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..