A source of high-quality alternating power to sensitive electrical equipment. An ideal uninterruptible power supply (UPS) provides a critical load with power when the main supply suffers from an over- or undervoltage condition causing interruption of the supply. It thus ensures power without break for the critical load. The UPS should be reliable and highly efficient, requiring low maintenance, low cost, and light weight.

Uninterruptible power supplies were originally designed to furnish reliable and high-quality continuous power to large computer systems. They are now applied to a wide variety of critical equipment, such as medical facilities, life-support systems, financial transaction data storage and computer systems, telecommunications, industrial processing, and on-line management systems. UPS ratings range from a few hundred volt-ampere (VA) up to several thousand kilovolt-ampere (kVA). Three types of UPS systems are available: static, rotary, and hybrid static/rotary.