VAs (volt-amperes) are one way to measure the capacity of an
uninterruptable power supply (
UPS). The VA is the number of volts the UPS can support multiplied by the number of amperes it can support. Yep, you got it - VAs are watts. After my great search for the definition, they turn out to be
watts.
When you are choosing a UPS, you need to consider the VA rating and the run-time (and whether it's at half load or full load). For example, if one UPS can run for 45 minutes at 300VA, and another one can run for 5 minutes at 600VA, then the first one is better, even though it has a lower rating. But, if you will be running more than 300 W off it, it's probably better to get one with more VAs.
When choosing a UPS, you need to know how much power your computer uses. An easy way to find this with no additional equipment is to find either the wattage or the voltage and amperes used by each component and convert them to VAs (1 watt = 1 V A, and 1 V * 1 A = 1 V A). Of course, you
can take shortcuts - if you don't want to do this for everything inside a computer, you can just use the power supply rating to know the maximum it will be using (although you may upgrade your power supply in the future).
If you expect to add devices in the future, you should add a little to your final result (remember that if you took the power supply rating shortcut additions to your computer won't count, unless you plan to overload the power supply). This will give you the final VA rating you are looking for, and then you just need to determine the runtime of the UPSes available where you are buying. You can either determine the real runtime by calculating it from your computer's VA rating and the runtime (taking into consideration the load it's at - some are half-load to make it look better), by finding the runtime for your computer of one UPS and then calculating relative runtimes of the other ones, or just using pure relative runtimes (this one run twice as long under the same load and I can afford it, so I'll take it).
There are, of course, other ratings for a UPS, such as how well it filters the power (rated in
dB), but most people will be buying them for the power backup features, so the VA and runtime ratings will be more important to them. Of course, that "25,000$ surge protection insurance" can't hurt :).
Many thanks to http://flthlpdsk.chinalake.navy.mil/COMPUTER/Tutorial/ups.htm for answering my question and more