UPS systems ensure a controlled flow of energy to a given application. They protect critical operating equipment and sensitive equipment that is vulnerable to loss of power by providing a filter function. This shields equipment against voltage distortions and fluctuations in frequency or amplitude. In addition, UPS systems ensure an uninterrupted flow of power to electronic devices in the event of a brief or even longer power outage.

The three general categories of modern UPS systems are on-line, line- interactive and standby:

• An on-line UPS uses a "double conversion" method of accepting AC input, rectifying to DC for passing through the rechargeable battery (or battery strings), then inverting back to 120 V/230 V 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.
• In a standby ("off-line") system the load is powered directly by the input power and the backup power circuitry is only invoked when the utility power fails.

UPS systems are in use in many areas, including
Telecommunications, Data centers, Process supply in the chemical and petrochemical industries, Aviation, air traffic control, Power supply in hospital, to operating rooms and intensive care unit