(FORTUNE Magazine) – Winter ice storms. Summer thunderstorms. The electrician working in the office next door. The guy digging with a backhoe down the street. They're all potential enemies of your computer.

Sooner or later, one of them will knock out the power to your home or office machine. When the electricity dies, a lot of things can happen--all of them bad. At best, you'll lose whatever changes you've made to the letter, spreadsheet, or report on your screen. If your computer is writing to its hard disk when the lights go out, there's a good chance you'll scramble some or all of the drive. At worst, if the power comes back with a surge, it can fry delicate electronic components and render your entire PC past tense.

But a relatively inexpensive gadget can protect you from blackouts, brownouts, surges, and other electrical demons. An Uninterruptible Power Supply (UPS for short) provides a battery backup that will keep your computer running for a few minutes when the electricity dies, giving you a chance to save your work and shut down the system in an orderly fashion.

Once loved only by corporate information-systems techies, the UPS has evolved into a consumer product as well, available in dozens of variations that range in price from $100 to $2,000 or more. One of the slickest for home and small-office use is the $180 UPStart from SL Waber of Mount Laurel, New Jersey. It not only provides battery backup for your computer and monitor but also acts as a surge suppressor, line conditioner, and one-switch power strip for your printer, modem, scanner, and other peripherals.

The UPStart includes software too: a program for use with Microsoft Windows called Electronic Bookmark. In the event of a power outage, it is supposed to sense the problem, save works in progress, and shut down your computer before the battery runs out. When the power returns, the software is designed to restart your programs and reopen your files right where you left off.

Unlike most UPS units, which are squarish lumps that sit on the floor, the UPStart is a flat, stylish box about 1 foot square and 2 inches high that fits under your monitor. On the back, you'll find two outlets for computer and monitor that are connected to separate 250 volt-ampere lead-acid batteries. (Each battery can deliver just over two amperes at standard household voltage, enough for an ordinary monitor or PC.) Three additional outlets aren't backed up by batteries but have circuits to protect from surges and spikes. They're intended for your printer, scanner, and other noncritical peripherals. Panel switches on the front of the box let you control each outlet, or you can start up the whole shebang with a master switch.

There are also two phone jacks--one for a cord that runs to your wall jack and another for a cord from the UPStart to your modem. The box will ward off power surges in the phone line, a frequent killer of modems. Finally, you'll find a cable that runs from a port on the back of the UPS to a serial port on your computer. It enables the Electronic Bookmark software to monitor for a signal from the UPS when the power goes out.

To install the UPStart, just plug it into a wall outlet and wait an hour while it charges. Then plug your computer, monitor, and other peripherals into the UPStart and run the serial cable to your PC.

Since I couldn't conjure up a thunderstorm to test the UPS, and it's still too warm for snow or ice to knock out the electricity in these parts, I did the best I could by yanking the power cord from the wall and plugging it in repeatedly. I also went down to the basement and flipped the electricity off and on at the circuit-breaker box.

The UPS worked exactly as advertised. The screen never flickered as the battery backup kicked in, and there was enough power to keep the computer running for five minutes, just as SL Waber claimed. If you're at your desk when the electricity dies, that's plenty of time to save your work and turn the system off.

However, I wouldn't depend on the Electronic Bookmark software to do all this for me. First, not everyone has an unused serial port--I had to disconnect a scanner to test the program. Second, since the software works only with Windows 3.1 or Windows 95, if you're a DOS or Macintosh user, you're out of luck. Even if you do have Windows, you may run into trouble. Although Bookmark is configured to work with Microsoft Word, Excel, Ami Professional, and several other programs, it wouldn't consistently save open files on my computer or reopen my files once the power came back on. (There's a lesson here: Never, ever leave your machine, not to answer the door for FedEx, not to pour a cup of coffee, not to walk the dog, without saving what's onscreen.)

Even so, the UPStart is a good value. While it's not the cheapest unit on the market, it does what it's supposed to do--keep you in business even when lightning strikes. For information, call SL Waber at (800) 634-1485 or (609) 866-8888.