Other influencing factors
Other than the underlying UPS technology, the main factors influencing your choice of product will be power output and back-up time.
The power output will be specified in VA or W, and this figure dictates how much equipment can be connected to it.
Just add up the ratings for your PC and any other equipment you want to be able to power (you should be able to find this information on a label on the rear of the equipment) and ensure that you buy a UPS with at least this capacity.
Do bear in mind, though, that only a huge UPS will be able to power your equipment for more than a few minutes from the battery. In reality, you'll probably only use it to save any files that were open and shut down Windows.
You're unlikely to need a UPS that will power your printer, scanner and so on as well. The back-up time is a measure of how long equipment will run on battery power.
This depends on how much power you're drawing from the UPS, so don't take notice of claims that a UPS provides 'up to' a certain time.
Time at full load
Instead, look for the time at full load. For some supplies, this figure may be fairly short – one of the products reviewed here provides just two and a half minutes at full load.
A UPS that allows you to work on through an hour-long blackout will be huge and vastly expensive, but all you need is enough time to shut your system down in a controlled manner.
Most UPSes sound an alarm in the event of a power failure, so you can take the necessary action. Some also have software that closes the system down automatically if power is lost.
There are a few other things to look out for. Many UPSes feature additional sockets to provide surge and spike protection with no battery backup. These could be used to provide protection for a printer, for example. It's also common to find surge and spike protection for data lines entering the PC.
Of most importance is the telephone line, although some products can also protect a network connection. It's hard to get excited about a piece of equipment that normally sits in the background doing nothing, but the benefits are clear.
If you do lose data because of a problem with the mains, you'll be kicking yourself.