A UPS is designed to make sure the PC gets the power it needs at a consistent level from the outlet or from the battery inside the UPS. The UPS is usually a pretty ugly and pretty heavy box that you hide underneath your desk. You plug it into the wall and your PC and monitor into the UPS. The UPS contains a battery that is constantly being charged to stay at maximum capacity. When the power drops below a certain level (a brownout), or fails completely (black-out), the UPS immediately (within a millisecond or two) kicks in and starts supplying the PC with power from its battery.
Of course the battery in the UPS does not last forever, and it alerts the user via an audible alarm that the power has failed so that the user can save any data and shut down the PC gracefully. BTW – Another good indicator that the power just failed is when all the lights go out and you hit your shins on the cast-iron frame of your antique coffee table while clawing your way through the dark trying to find the flashlight whose batteries are empty.
A good UPS can be connected to the PC via a serial or USB cable. You install special software on the PC, which enables the UPS to automatically shut down the PC in your absence in case of a power outage. This is very convenient if you leave your PC on all the time or even just step out for lunch.
In addition, a UPS usually has a good surge protector built in, that actually works and prevents power surges from travelling into your PC through the power line. A good UPS will also have a surge protector for your phone line to prevent a power surge entering your PC through the phone line plugged into your modem.
Newer UPS’s have usually multiple outlets (6 or 7). Half of them are battery backed-up, meaning every device plugged into one of those outlets will be supplied with battery power in case of a power outage as well as surge-protected. The other half is surge-protected only. In the battery backed-up outlets you should only plug in the most important pieces: The monitor so you can see what you’re doing and the computer itself. These are the only two things you need to shut down your PC in a critical situation. Everything else, such as printer, speakers, etc. should be plugged into the surge-protected outlets.
Tip: Never plug a power strip into one of the UPS outlets. Not only will you reduce the back-up time provided by the battery, you also introduce a weak link in the chain which can pose a safety hazard.