Plugging in your information, the selector suggests a 800VA UPS (Best Price), a 1200VA UPS (Best Value) and a 1500VA+ (Best Performance). The ""Best Price"" one supposedly gives you a runtime of 13 min, ""Best Value"" = 21 min., and ""Best Performance"" gives you 50 min."
"So, if you run a calculation here, you will find what the actual power consumption of your rig should be.
According to the calculations, mine draws under 250W, (even though it has a 550 W PSU) so I got this 250W PSU.
It is currently working fine for me - its protected from power failures, and isn't overloaded.
I could not connect a monitor to it as well, because that causes it to beep and shut off (ie it detects power draw is over 250 W and gets upset, as its supposed to by design)."
"There are meters you can buy that measure the amount of AC power being used by a particular wall socket. The one I own is called a ""kill-a-watt."" You plug the meter into the wall socket, then plug your device into the meter. The meter is then in series in the circuit so it can measure how much current you're drawing. If you use one of these with your computer, you can watch how much power it uses while it boots, and how much it uses while you're web surfing or typing in a word-processor.
What you will probably find is that your computer uses a surge of power while you're booting, but much less after that. This is usually the constraint that forces you to use a power supply with a certain amount of power: if it had less, you wouldn't be able to boot.
If your power supply is 700 W, then the power you actually use while booting should be less than that, and the power you use after booting should be much, much smaller than that in turn. If you really want to know exactly how much you need, you have to use a meter. You can't just calculate it, because power use depends on every device you have connected (hard disk, sound card, ...), and the power they consume varies depending on what the computer is doing.
You only need the UPS to keep you from losing your work, so you don't need it during boot."
