[michaelt]

> The UPS consumes almost nothing when AC is on, so that can't be that.

Back in the 1990s, one could buy a "double conversion" UPS that converted AC to DC then back to AC, at all times. This was, supposedly, the best type of UPS (in my experience they were also the least reliable)

[throw0101c]

> Back in the 1990s, one could buy a "double conversion" UPS that converted AC to DC then back to AC, at all times. This was, supposedly, the best type of UPS (in my experience they were also the least reliable)

They are "best" in the sense that your output is completely decoupled from your input so you got the most protection from any electrical noise. The trade-off is lower efficiency (AC-DC-AC roundtrip) and more battery wear (it's constantly 'in use').

Any >10kVA UPS is probably double-conversion/online.

[dreamcompiler]

If built correctly this design also suffers no transition transients. You can switch the external power off/on all day and downstream equipment will never see a glitch.

[jsiepkes]

As far as I know the more expensive UPS models are all still "online" (ie. double conversion) UPS'es.

These are also the only variants which will protect you against things like a phase ending up on neutral in a 3 phase power system. I've seen this happen twice. Fried a lot of equipment.

[ErroneousBosh]

Our UPSes at work are like that, but the smallest is the size of a sofa and the largest is the size of a minivan.

You can put them on street power, you can throw them over to the diesel genny, back onto a secondary genny, back onto street power, and the output won't even ripple.

[Aurornis]

This is called an online UPS and it's still a thing.

It's not a good option for home use because it's always sending power through an inefficient path. The devices we use have power supplies that can handle transients and fluctuations.

[scottlamb]

Where is it a good option? I'd expect datacenters to be more focused on efficiency, not less.

[felixgallo]

Music, medical, engineering. Wherever the waveform consistency and noise floor are the optimization point.

[rdbl27]

Datacenters are focused on "never letting the equipment go down for any reason."

If they can do that efficiently, that's great.

If they have to choose between efficiency and outage risk, they always drop efficiency. That's why they use exclusively online UPS.

[scottlamb]

> Datacenters are focused on "never letting the equipment go down for any reason."

Disagree. I'm most familiar with Google. For a long time at least their UPSs were built in to each machine and not super reliable. They built entire datacenters without cooling with the understanding that for a few days a year it may be too hot for human access. The reliability is from failing over to other equipment—machine, cluster, even region.

Small/on-prem/co-location datacenters may make totally different choices, but I think the major cloud/AI providers would be similar to Google in this respect.

The sibling comment about other industries was enlightening to me though.