An Uninteruptable Power Supply is a device that is designed to provide electrical power to critical equipment in the event of brown outs or black outs of the normal electrical service. They exist in a wide variety of sizes, shapes, and capabilities, but are all based on the same principles. They employ a battery that stays charged while the electrical service is in operation. In the event of a power event such as a brown out or black out the battery takes over and supplies the required power to keep the equipment online. In the modern day era of critical computer (and other) systems it is common to find these devices in most businesses and a growing number of homes. Because of their cost, however, their use is usually limited to critical systems that need to run constantly. Obviously a battery has a limited time it will operate, but the idea is to cover short-term power events and/or at least provide some time to properly shut down volatile systems.