So are you suggesting that ANY size/capacity of UPS will power all my equipment for long enough to shut them down properly?
No, you need to add up the VA rating AND the wattage of ALL your components, that will be on at any one time. The UPS must be able to provide the VA AND continuous watts for everything.
The VA rating is different for Watts, becuase of the inductive nature of power supplies. The current is out of phase with voltage, so there is apparent power and true power. The UPS must provide enough current to provide the current required to fulfill the apparent power requirements, and be able to provide the continuos true power in watts.
This whole area is very misunderstood, because the nature of the flow of current in inductive circuits requires an understanding of vector triangle and trigonometry.
However if you follow the instructions at the beginning of my post you will not go wrong.
You should be able to find the VA ratings and continous wattage ratings in the manual and or on the back of the device.
I support the use of APC products. I also live in a rural area and a UPS is vital. In my small system downstairs, surge protectors did not do the job, and had to go to a UPS for that also.
I use refurbed APC smart UPS systems. They have been fine. The crucial thing in addition to power, is response time in the 1 to 3 msec range for sags and peaks. The closer to 1msec the better.
There is more poor information on this topic on this important issue, than just about anything else.