An uninterruptible power supply (UPS) is an electrical system containing storage systems (e.g. batteries, flywheels) capable of supplying high-quality electrical power without interruptions. The input of the UPS interface is connected to the main electrical supply, and the output is connected to the customer electrical load.

A UPS has many advantages. It provides protection against all types of power supply failure, filters a vast range of disturbances found in the mains supply and, as such, provides more sensitive loads with a clean power supply. With extra storage capacity, it can be designed to support the critical load (for instance, when an alternative supply such as a standby generator is brought on-line) for up to one hour.

Given the importance of communication technologies and online processing (e.g. online banking), UPSs are more and more common in computer rooms, data centres and server areas to secure and maximise systems availability.

Replacing an old UPS can achieve great improvements in energy use and management. A new unit can, for instance, help to preserve battery life and maximise runtime. Highly-efficient UPS can cut energy losses by 30-55 %.