And watch out for UPS manufacturers that lie. The one I bought recently was calculated to last at least 30 minutes and said it would last 'up to 40 mins on half load'. Switches itself off, even when the mac is asleep, after about 7 or 8. If I leave the mac awake it does the same. The Belkin one I had before did the same after a few years. The first Belkin I have which is 10 years old lasts 30 to 40 mins with the same load!! I have a feeling that they're all designed to switch off quickly now so they're all useless to me as I want one that will let me leave the computer asleep for an hour or two as generally powercuts are either a couple of mins or a couple of hours.
General idea with a UPS is to go for one which is for twice the wattage than you have plugged in as they last much, much longer on half load.