Which ups should you get for a 550w power supply - 600va or 800va?

Answer 1

A computer power supply does not pose a seriously complex load ('complex load' in its electrical meaning); in other words, a computer power supply wattage (e.g. 550W) will be approximately the same as is VA value.

However, a computer's power supply is typically rated after its nominal output power. Since it can't operate at 100% efficiency, the input power will be higher than 550W under maximum load.The power supply's technical data should tell you its rate of efficiency or its input power under maximum load, and you can chose the UPS accordingly.

You could also chose a less scientific approach and justify the purchase of the 800VA, the 600VA, or even 400VA UPS: The bigger UPS will be overrated for this particular power supply, but might provided the better investment as it can support additional (small) devices and is likely to be sufficient in the event of a computer replacement (which might have different power requirements).

However, the 600VA unit will probably be just fine for this particular case.

You could also argue that the 550W power supply is a nominal power rating; it is the power the power supply is specified to sustain under full load. Your computer will very rarely come anywhere near this, and you could argue that high power number-crunching or DVD writing may not be required during a general power outage, making a less expensive apparently underrated UPS sufficient to sustain operations or data for the expected duration of a general outage.