UPS "sizing" is always a tricky question. Even for the experienced. Size depends on direct connected devices, one's expected future growth, one's minimum "run" time and one's maximum "run" time.
For Backup UPS "sizing", may I suggest running the APC Sizing URL. At end of their selections / questions, it recommends a "good, better, best" product size. For example, 900VA, 1200VA or 1300VA unit. For me, I don't pick the bottom and I often don't pick the top. Most of the time, I pick the middle "size". With that "VA" size chart in hand, I then decide to purchase APC product or perhaps a different brand product (of same size). To me, knowing the "sizes" is the first question answer.
For APC Backup UPS, sizing (for your own system setup), surf:
http://www.apc.com/tools/ups_selector/index.cfm
See their "PC or Workstation" column.
Select "Configure now" button.
If your exact iMac device is NOT listed, pick simular "power need" device. For example, Desktop with 30" monitor with Intel i7 processor.
Continue to select your other unique items.
Select "show UPS solution".
For my selection choices, it recommended:
Good: APC BACK-UPS 900VA 120V
Better: APC BACK-UPS RS
1300VA LCD 120V
Best: APC Smart-UPS 1500VA LCD 120V
For me, I bought "APC BACK-UPS RS 1300VA LCD 120V" unit. It's been working great for the last 7 months...
Good luck...
.