How to choose a UPS / calculate power for a new PC

"APC UPS Selector
Newegg PSU Wattage Calculator

Obviously, these tools are made by people who stand to make extra profits by skewing the numbers, but I have found they are pretty decent to work with."

"Some new information for an old thread. Your UPS should be capable of providing the max potential draw of your PSU. I mean, the entire point of the UPS is to protect your data so it should not be the weak link in the chain of components. Also, the price jump to a higher rated UPS isn't that much in the scheme of things.

Another important thing to consider when choosing a UPS is whether your PSU has active power factor correction. These active PFC PSUs are more demanding of their input and can shut down when the UPS kicks on in a power outage. The quick reason - consumer UPSs typically output a squared sine wave approximation and an active PFC supply expects a smooth sine wave. When switching to a stepped wave, they might see a zero power state and just shut off. Not always, maybe sometimes, maybe never depending on the sensitivity of the components... More info on this thread.

As applies specifically to this thread and wattage ratings, a PSU with active power factor correction may actually draw more power when necessary to correct for misaligned current. This is possible anytime as power delivery is never perfect, but much more likely when the current is riding a squared off wave. So while you certainly need to provide for the power demand of all your components, you may need more headroom than you expect if your PSU overdraws to compensate for the output of your UPS.

Better to be safe. Get a UPS that meets or exceeds the wattage of the PSU. Also, buy a UPS that can safely operate with active PFC PSUs (which means the UPS delivers a pure or at least better-than-square wave)."