DC vs AC Coupling, a thought I had

I am in a position where I want to add solar to an existing UPS install. My installer gave me two options:

a. DC Coupled with a RS MPPT or
b. AC Coupled with a Fronius grid-tied inverter (ACOut coupled)

I was just thinking, if I have a 5kva Inverter and add the RS MPPT then during grid failure I can have at most 5kva output, right?

Now for the AC coupled, would I get the 5kva from the inverter + let’s say 5kva (4.0kW max) of the ac coupled grid-tied inverter if I need the capacity?

Yes, in theory. You would get 5kva from your battery inverter plus whatever is available from the PV inverter at the time, which is not necessarily 4kW and this is the problem. You can’t design the system assuming that there will be PV available during load shedding and therefore undersize the battery inverter.

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I will advise you not to do that. It is best that the Multi is able to carry the full load, because the Fronius unit may disconnect for whatever reason. For example, I’ve seen the Fronius disconnect at the moment of a grid failure (because it sees a drop in the voltage), and then it needs a little time (about 60 seconds usually) to reconnect. During that time the Multi must be able to carry the full load.

Also imagine this scenario: The batteries are full, so the Multi pushes up the frequency to around 52Hz to throttle the Fronius down. Then a large load (let’s say 8kW) starts up. While the Multi pulls the frequency down (which it cannot do too quickly, otherwise the Fronius will see it as an islanding event), the Multi has to be able to carry the full load.

So no, don’t count on this actually working. Rather consider at what time the power is more likely to be used (during day time as it is generated, or at night). If you use most of the power immediately, go with the Fronius. If most of it is stored for later use, go with the MPPT.

@plonkster @Stanley Yes, both points taken and No, the system is not designed to take advantage of the possible Fronius power above the 5kva battery inverter limit, it was just something I thought off while staring the two options in the face. Most of my power is used during the day (pool and all + politics) so my first choice is the Fronius.

I think it will make the system a little more stable overall with larger loads.

I presume your inverter (UPS) doesn’t have a MPPT already built in for the PV input?
(I thought this was dog standard…)

Nope. It’s a Quattro.

I see you thinking of Fronius. Just for interest sake I see that thw Solis range of PV invertres also do well with FS.