Choosing a desktop Uninterruptible Power Supply (UPS)

Local power quality and what gear you've got drive the desktop UPS decision

by Daniel P. Dern - A (good) surge protector can protect electronic devices that can tolerate losing power, whether for a fraction of a second, or a few hours, like printers, scanners, or a television. (Unless it's being used for something that you can't afford to have be stopped or interrupted.)

[ Using and caring for your desktop UPS ]

But to ensure your computer, external hard drive/file server/media center, cable box/DTR, router, answering machine and other devices not only are protected but also don't stop, you need an Uninterruptible Power Supply (UPS).

UPSs fall into the same categories as backups: 1) things that desktop computer users need, and 2) things that are far easier and less expensive to get ahead of time than to discover you should have. For desktop/SOHO users, UPSs are available for anywhere from $50 to $200. Because getting the wrong one may be as bad as not having one at all, here are 5 tips for selecting a desktop UPS.

[ See also: What to look for in online backups ]


One of the two ways that UPSs are sized is the amount of power they deliver, usually stated in Volt-Amps (VA).

If the computer, monitor and whatever else you plug into the UPS' Battery Backup outlets call for more power than the UPS can deliver, the UPS won't work when there's no power; they're not set to prioritize based on outlet or whatever.

A good UPS will sense that you've plugged in more stuff than it can provide backup power for, and fail or alert instantly to let you know you've plugged too much in. But some won't, so be sure to try it out, with everything powered up, but no unsaved files open, by removing the UPS plug from the wall, and see what happens.

Higher-power UPSs cost more than lower-power ones, so you need to strike a balance between power and price. Also, vendors can calculate how much power a device uses in different ways, so you can't simply add device power numbers up. Plus, some devices draw more when starting up, or in heavy use.


How long will the UPS supply power?

That UPS battery isn't a Green Lantern Corps Power Ring that's good for 24 hours. Desktop UPSs are typically good for five to twenty minutes of power, depending on how big the battery is, and how much/little stuff you're powering.

You should be looking for enough power to weather a brief outage, and let you close out your applications in a calm, orderly fashion. If you want to keep going when the power's out for longer, you're best off with a notebook computer -- keeping in mind that the UPS may not power your broadband/WiFI for more than an hour or two.


How does the UPS decide when to spring into action?

In terms of how UPSs work, there are three main "topologies,' in increasing order of tolerance for power problems (and in price):

STANDBY - Here, the battery power is used only when the voltage drops below a certain level, like below 90 volts (or completely goes away). This is the least expensive type of UPS, it's what most people get.

strong>ON-LINE INTERACTIVE - This type of UPS has a transformer, so if the AC voltage gets too low (or too high), it brings it back within a target range, and only "goes to battery" when the incoming voltage drops too low or goes away. Get this if the power where you are is somewhat flakey; "going to battery" frequently is bad for the battery, reducing its capacity and usable lifetime.

strong>DUAL-CONVERSION, a.k.a. ON-LINE - Here, the UPS is always taking the AC power, turning it into DC and then back into AC. Get this if the power where you are has lots of interruptions, sags or micro-outages...or if you have equipment that needs sine-wave-like power (per below)


What the UPS power output looks like

If you look at the alternating current from your wall outlet on an oscilloscope, the power level looks like a sine wave.

The UPS battery is putting out direct current and the UPS' circuitry is turning that into alternating current. The least expensive UPSs are simply burping out "square wave" A/C), slightly more expensive UPSs are putting out A/C that looks like stair steps, and the most expensive ones put out A/C that's a sine wave looking the wall current or close to it.

Many devices can use the cheap square-wave power just fine. For example, your computer and monitor, which have power supplies, probably can. But many devices can't, like some wireless routers; they won't work right, might not work at all, or might even be damaged. So you need to do a little research (or testing).

Dual-conversion UPSs cost more, but the difference in cost will be a lot less than not being able to work, losing work in progress, or damaging your equipment.


All UPSs have outlets providing battery backup power, meaning that when there's a power problem, these outlets will still push out power. These outlets are where you plug in your computer, primary monitor, cable/router box, external hard drives... the gear that you want to keep running when the power blinks or goes out.

Some also have "surge only" outlets that provide whatever protection the UPS has against surges, spikes, and noise, but not against dips, brownouts or outages. Plug printers, scanners, and other gear that you don't mind stopping when the power goes out into these outlets.

Some UPSs also have Ethernet, cable and/or phone jacks, letting you also protect these lines against surges and spikes.

A plain old standby-type UPS with enough power may be good enough for you -- unless your utility power is somewhat flakey, or your cablemodem and router need better power.

Daniel P. Dern is a freelance technology writer based in Newton Center, MA. His web site is and his technology blog is

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