An uninterruptible power supply (UPS) is a device that provides battery backup when the electrical power fails or drops to an unacceptable voltage level. It is used to keep computers and equipment safe when there is a loss or significant reduction in the primary power source. A UPS commonly includes a battery that is kept charged and ready, and when power fails, the battery supplies power as long as it lasts. The UPS can be set up to alert file servers to shut down in an orderly manner when an outage has occurred, and the batteries are running out. There are different types of UPS systems, including standby, line interactive, and online UPS. The most commonly used type of UPS is the full-time or full double conversion UPS, which is also the most effective. UPS units range in size from ones designed to protect a single computer without a video monitor to large units powering entire data centers or buildings. The main advantage of an online UPS is its ability to provide an "electrical firewall" between the incoming utility power and sensitive electronic equipment.