the unit's performance drops "off", it may indicate the refrigerant charge is low. This condition wastes energy because the air conditioner operates but it is not able to cool the space.
During the heating season, either remove the unit and tightly seal the window or cover the air conditioner with an insulating cover.
When purchasing a new unit, look for its energy efficiency label and EER rating. A unit with a high EER usually costs more to purchase, but costs less to operate. Unless they are rarely used, over its 10-to-15 year "life-span", high EER units are extremely cost-effective and provide quick paybacks.
As a general rule, small units and portable air conditioners are considered "energy efficient" if they have an EER of 8 or above, although most later model ones are higher than that. A large window or through-the-wall air conditioner is highly efficient only if the EER is 10 or higher.