question

You are not off grid. Power shift and < 20 ms switch over to AC out 1 on grid fail is standard.

Be aware it is not a UPS there is a switch over to inverter but < 20 ms.

Depending on the "critical load" a multiply Ii GX (all in one) will do it,

Beauty is even a small PV (1 or 2 panels and a microinverter) kann be easily AC coupled later.

I always recommend grid parallel, critical loads via multi rest as is on grid,

You just don't have any PV inverters or MPPT and the inbuilt CT is fine.

lifeingalicia, I'm not sure I fully understand your answers so I will go over them to see if we are on the same page.

-About the <20 ms part of your answers.

The way I interpreted that part is that you're warning me that the multiplus (original one, the II, the II+GX) is not a true ups and that the internal transfer switch can take up to 20 ms to transfer. Hence some kind of loads might not continue work normally after the switch. This is fine by me. Computer and electronics should have any problem with the switch. And for the loads that may need manual restart (like, for example a dishwasher) after a switch, I'm fine to do it. The important thing is that the switch does not fry anything, the fridge for example. But usually they have a protection that make them restart a couple of minutes after a power interruption.

-About the off-grid, on-grid part of your answers.

Maybe there is a problem with terminology on my part. But they way I've understood the definition of an off-grid application is not "there is never going to be electricity coming from the mains into the AC-input". The way I understood the meaning of an off-grid application is that the inverter is set so that it cannot assume that it is connected to an electrical grid that can take current back-feeding. In this case the inverter just uses what comes in from the AC-input (whenever there's is something coming in, whether it be from the mains or from some form of generator) and uses it the way it is instructed to do (e.g. charge the battery, feed the loads etc). This is an off-grid application to me.

On the other hand, an on-grid application is when the inverter can assume that it is connected to a grid that can take back-feeding and the inverter is able to do so (so that you're paid for what you produce with your PV). In this case we have all the stuff with the country regulations, the need for anti-islanding etc.

Having said that, my concern with that double asterisk remark from the pylontech document was that is seemed to say

"Beware that if you are in an off-grid application" (the way I defined "off-grid", that is no back-feeding) "then to keep awake the system and the batteries you still need an mppt attached".

And since my application will not have any PV attached (and hence no MPPT either) I took it as a problem (because certainly I won't buy an MPPT just to keep the system on).

Great - yes <20ms is nothing but it is not a true UPS.

Electricity that is produced has to go somewhere.

So in off grid installs it is vitaly important that any PV or any charging comming in can be regulated / shut off if bats are full. Thus a MPPT with communication to a BMS is required. There is no back feeding excess.

This is also why rule 1.0 exists.

Imagine Grid or gen fed AC input 2Kw - bats are full - PV running at full tilt - 3Kw - suddenly load drops - ---- - puff.

Well managed bats can take a sudden burst influx whilst regulators kick in - but only just.

If not well designed / managed frequency and or voltage go sky high.

No issue when grid connected - AC reverses at half wave - as there is more or less no impedance voltage goes up minutely - no problem.

So yes your asunption is OK just remember electricity has to be consumed,stored or switched off(regulated).