An uninterruptible power supply (UPS) is a device which maintains a continuous supply of electric power to connected equipment by supplying power from a separate source when utility power is not available.

Uninterruptible Power Supply (UPS) differs from an auxiliary power supply or standby gas generator, or diesel generator, which does not provide any instant protection from a momentary power interruption, however could be used to provide uninterrupted power to equipment for 1 - 20 minutes until a generator can be turned on. Integrated systems that have UPS and standby generator components are often referred to as emergency power systems.