> 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)

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throw0101c 2 hours ago | parent | next [-]

> 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 an hour ago \| parent [-]
		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.

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jsiepkes 2 hours ago | parent | prev | next [-]

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.

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Aurornis 2 hours ago | parent | prev [-]

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 an hour ago \| parent [-]
		Where is it a good option? I'd expect datacenters to be more focused on efficiency, not less.