Lab

UPS devices

Today's PCs are very stable, and even if an application crashes, it shouldn't cause you to lose work in any other applications you have open. If you have a desktop PC, though, there's still one event that can damage your files and even Windows itself: a power cut.

Computers need to shut down properly, so a sudden loss of power can damage files on your hard disk. Fortunately, you can protect against this danger. An uninterruptible power supply (UPS) sits between your PC and the mains. In the event of a power cut, its battery keeps your PC running, giving you time to shut it down safely. UPS devices can also protect your PC against power surges or brownouts, monitoring the power that enters your PC and adjusting it as necessary.

We've tested eight of the latest UPS devices, putting each through its paces to see how long it can keep your PC going when the power fails. We'll find a UPS that will save your work and your money.

There are several factors to consider when choosing a UPS. You need to take into account how much power it needs to supply, and how many different pieces of equipment you want it to keep running in the event of a power cut. You should also consider whether you'll always be near enough to your PC to save your work before the battery runs out, or if you need the UPS to do this for you. This guide will help you find the perfect way to safeguard your work when the lights go out.

Power up

There's no point in having a UPS unless it can reliably power your PC and monitor. For this reason you should consider carefully how much power you need your new UPS to supply. The amount of power your PC requires, measured in watts, depends on the components inside. Your UPS should be able to supply this amount of power, plus a safety margin of around 40 per cent.

UPS manufacturers state maximum power outputs in volt amps, or VA, and also in watts. Annoyingly, this watts rating is often hidden away in the product manual and not quoted in product specifications or on the box. As volts x amps = watts, you would expect the figures to be the same. However, this is not the case when powering PC equipment. PC power supplies draw two kinds of power from the mains: consumed power and reactive power. Consumed power is used by the PC's components, while reactive power is stored in magnetic coils and returned to the mains. When a UPS is powering your PC from its battery it has to provide both types of power, which puts more of a strain on the battery. For this reason, the amount of watts of power a UPS can provide is lower than its VA rating.

The easiest way to find out how many watts your PC requires is to use a power meter, available for £27 from Maplin: see http://tinyurl.com/2ccf8y. Add together the number of watts drawn by your computer, your monitor and anything else you want to connect to the UPS, then add 40 per cent to see how much power your UPS should be able to provide.

Socket to 'em

Another important consideration is the number of power sockets your UPS needs. Three of the UPS devices in the group use standard UK three-pin sockets, so it's easy to connect any equipment with a standard plug. The remaining five units use female IEC, or kettle, sockets and come with female-to-male IEC cables. These are fine for powering your PC and monitor, but they won't enable you to connect many printers or other peripherals.

Besides the power sockets, which provide both battery power and surge protection, many UPS devices have additional sockets that provide only surge protection. Many UPS units also have RJ-11 and RJ-45 telephone and Ethernet plugs, which enable you to protect equipment such as broadband routers or telephones from surges coming down the phone line, and protect your PC from any power spikes that pass through your home network.

All the manufacturers in the group offer financial compensation if any equipment connected to the UPS is damaged by a power surge. The amount of compensation on offer varies from £12,000 to £200,000 for the top-of-the-range Belkin 1200VA Universal UPS.

As well as power surges, a UPS can also protect against dips in the mains voltage, or brownouts. If the mains voltage gets too low, your PC may crash, so a UPS will attempt to compensate by adding power. The way it compensates depends on whether the unit is a standby UPS, or line-interactive. A standby UPS doesn't provide any battery power until the voltage drops below an acceptable level, and will then switch straight to battery power. A line-interactive UPS will attempt to use its battery to boost the mains voltage to an acceptable level, and will switch entirely to battery power only when it can no longer provide a large enough boost. This voltage boost technology is known as Automatic Voltage Regulation (AVR). Thanks to AVR, a line-interactive UPS will tolerate much lower voltages than a standby UPS, and will switch to battery power less often, prolonging the life of the battery.

Stay alert

When the power goes out and the UPS switches to battery power, it will bleep intermittently to warn you that you need to shut down your computer. This is fine if you're within earshot of the device and can get to the PC in time to save your work, but if you're not around, it's less helpful. Six of the UPS units solve this problem by connecting to your PC, which enables them to shut down the PC automatically if the battery level falls too low. They can even save any open files before turning off the computer, safeguarding your work. Some can even send an email alert, warning you or an administrator that your computer is shutting down.

Five of the UPS units in the group connect to your PC with a USB cable, but Trust's 1200VA Management UPS uses a serial cable. Some modern PCs lack a serial port, or the port may be disabled in the BIOS. If you're thinking of buying the Trust 1200VA UPS, check that your serial port is present and working correctly.