Lab

Uninterruptible power supplies

Technologically advanced as it may be, your desktop PC ultimately depends on its mains cable. In the event of a power cut, the system will turn off instantly, vaporising the contents of its memory (RAM). Any work you've done since your last save will be lost. Worse, an unexpected shutdown could also damage files on your hard disk. It's not common, but you might have to re-install software and you could even find documents are irretrievably corrupted.

Fortunately, you can guard against this by plugging your PC into an uninterruptible power supply, or UPS. This is a modestly sized box containing a large battery. The UPS in turn plugs into the mains, which powers your PC as normal while also keeping the battery charged up. If the power goes out, the UPS switches automatically to powering your PC from the battery. It won't last more than a few minutes, but that should be enough for you to save any current work and shut down the system safely.

As well as keeping your PC up and running if the power goes off, a UPS will direct any extra voltage to earth in the event of a power surge. If for any reason damage still occurs to connected equipment, the UPS manufacturer will usually compensate you up to a certain value, as long as you've used the UPS correctly.

Today's UPSes can be a little smarter, too. The unit will beep continuously while the power is off to let you know there's a problem. In case you're not around at the time, some can automatically save your work and shut the PC down. They may even send you an email to let you know there's a power cut, which is handy if your home power goes down while you're out at work.

All the units on test have at least two power sockets, so you can run your PC and monitor. Some have normal 3-pin UK mains sockets, so you can plug any device into them, while others have 3-pin IEC or 'kettle' sockets, taking smaller plugs, and come with cables to connect these to your PC and monitor.

Socket to me

All UPSes also have built-in surge protectors. These protect against power surges coming down the electricity line, which could damage your PC's components. Some of the sockets on each UPS provide both battery backup and surge protection, while others give surge protection alone, for devices that don't mind being powered off unexpectedly but could still be affected by spikes, such as your printer or cordless phone base station. Many UPSes also have telephone line and network surge protection sockets, which protect equipment against surges coming down the phone lines (which are susceptible to lightning strikes) or over your home or office Ethernet network.

Some pricier UPS units also provide automatic voltage regulation (AVR). This protects against dips in mains voltage, sometimes known as brownouts. Modern computers can cope with a certain amount of voltage fluctuation, but a large dip could crash your PC crash, so to stop this the UPS uses its internal battery to boost the voltage to an acceptable level when necessary.

We tested eight UPS units with various specifications and prices. Before choosing a UPS, you should consider how much power you need it to supply. All UPSes have a power rating, given in volt amps, or VA, and watts. You need to choose a UPS that can deliver enough power for your particular PC.

Our benchmark results tell you how long each UPS managed to keep our PC going while at idle and while performing some intensive video encoding and image editing tasks. In use, the actual length of time you would have in which to respond and save your work would be somewhere between these figures.

Different models impressed us in different ways: some kept the system running for ages, some had very slick software, while others had more powerful features. Whatever your need, there's a UPS here to make sure your work doesn't disappear when the power does.