For those who want a simple just-tell-me-the-bottom-line answer for their UPS needs, American Power Conversion (APC) has a nifty UPS selector
that yields three product recommendations (all of them APC, of course) listing them as: best price, best value, and best performance. If nothing else it provides one with a rough idea of what VA range you need to be looking for.
Generally speaking, the VA output rating of the UPS should exceed the load by at least 50% to 100%, so how do you calculate your expected load? The link to BC Hydro’s UPS Guide
contains a six-step worksheet using hardware wattage to help determine a UPS for your needs. Very briefly, you simply obtain the wattage of all devices to be connected, bearing in mind that printers are energy hogs and should not
be operated on battery backup ever
, and then compare this to the total wattage listed by the UPS manufacturer. This will get you into the ballpark area of your UPS needs.
However, the UPS VA rating is not the same as power drain, measured in watts, from the hardware. Some literature suggests a conservative factor of Watts × 1.6=VA, while others list VA × 0.85 to 0.7=Watts. There is no way to accurately know without actually measuring the draw of your hardware with an ammeter. So where does that leave the electronically challenged? As mentioned in the JetCafe
link, obtaining a UPS whose VA is equal or greater than the total electric load rating of all hardware will err on the side of caution, and be properly conservative.