How do these devices communicate so they work together? e.g. if you have more than one PV inverter how do they both get the command to back off if excess power is being generated so they don’t feed back to the grid?
PV inverters synchronize to the supply, they work together by virtue that they both receive the same AC supply. No reference AC and they shut down.
This is a secondary function. Now, we are talking fancy footwork and comms channels.
When a PV inverter is off-grid the hybrid frequency can be varied to curtail power, or as with some Fronius and ABB inverters there has been an additional collaboration developing a comms protocol.
The fact is when grid-tied and without additional functionality, they would merrily pump the excess power into the grid. They don’t need all the additional doodads to do this. And some of them aren’t even equipped to do it. ( Although frequency-shifting power curtailment is basically standard throughout now).
Short answer: Sunspec.
Sunspec is a protocol (usually running on top of modbus-TCP, but sometimes modbus-RTU) that is standard enough that any PV-inverter that supports it can be used to talk to other equipment that also supports it.
Sunspec implements a “model” (that is what they call the various blocks in memory) called “immediate controls”. It is model 123. It has a power limit field that you can write to. Any inverter that supports that can be limited. But because not every inverter implements it properly/the same, in a Victron system only Fronius and ABB is supported (we know they work well).
Edit:
- SolarEdge supports sunspec, but not the immediate controls model, so can’t be limited.
- SMA supports sunspec, but not all their inverters support immediate controls unless you enable it through some other SMA-specific routine, and it may be firmware specific. I don’t know.
- Fronius and ABB support sunspec and the required immediate controls model.
- Solis does not support Sunspec.
And the Fronius will work off grid anyway so not much thought. 10m of AC cable maximum needed…
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And maybe some CAT5.
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Plan to use 2 of these (as per @plonkster 's advice)
https://www.takealot.com/tp-link-5ghz-300mbps-13dbi-2x2-outdoor-cpe510-dual-pack/PLID70396875
You need to also talk to @JacoDeJongh , he installed two of these on a farm where he needed an internet connection across a Mielieland… and from what I heard it worked well.
The only thing I cannot vouch for is how long they will last. It’s plastic. The sun will probably kill it eventually. But mine is mounted in the shade, so it should be fine.
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I had a couple of those, I set them to country code:Columbia or something. Their transmit power is nobbled to whatever the country regulations are. I think I chose Columbia, anyway it gave them ten times more power than a ZA country setting.
I didn’t use them as a pair just to make two large wifi hotspots
They worked well, … when they worked, but forever needed reseting or setting up again.
I ended up stringing UV proof CAT6, with router hotspots, far more reliable.
If I did it again, I’d use fibre optic and a data converters either end. There is too much lightning where I did this, and you have to have your earthing spot-on to electrically link separate buildings with a piffling copper comms cable.
I’ve blown a few network switches since then.
If you want to save yourself heartache just use fibre straight off.
Mine’s been rock-solid. Granted, it also only provides Wi-Fi to the backyard. And serves as a “repeater” on a long cable run which has my main solar system’s CCGX at the end 
Like I say my environment, is fraught with thunderstorms, dips and power cuts.
Wireless never seems to be as reliable as hardwired, which is acceptable for some things like internet browsing, but not so good for “must work” things.
For a little bit more you can get great quality with the Ubiquiti NanoStations which I can also recommend
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Would this be a direct connection (no wireless)?
Fibre is amazing and has transformed comms especially here in SA.
Yes, reliable, fast and a non-electrical connection. Just put a little gadget box on each end and you are back to CAT 5 within the building again. I have got the gadget boxes, but I still use overhead CAT6 for the long span and just a 1m fibre patch cable between the gadget boxes at one end.
It still gives me the electrical isolation, the problem is this is at my sister’s in SA, (I am in Ireland), and her internet guy doesn’t know why the fibre is there and keeps on taking it out of the circuit.
Then along comes a thunderstorm and the ethernet switch is blown again.
I swore next time I do this the whole span will be fibre. I can’t remember the pricing difference but that UV resistant CAT 6 isn’t cheap anyway.
Here is some of my email enquiry before I bought the boxes, if people are interested:
FS.com was the suppliers
Overhead fibre can’t be that expensive compared to CAT 6 equivalent. The only issue with fibre is the splicing of course…
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these days
Plenty of premade options up to 150m it seem at 10m ish increments.up to 100m. I daresay you could order a custom pre-spliced length.
I know there are patch leads (with ST connectors) available but I haven’t seen an outdoor version…
A quick google:
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Pv inverter along with Multi
My situation …being in Zimbabwe everything is difficult. Fuel…water…gas. We can have Load shedding for days and power cuts triggered by faults for months!
Of course the sun is free and is not anyway controlled by the govt
MY 2 CENTS
we use induction quite a lot…and an aircon quite a lot so having a pv inverter during the day means that our multi 2 5kva operates much cooler and more efficiently in passthrough mode. We need that as back up repair service is not as great as you in RSA
An mppt is great for direct battery juicing up and night time we control our loads as we do not want our inverter to operate close to peak capacity except when non inductive loads surge to start up etc. Fridges are always baseline loads less than 400watts. So even at 800watts the multi 2 is as quiet as one would appreciate.
Gas is always a backup …but we may go for days without supply…and even if we do its 33rand a kg!
I know of a splash surburb that has no city water for a decade…and borehole levels become erratic 6 months of every year
Gennys are great…but fuel supply is not so great…expensive and our very on Zanu Pf is blending more ethanol that makes these gennies a pain to maintain
In conclusion…i think we Zim guys are lucky. Our hardship has not only forced us to go off grid by choice…but WE CAN SHOW YOU how it can BE DONE as we have no other OPTION!!! Its great. But a PAIN
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Hey, “EFF”, tell your mom the “DA” is sending her our regards. 
Is she enjoying the induction plate?
Guys, @Skye is my friend Reed in Zim. He bought my original Phoenix way back. Since then we have perfected the art of “horse-trading”, just like in the old days, between our countries.
I have an induction plate, he wants one, or he needs a ladder from builders, or a drill, maybe some tubes for his bike, I go and get it, then I drive to a Zim bus stop, give it to someone, picking up anything from a MP 500va, or an MP 800va or maybe some 2nd hand 2nd Life lithium cells in return.
It is astounding what the Zim busses, if you are connected, can do for “trade” between SA and Zim.
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