Evaporative coolers cool outdoor air by passing it over water-soaked pads, which can lower the air's temperature by 15-40 degrees. Evaporative coolers are installed as an alternative to air conditioners in low-humidity areas because they cost about 50% less to install and use about 25% of the energy. They do, however, require more frequent maintenance and windows must be partly opened to allow the warmer air indoors to escape as it is replaced by cooled air from the evaporative cooler. This also means that fresh air is constantly circulating in contrast to the recirculated air that is used in a central air conditioner.
Two stage evaporative coolers are often more efficient both as to energy and water usage, which is important in hotter, drier climates, and don't add as much humidity to the home as a single stage cooler would. In addition, choosing one with a vent-only option would allow the evaporative cooler to be used as a whole house fan during milder weather. (Energy Savers)
Estimate your potential to save energy and money by installing this product, please see the U.S. Department of Energy's Home Energy Saver calculator.