I see that a lot of Eaton models (both ECO and Pro lines) are not actually pure sine wave.

The recommended ECO models that I’ve seen, is basically offline/standby not line interactive. When do I really need line interactive ones, how do I know if I need that kind of voltage regulation and if I am living in a big city, does it really matter?

The only model that actually make sense price wise compared to the Eaton in the EU, is the Cyberpower cp1500pfclcd, which does have both sine wave andis line interactive.

In general I would like to have a router, 3d printer, and a Synolgoy NAS on it, maybe my PC.

I’ve read this comment from another thread, aside this guy people also be shitting on anything but online topology of UPS which is much more expensive:

… line-interactive UPS designs. … They deliver street-power to the attached devices through some basic surge-protection until they detect a loss of street-power, then they switch to battery very quickly.

That is not what the term line interactive means, though there generally is some surge protection in the UPS as well.

Line interactive means that the UPS when running from street power semi-regulates the output voltage to keep it in a more consistent range. It does this by using the transformer in the UPS to “boost” or “buck” the output voltage. Even very cheap UPSs of this type have a minimum of one buck and one boost, and some have more.

Prior to line interactive, non-online UPSs were called standby UPSs, and line interactive is a standby UPS with a bit of clever tap-changing to increase the value and thus the sell price of inverters.

Opinion: line interactive offers no advantages over old-school standby UPSs, as modern kit has switch mode power supplies which are very tolerant of rubbish power, so semi-regulating the power prior to the load is a waste of time. But… the market has decided, and now standby UPSs have vanished, and line interactive is everywhere.

In terms of on-line UPSs:

Street-power flows into the unit, and is pushed through a transformer to feed the batteries. Power is pulled from the batteries through another transformer to power the attached load.

That statement implies something that doesn’t happen.

The hallmark of a double-conversion or online UPS is that the load is (almost) always supplied from the inverter, and thus there is no switchover seen by the load between street and battery power.

But, the DC power does not flow “through” the batteries; the batteries are kept charged as per any UPS, and the inverter gets is DC from the input converter. When street fails then the batteries now have to supply all the current, they get the same surprise that batteries in every UPS gets.

How true is this?