The efficiency of these units is given as an EER number. EER stands for energy efficiency ratio and is the cooling output of the unit divided by the power required to produce the rated cooling. For a given size unit a higher EER number means less electricity is required to run the unit.
These units come in a variety of models. There are cooling only units, cooling units with electric heat, and heat pump units. They can be either mechanically or electronically controlled.
The latest innovation is called low profile window air conditioners. These units are shorter and wider than the typical unit. This allows the window to be more fully closed which allows a more normal view out of the window.
There are also many options such as anti-bacterial filters, charcoal filters, and electrostatic filters. Some units can be operated by remote control.
When you go to purchase them you need to take some information with you. You need to know the size and type (casement or double hung) of the window it will be installed in.
These units come in 120 volt or 220 volt. Therefore, you need to know what voltage you have close to the window as well as the available amperage of the electrical circuit. This can usually be found by the type of receptacle that will be used. These units should not be plugged into an extension cord!