UPS Selection Advice

I just had a 6 cam / 8 ch PoE system installed at a home and I want to add a UPS that could power the system for let's say up to 30 mins.

The NVR requires around 25 watt (total PoE output 120W) and each of the 6 cameras around 7W each but I do want to have some extra room in case I decide to replace 2 cameras with 12W models.

How can I easily determine what UPS spec (how much VA) I should be looking at for this system? Is there a good guide available online to determine my requirements?

Thanks in advance!

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