A room air conditioner is a small enclosed cooling device that is able to be mounted in a window or through a wall for venting and for air exchange. Recently, there has also been an introduction of portable air conditioning units, which are housed in cases that sit on the floor near a window with a tube for venting. These portable units usually have casters mounted to the bottom of the case allowing them to easily move from one room to another as needed.

The purpose of air conditioning equipment for a room is to cool the air within a small, confined area and they are not intended to be used as a "whole house" air conditioning system. They are most often used in houses or apartments where people are renters; therefore, they are not permanently installed and can be removed and taken to a new location if the occupant moves.

Typically, a room air conditioner will have a control panel mounted somewhere on the front or on top, in order to give the user easy access to the controls. Generally, there are independent controls for setting the target room temperature and for controlling the fan speed for circulating the conditioned air around the room.

Most people are not aware that room air conditioning units do not "technically" cool a room. It is more correct to say that room air conditioners remove the heat from the air and then moves it elsewhere to where the unit has been vented.

A pressurized refrigerant, R-22, flows through the air conditioner unit when it is turned on. While the unit is on, the condenser will pull the refrigerant gas into the air conditioning system and then pressurize it, which raises the temperature of the gas. This heated, high-pressure gas travels next to the condenser coils, which are located on the outdoor portion of the unit where the fans distribute the heat that has collected.

The cooled gas then becomes condensed into a liquid, which moves to the interior part of the room air conditioner to the evaporator coils. At this point, these coils can absorb the heat that is in the room. The fan then moves the air throughout the unit. The thermostat controls the cycling of this process to maintain the desired temperature in the room.

Some room air conditioners have mechanical controls, such as knobs and push buttons. The newer air conditioning units are controlled with electronic touch-pads that tend to provide more functionality and can regulate the mode the units operate in, the temperature set point, a timer for automatic shut off and powering on, and various other conveniences, most of which are designed to make the unit as energy efficient as possible.

In order to help consumers choose the best room air conditioner unit, small air conditioners are rated for their energy efficiency level by EnergyStar.gov, which is a service of the U.S. Dept. of Energy. The Energy Efficiency Rating (EER) is the cooling capacity of the unit, in terms of BTUs divided by watts of electrical power used by the unit. To have an Energy Star rating, a unit has to have an EER that is 10 percent higher than similar models.