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!

I have found APC's product selectors fairly useful for this. They are helpful because they factor in load efficiencies / battery type factors into their formulas, so really your inputs are limited to just what you've supplied above (total wattage, backup time desired, expansion room needed). The result is something similar to this:

The output is an APC-specific model, but you can use the raw specs of their recommendation (ie: 1300 VA) and shop models from other mfgrs.

We only use APC brand units. They are well worth the money.