APC Back-UPS Pro BR1000G
The iMac has a built in 310-350W psu inside. The 500VA-650VA ups all are just around 300-330W max load and the ups will shut down in less than a minute (probably few seconds if battery is not fully charged in between power cuts) even on a full battery if you buy one of those. So i did not recommend any 600VA or around that mark ups. This 1000VA rating ups will give you at least 15 minutes to finish up work or wait till the power comes back on instead of rushing to switch off pc. You could even attach a printer and take prints off quickly if you want if there's no power. (Ups companies say connecting printer is not recommended, but i've had to take photocopies in a pinch when there was no power and it works).