An uninterruptible power supply, also known as an uninterruptible power source, UPS or battery backup, is an electrical device that provides emergency power to equipment when the mains power fails.
A UPS backup can be used for protecting any type of equipment, although they are normally used to protect computers, data centers, telecommunication equipment or other electrical equipment where an unexpected power disruption could cause injuries, fatalities, serious business disruption and/or data loss.
UPS backup units range widely in size from units designed to protect a single computer without a video monitor (around 200 VA rating) to large units powering entire data centers, buildings, or even towns or cities.
Uninterruptible power supplies differ from an emergency power system or standby generator in that it will provide instantaneous or near-instantaneous protection from input power interruptions by means of one or more attached batteries and associated electronic circuitry for low power users, and or by means of diesel generators and flywheels for high power users. The battery running time of most uninterruptible power sources is relatively short, often in the range of 5–15 minutes being typical for smaller units—but sufficient to allow time to bring an auxiliary power source on line, or to properly shut down the protected equipment.