There is no official recommendation from Agilent, but even if there were, it would depend on what modules are in your system. I would suggest looking at the user manual for each module and see what the maximum power consumption is, then find a UPS that can handle a bit more the total power consumption of all the modules.
We use an APC Smart-UPS RT 5000 as back-up. It's over-kill for the LC and computer, but was originally purchased to back up the QToF. I do NOT recommend this to back up an Agilent QToF, as we found out that the 208V coming out of the transformer is the wrong phase.
We had a power outage for about 30 minutes this week, and the back-up had plenty of reserve to cover the short time without utility power. We're running a 1200 LC with multi-sampler and cooling option.
I went ahead and flagged the “Correct Answer” on this post. If you still need help though, please let us know and we would be happy to continue working with you.