I've got a UPS which I've had for 3 years as we get occasional power outs for 2-3 mins.
Most major manufacturers give you a table to work out current consumption for computer, monitor etc.. Add them all together (ignore printers as they take quite a bit). Then when you wire them up you need to ensure you only connect those you want protected after the UPS: you need to plan your HT cabling and buy more 4/5/6 way sockets..
I only just found this thread...
oooh Helen, and me a co author and Physics teacher!
(I'll go off and sulk now <g>)
PC world have some quite nice ones too - UPS are rated (usually) so you can work out the battery life based on the total power consumption (Watts) of your system. Big question is whether it's to cover for minor spikes etc or actual power loss.