Line-interactive UPS is best choice for most computers

This time around, let's talk in detail about uninterruptible power supplies -- abbreviated as UPS.

By now, most folks know that a UPS provides the electricity to allow your electronics to run when the power goes out. Most are designed simply to give you enough time -- usually minutes -- to gracefully shut down your gear when the utility power goes out. Many are sold with software that allows you to automate this process.

Virtually every UPS provides more than adequate surge-suppression capabilities.

All the major UPS manufacturers provide charts, usually on the Web, showing the expected run time for their units, based on how many and what type of devices you have plugged in. As a result, many folks make the incorrect assumption that all UPS devices are created equal and the only thing you should worry about is how big a battery it provides.

But we believe the most important factor to consider, and one which is often overlooked, is the category, or topology of the UPS.

There are basically three categories of UPS to be concerned with: standby, online, and line-interactive.

» Standby UPS basically passes utility power straight through to your devices (via surge-suppressed outlets), until a source power anomaly occurs. These might be a power failure, surge, spike, line noise, or over- or under-voltage.

When such an anomaly occurs, the UPS switches to battery power. Usually, this is switch is unnoticeable, although very sensitive gear sometimes reacts poorly.

Standby UPS devices are sometimes called "offline" and are usually the least expensive (often $50 or less).

» With an online UPS, output power is always supplied by the battery. The battery is charged with utility power. As such, there a switchover between battery, and utility power never occurs.

Further, the output power is regulated and filtered to be clean and consistent (i.e., no noise or over- or under-voltage).

» Online UPS devices are the most expensive (starting at around $1,000 and running into the tens of thousands). Also, be aware that these units normally run very hot.

A good compromise between the two is a line-interactive UPS. Under normal operation, a line-interactive UPS regulates and filters utility power before providing it as surge-suppressed output power. As a result, the line-interactive UPS only switches over to battery in extreme cases, such as complete power failure, minimizing potential problems with very sensitive gear and extending battery life.

The line-interactive UPS is priced somewhere between the standby (cheap) and online (expensive) models; we've seen some in the $100 range or less.

Based on utility, as well as cost, we highly recommend line-interactive UPS for most home, small- and midsize-office computer-oriented environments. For the difference in cost, we really don't see a standby UPS as appropriate for use with computers.