Idle as an operating state for the Bullet is undefined but (I think) there are only 2 operating modes related to what you want-
- OFF mode uses about .3 A @ 120 VAC… less than 40 W.
- PREHEAT uses P7 when getting to temperature, then when it maintains that temp it varies between P0, P1 & P3.
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.
(1) A generator can be substituted for the solar panels as can a wind generator.