Hmmm ...
If the UPS is 600VA then actual power handling will be less in Watts.
If the UPS is 600W then the actual apparent power available is more in VA.
There is something called Power Factor (PF) or vaguely efficiency at play in all UPS and device power supplies too.
VA x PF = W
VA - Rating of UPS
PF - Power factor (number between 0 1nd 1) of efficiency in %
W - load that it can support
Most small UPS PF is between 0.5 and 0.8 (60% - 80% efficiency). Even though they claim > 80%, I would take it with a pinch of salt.
Lets take an example
APC Back-UPS 600 (relevant specs below)
Output power capacity - 360 Watts / 600 VA
Max Configurable Power (Watts) - 360 Watts
Battery Volt-Amp-Hour Capacity - 60
(really don't know how they specify this; as the common measure for battery is Amp-Hour at a nominal Voltage, not Volt-Amp-Hour; assuming it is a 12V battery then its capacity is 5Ah)
This UPS has a PF = 360/600 = 0.6 (60%)
Now for run time (from their own website)
Load Time
50W 40min
100W 25min
200W 6min
300W 2min
As you near the max rating in Watts, the run time decreases drastically.
Now lets take your device (router). It uses a wall-wart for AC/DC conversion. Wall warts can be good or bad. Most are not efficient, meaning a portion input power goes in heat.
At 12V/1.5A it is an 18W hog (gigabit Eth and hi-speed wifi; not exactly a power friendly combo); add wall-wart power loss of 7W. That bring it to 25W load.
On the above UPS example, you may get about 1.5 hours. On a home 850VA UPS with a larger battery (say 150Ah) you can get a much much longer run time. Not because of the UPS rating but beacuse of the battery.
Hope explanation this helps. This is the best I could come up with the available info.
Cheers,
Raghu
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