How To Calculate the Backup Time of a UPS

I have seven years of experience in three different companies and have worked with equipments as complex as the RADAR to as simple as a UPS. Yes, that’s where I am working these days. I oversee maintenance and repair of UPS(es) installed in all the airports of Pakistan so yes if you are in any Pakistani airport when the lights go out and the UPS doesn’t come up, you know who to blame. Recently while testing a UPS I wanted to find out if there was a way I could estimate the backup time of a UPS without actually going through the whole testing procedure carried in my workshop. Some Googling revealed that indeed there is a method of finding this out. Since I want to keep my readers aware of whatever new stuff I am learning, here goes:


For the non engineers, volts is not the AC 220V but the voltage of your batteries which is usually 12V or 24V. The second thing is the nubmer of batteries. In homes, we usually have just one battery.

Amperes is the current rating of the battery and power factor is the power factor rating mentioned on the UPS. As a safe rule, you can take it 0.8 or 0.9.

The load has to be in VA=VoltsAmps means if you have a load given in watts. Divide it by 0.8 to get the VA of that load.

If you have any questions, feel free to ask as I know this post may be hard to understand for non technical people.

You can also use this very helpful website to do this calculation. This requires just plugging in the numbers and you are good to go:

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