Network users take electricity for granted until the supply suddenly shuts off. Then a reliable uninterruptible power supply becomes supremely important.

Such power supplies, known as UPSs, are basically large, manageable batteries. Their chief specification is run time.

In testing UPSs, the Government Computer News Lab considered several other factors, too. How many outlets did each UPS have? How many plugs were directly battery-attached as opposed to giving only surge protection? And how easy was it to monitor each UPS with the included software?

We set up a test workgroup of three desktop computers and a monitor, enough to tax but not overload our test UPSs, which were rated between 1,000 and 1,500 volt-amperes.

Most computer power requirements are stated in watts, although VA gives a better indication of capacity. Watts multiplied by 1.4 gives VAs. Our largest test workstation drew 220 watts, or 308 VAs.

Some monitor power requirements are in amperes, which can be converted to VA by multiplying by 120. A 1.3-amp monitor would draw 156 VAs from a UPS, for example.

No matter what devices are connected, their VA load should be less than the UPS's VA rating. A 1,000-VA UPS can support three average desktop computers plus a monitor, or two servers and a monitor.

The total power demand of our test network was 950 VAs -- close to but less than the top VA ratings.

The Para Systems Minuteman SmartSine S1400 was the best all-around performer with the longest run time. It sat on the floor and, at 6 inches by 17 inches, took up little more room than a shoebox.

The SmartSine was the only unit in the review with eight plugs for devices; others had six or four. All of the plugs connected to the battery unlike other units, some of whose plugs gave only surge protection and not backup power.

The SmartSine was the slowest to drain in our full-blackout test. The test network stayed online for 43 minutes 47 seconds -- the best time by a few seconds, and more than double that of some rivals. Considering its price, the SmartSine easily earned an A grade and a Reviewer's Choice designation.

Many users forget that power can also spike through Category 5 cables. The SmartSine covered this angle with a network surge-protection port in back.

There was a serial cable for management, but we were disappointed not to find a USB port. The included Sentry II software can manage multiple UPSs, but only one at a time. The SmartSine also could be managed remotely.

Any user with this box under the desk should find the software quite satisfactory for automatic shutdowns and preprogrammed responses to power events.

The Tripp Lite SmartOnline 1000 Rack/Tower UPS, another Reviewer's Choice, came with the best management software in the review. The black box could stand like a tower system or slide into a rack with two units of space available.

Six device plugs all connected to the battery for backup power. A detailed load meter on the front was a bit harder to read than those on the other units. In a power failure, the dual-function LEDs indicated something different from their normal operation. Most other UPSs had one set of displays for current load and another for percentage of charge remaining. A standard RS-232 serial port was available for managing the UPS. Setting up responses to power events with the software was easy.

Tripp Lite was the only vendor in the review to fully embrace the concept of multi-system, multi-vendor UPS management. A glance at the network management screen showed which devices might need new batteries or were in danger of failing or experiencing some kind of power event.

Despite the seemingly low VA specification, the SmartOnline kept our test network running only a minute shy of the top performer, which had a higher capacity rating.

The Belkin Universal UPS 1000VA was tiny in both size and cost -- the only one close to the $100 price range. At just 12 by 5 by 8 inches, it was dwarfed by the other test units.

Of the six plugs in back, four connected to the battery and two were surge-only. USB and serial ports were present for management, plus a network surge protection line -- mislabeled "Tel/fax/modem" even though standard phone connectors won't fit Ethernet ports.

The Belkin could not hang with the larger units in run time, but it did keep our test network going for 16 minutes 13 seconds. It wouldn't be well-suited to protect a large server, which drains a lot of power and requires 25 minutes or more for full shutdown. But for an office cubicle, the Belkin should be ideal. This powerhouse earned our Bang for the Buck designation.

American Power Conversion's Smart-UPS 1500 Rack Mount slid into a server rack and took up two units of space. The front LED panel showed all the basics such as power load, power levels and whether the type of current was correct.

The Smart-UPS 1500 was the only system in the review to arrive with the battery disconnected. That's the right way to ship batteries, but only APC did so.

Here's why it was right: If a connected battery discharges during air shipment, and the UPS starts beeping about its low-power state, airport security folks can get extremely nervous.

In the past, reconnecting the battery was a bit of a chore, but APC has gotten around that by shipping the unit without the faceplate attached. Now reconnecting the battery is as easy as plugging in a lamp.

The Smart-UPS 1500 had six plugs, all battery-connected. There were USB and serial ports for management. Other UPSs on the network were visible through the software, but it could manage only APC devices.

The Smart-UPS 1500 kept our test network going for 34 minutes 31 seconds. That's relatively long, but APC has previously aced the run-time category in our UPS reviews. This time it came in 10 minutes behind the top performer.

Nevertheless, the Smart-UPS 1500 had reliable power and good management, plus the best hardware-based display in the review.

Can a UPS be sexy? If so, the MGE Systems Pulsar Evolution 1500 is the supermodel of the UPS world. The slick, 2-inch-thick, blue rack-mount system had a very readable front LED panel, even better than the APC's. The glowing green power button shed enough light to read all the text on the display, even in a pitch-dark server room. That's a big help during a power failure.

Other functions fell a bit short, however. Although there were USB and serial ports for management, the Pulsar had only four battery-connected plugs, fewer than even the tiny Belkin. There was also a network surge protector port.

The Pulsar kept our test network going for only 24 minutes 39 seconds. MGE Systems removed a lot of the heavy magnetic components to make the UPS thin and lightweight, cutting down on run time.

The Tripp Lite SmartPro 1500XL Net UPS was the boxlike brother of the SmartOnline 1000. Both Tripp Lite units had the same management functions, but the 1500XL Net UPS costs more and did not perform nearly as well. There were six battery-connected outlets plus USB and serial ports for management. The main problem we encountered was a string of load-imbalance error messages, even though our test load should not have taxed the UPS. We ignored the errors, and the power stayed up for 31 minutes 3 seconds anyhow.

When we reduced network size, the error messages stopped.