As I understand, the Bullet draws 1500watts on P9. What is the power draw when it is idling?
This is again related to our unstable power conditions in town. I guess the firmware updates happen when the roaster is idle, so, if the power draw is nominal, I could hook it up to a 800VA UPS and at least get the roaster updated without worrying about a power cut.
Idle as an operating state for the Bullet is undefined but (I think) there are only 2 operating modes related to what you want-
F/W updates (again for the Bullet) must be done in OFF mode. Your computer may be a bigger load at that point, but your planned 3KVA UPS will handle the job easily. Just remember that a UPS does not function as a line regulator. Power has to drop out (drop below the trigger point for brown-out) before it substitutes the battery for the powerline, so there are transients to deal with.
I whine about our local power provider but they are saints compared to what it sounds like you’re dealing with!
As an aside, you could consider a photovoltaic(1) system that utilizes a bank of batteries, chargers and a sine wave inverter. The system runs off the batteries all the time and the power line (and solar panels) keep the batteries charged. You must have local/nearby suppliers that can recommend a system design for your needs.
Bruce
(1) A generator can be substituted for the solar panels as can a wind generator.
Just plugged my (new) Bullet in to a Kill-a-watt -
7.4 watts at idle.
The new V2 Bullet shows a significant improvement in power loss over the earlier V1/V1.5 examples. When I noted the power consumption above on my Kill-A-Watt with the Bullet in Off mode (0.3 A @ 120 VAC), I had the older V1 power board. I’ve since upgraded and now see really low stand-by power as you do. Power loss on the board, especially while roasting, is greatly improved. The Kill-A-Watt tells all its secrets! 
Bruce