Electrical outlets seem like a pretty simple proposition: You plug your stuff into them, and they work. (Even when you travel to other countries, with a little help.) But what we're not seeing our computers deal with is the little blips and interruptions coming out over the grid, all the time. Your average computer sold in the U.S. is meant to work off 120-volt AC power, oscillating at 60 Hz, but often that's not exactly what it's getting. Even if you never see the effect of those slip-ups and surges, the spikes and sags and tiny little brown-outs, the millisecond bumps in oscillation, your computer is feeling them.
The Institute for Electrical and Electronic Engineers tells us the typical AC outlet in North America gets a surge more than 600v (that's five times what your computer wants, and twice as much as what's dangerous to the equipment) about 13 times daily, and one more than 3000v (10 times the recommended maximum) about three times a week.
In fact, it's estimated that 47 percent of computer problems can be traced back, eventually, to this so-called "dirty power" [source: Superuser] An Uninterruptible Power Source, or UPS, is designed to keep the power coming steadily and constantly, even in the face of a total power outage, by using a battery and modulation system to filter the electric power. Essentially, it means your computer is always running off the UPS's battery, which the UPS is always recharging. That means there's no switchover time, like you'd imagine with a generator or other backup.
If you're convinced, just remember to keep your UPS grounded, replace the battery every three years -- you can get the replacements when you buy the unit, for convenience's sake -- and don't plug your printer or any other external devices into it. And, as is often recommended, it's probably a good idea to let the battery completely die at least a few times a year and recharge it from baseline.