An uninterrupted power supply‎,‎ also uninterrupted power source‎,‎ UPS or battery/flywheel backup‎,‎ is an electrical apparatus that provides emergency power to a load when the input power source‎,‎ typically mains power‎,‎ fails‎.‎ A UPS differs from an auxiliary or emergency power system or standby generator in that it will provide near‎-instantaneous protection from input power interruptions‎,‎ by supplying energy stored in batteries‎,‎ super capacitors‎,‎ or flywheels‎.‎ The on‎-battery run‎-time of most uninterrupted power sources is relatively short ‎(‎only a few minutes‎)‎ but sufficient to start a standby power source or properly shut down the protected equipment‎.‎

A UPS is typically used to protect hardware such as computers‎,‎ data centers‎,‎ telecommunication equipment or other electrical equipment where an unexpected power disruption could cause injuries‎,‎ fatalities‎,‎ serious business disruption or data loss‎.‎ UPS units range in size from units designed to protect a single computer without a video monitor ‎(‎around 200volt‎-ampere rating‎)‎ to large units powering entire data centers or buildings‎.‎ The world‎'‎s largest UPS‎,‎ the 46‎-megawatt Battery Electric Storage System ‎(‎BESS‎)‎‎,‎ in Fairbanks‎,‎ Alaska‎,‎ powers the entire city and nearby rural communities during outages‎.‎