Pure sine wave UPSs are not that expensive anymore man. I think the biggest "desktop" pure sine wave cyberpower sells (1500VA/1000W, CP1500PFCLCD) is <$300 now. I have a couple of them, they are great.
It's not about pure sine wave - it's double conversion. Only double-conversion would actually isolate the equipment from EMI on the line. Without that pure sine wave won't do squat for EMI.
And one of those, even the cheapest ones, run for about ~$900. And they are LOUD.
Are they loud because they're double-conversion or are they loud because they're designed for server racks? When I search for double-conversion online I can practically only find rack-mount solutions.
They're loud because unlike a regular UPS they need to run continually to convert the power back and forth. That generates a lot of waste heat, which fans must remove.
I wonder if modern motor and power control tech could be adapted to make a desk-side motor-generator set that is efficient enough to rival an always-active, dual conversion AC-DC-AC UPS.
How efficient could a small AC->motor->generator->AC chain be with a modest flywheel mass to provide cycle-to-cycle stability?
Could it ever make sense to put one of these after a standby UPS so the output is always filtered by the motor-generator but the UPS only has to kick in for outages?
One advantage of a motor-generator set is that it's relatively easy to get high-voltage isolation using an insulating shaft. It might be possible to build something that could survive nearby lightning strikes to the incoming AC line. I don't think any standard UPS can do this.
The portable lithium battery "powerstations" double as great double-conversion UPS in addition to their intended outdoors (camping, beach, etc) activities, and depending on capacity go for less than 900 USD. It's only noisy when fast charging or providing high currents.
Definitely had various computer equipment plugged in to ours and it was great (I didn't specically test for EMI).
And one of those, even the cheapest ones, run for about ~$900. And they are LOUD.