Guidance on Battery Backup sizes?

I have looked at several of our Dell desktop computer models, identified the size of the power supply in each one and then consulted an Internet-based Power Requirement calculator, but in the example of a Dell Precision 7910 Tower (1300 Watt Power Supply), the calculator indicated the "Suggested UPS Capability" was "(Out of Range)".

Here are several other examples in the event anyone else is interested:

........... Power Supply Size ... Min. UPS Cap. (VA) ... Sugg. UPS Cap. (VA)

OptiPlex 9020 ... 255-290 Watt ...... 357 ...... 500       
            
Precision T3600 ... 425 Watt/635 Watt .. 707/1001 .. 1000/1400
            
Precision 5810 ... 725 Watt ......... 1172 ..... 1750
Precision 7910 ... 1300 Watt ........ 1932 ..... (Out of Range)
            
250 Watt PC + 40 Watt Monitor ....... 406 ...... 650
725 Watt PC + 80 Watts Monitors ..... 1127 ..... 1750
425 Watt PC + 80 Watts Monitors ..... 707 ...... 1000
635 Watt PC + 80 Watts Monitors ..... 1001 ..... 1400

So the question remains on which "size" battery backup to buy for a "beefier" desktop computer?

1000 VA, 1300 VA, 1500 VA, or larger?

I can definitely appreciate the idea: The larger U.P.S. unit brought, the longer the system will remain online during a power outage, but there are budgetary limitations and concerns to consider as well.  In some business environments, work lost = billable hours lost, so the contractor or consultant (that us, gang) must consider how to prevent "work lost" due to lack of contingency preparations.

Thanks in advance for any suggestions, comments or observations!