Uninterrupted power supply (UPS) is an equipment which will provide emergency power to connected equipment by supplying power from the battery when utility power is not available.

UPS is not an auxiliary or emergency power system or standby generator. It does not provide instant protection from a momentary power interruption. It can only be used to provide uninterrupted power to equipment for about 5–15 minutes until an auxiliary power supply can be turned on, utility power restored, or equipment safely shut down. You should have sufficient time to shut down your computer before the UPS supply is shutting down if you do not have emergency power system or standby generator.

UPS is typically used to protect 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 come in sizes ranging from units which will back up a single computer without monitor (around 200 VA) to units which will power entire data centers or buildings (several megawatts).

There are three types of UPS available in the market, on-line, line-interactive and standby UPS.

On-line UPS uses a "double conversion" method of accepting AC input, rectifying to DC for passing through the battery (or battery strings), then inverting back to AC for powering the protected equipment.

A line-interactive UPS maintains the inverter in line and redirects the battery's DC current path from the normal charging mode to supplying current when power is lost.

A Standby ("off-line") UPS the load is powered directly by the input power and the backup power circuitry is only invoked when the utility power fails. Most UPS below 1 kVA are of the line-interactive or standby variety which are usually less expensive.

You can use standby UPS to protect normal office workstation but try not to use it for your network server where more information processing are going on. Use on-line or line-interactive UPS to protect computer with important data expeccially database server.