An Uninterruptible Power Supply (UPS), also known as an Uninterruptible Power Source, Uninterruptible Power System, Continuous Power Supply (CPS) or a battery backup 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. There are three distinct types of UPS: off-line, line-interactive and double conversion (also called on-line).