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As you put the finishing touches on that proposal--poof!--the lights go out, your screen goes blank, and you're left in the dark about just how much of your precious work was lost. Power outages are common, and no one can predict when one will hit. So unless you're ready to lose whatever you have in your system's memory any time the power fails, it's a good idea to safeguard your PC with an uninterruptible power supply. The heart of every UPS is a large battery that kicks on when an interruption occurs in the flow of AC power. Such batteries can typically buy you 5 to 15 minutes or more of additional computing time--enough for you to save open documents and shut down properly. And all good UPS units also protect against surges that can fry your PC's delicate electronics, another compelling reason for you to add one to your list of computing essentials.

For this review, we looked at three of the latest models from Tripp Lite: the $159 500 volt/amp (VA) Internet Office 500LP, the $299 450VA SmartPro 450RT, and the $169 500VA Smart 500 USB. We also tested a stylish new 500VA $199 Back-UPS Pro USB 500 Clear from APC.

Any UPS you buy should meet some basic requirements. A UPS with a rating of about 350VA is about the smallest you should use for a low-powered system with a small monitor. Both APC and Tripp Lite have guides to selecting the right size of UPS (see the links at right). Make sure your unit has enough outlets to handle your needs. Some of the outlets will be backed up by the battery; use them for essentials like your PC and monitor. Others will have surge protection only; use them for nonessential peripherals like printers and scanners. (Running peripherals off the UPS, especially power-hungry laser printers, can dramatically reduce battery run time.)

Power surges have also been known to travel down phone lines, so if you're connected to the Internet by phone line, even digital subscriber line, the UPS should provide in and out RJ-11 jacks with surge protection.

New UPSes Shape Up

The new breed of UPSes is starting to depart from the older cinder block look. The units come in surprisingly stylish shapes, and all are reasonably compact. All four offer both battery-backup/surge-protection and surge-protection-only outlets, signal lights, and audible alerts, and all have adequate battery run time. We tested the UPSes for endurance using a minimal load consisting of a sparsely configured 300-MHz Celeron test bed and a 17-inch monitor.

Three of the units, the APC Back-UPS Pro USB 500 Clear, the Tripp Lite SmartPro 450RT, and the Tripp Lite Smart 500 USB, will also shut your computer down in a controlled fashion in the event of a power failure--just as if you had selected Shut Down from the Windows Start menu. The bundled software periodically checks UPS and power line status via either a Universal Serial Bus cable or a serial cable. When the battery tells the UPS that AC is no longer available, the software invokes the Windows shutdown sequence before the battery runs out of juice.

The units all are backed by at least $25,000 of insurance for properly connected equipment in the case of damage caused by a power surge.