It would appear that VA/W are not the only factors in determining runtime on a given UPS, and that there are "hidden factors" to account for (similar to sensor size on camera's, and not just MP).
For example, I'm using sizemyups.com and putting in the calculator that I have a load of 1800W and I want that to run for minimum of 20 minutes on battery. On the results, it shows me I can size at 2000VA/1600W 120 VAC (20A plug/circuit)and get 29.2 minutes run time out of it. Selecting a model of 3000VA/2560W 120VAC (30A plug/circuit) gives me less run time, at 26.3. How could this be... on glance one would figure that the "larger unit" would provide more run time when keeping the load requirement constant. What is the "hidden factor" here? Is it simply more/bigger batteries in one or the other, and that VA/W really has little to do with...