I am also an APC follower, but...
The frustrating thing about choosing the UPS is that the most common rating scale you see is only 1/2 of what you need to know.
They're addressing the characteristic battery issue -- the larger the demand for current, the lower the total energy you can suck out of the battery, so they give you the current load that the battery can handle at its given voltage (120 for us yanks), usually in kVA, which is kinda like Watts but not really even though you learned P=VI in electronics.
Then the other 1/2 you need to know is HOW MUCH ENERGY STORAGE is there??? They sorta give it to you by way of a few points of the energy curve, meaning they say "10 minutes at 1/2 load". Well okay that is informative, and technically accurate, but only in comparison with another similar kVA rated device of course.
Belkin, to their credit, is clear about both the kVA rating as well as the Joules, which presumably is Joule delivery for very lowest load and you can guess the rest of the curve. At least that's based on the units' boxes I've looked at.
APC also makes it very unclear about why you should pay an extra 1/3 or more for the "Smart-UPS" model instead of the "Back-UPS" model. And they keep trickling features into the relatively new "Back-UPS Pro" line that used to be "Smart" only, so it's really super hard now. The product differentiation is a total mess, at least from my perspective, but the products themselves do everything in their specs perfectly well. Just mind those specs, use the APC website, and never ever buy thinking "oh, this one should be the nearly the same as that because they're nearly the same name" no way.