Add a 2nd Fronius Inverter with Solar NET or standalone connection?

Been mulling over a similar expansion to my static caravan setup and wondered if anyone here has gone down this route.

Currently running a Victron Multiplus-II 48/3000/35 (single phase, not the three-phase stack the Fronius crowd often use) with a Fronius Primo 3.0 on AC OUT, feeding a Fogstar 48V LiFePO4 bank. Works a treat under MG50 grid code with feed-in limiting configured through VEConfigure.

Now I want to bolt on a second Fronius — likely another Primo or possibly a Primo 4.6 to pull more off the south-facing roof panels. The question is whether to link them via Solar NET (Fronius's own daisy-chain communication bus) or just run them as completely standalone units both sitting on AC OUT.

My concern with standalone is that the Multiplus has no visibility of what each inverter is individually doing — it just sees total AC IN power. Feed-in limiting becomes a bit of a blunt instrument.

With Solar NET, the Fronius DataManager coordinates between units, and if you're also running Node-RED on a Cerbo GX, you can theoretically get much finer control. But I've read conflicting things about whether the second Fronius gets its own DataManager card or piggybacks off the first.

A few specific questions:

• Has anyone run dual Fronius units on a single-phase Multiplus setup (rather than three-phase)?
• Does Solar NET genuinely improve feed-in limiting behaviour, or is it marginal?
• Any gotchas with the MG50 grid code when both inverters are producing simultaneously?

Interested to hear what others have done — particularly anyone

Hey @EdHamilton, interesting one! I've got a similar setup and went through this exact dilemma last year.

One thing worth considering before committing to Solar NET is whether your Multiplus-II's AC coupling limits will cause headaches. Victron has that frequency-shift curtailment behaviour when the batteries are full, and Fronius handles it well enough, but adding a second inverter doubles the potential for things getting complicated during low-load periods.

If both Fronius units are relatively close together physically, Solar NET keeps monitoring tidier through a single data point. Standalone means more cabling but arguably more resilience if one unit develops a fault.

What's your approximate PV array size on each string? That might influence whether the Victron's AC coupling headroom even accommodates two inverters comfortably. Worth checking the 1:1 rule carefully for your battery bank size.

Two Fronius inverters on Solar NET is all well and good until one of them decides it doesn't fancy talking to the other — ask me how I know, three soggy nights on the narrowboat debugging DATCOM cables with a head torch.

Standalone connection is the boring answer but it's the one that actually works when you need it to. Less elegant, more reliable — basically the off-grid motto.

Also worth checking whether your Victron Multiplus-II 48/3000/35 can actually handle the combined AC input from two Fronius units without crying — the GX device config gets fiddly fast, and Victron's documentation on multi-inverter setups reads like it was written specifically to test your patience.

@OffGridGeek raises a fair point about Solar NET reliability — had a similar headache when I was wiring up the cabin a few years back.

What tipped my decision was load profile. If your peak demand is predictable (mine spikes hard when the EV charger kicks in), running a second Fronius standalone through a dedicated Victron input gives you cleaner separation and easier fault-finding when something inevitably goes wrong at 11pm on a Sunday.

Solar NET is elegant on paper, but I've found that when the Victron is orchestrating everything via ESS, adding another layer of Fronius-to-Fronius communication is just another thing to interrogate when VRM throws up a weird reading.

Standalone kept my system transparent. Two clear sources, Victron in charge, everything visible in one place.

Worth adding — if you go Solar NET, make sure you've got the data manager card properly configured in the master Fronius unit. Caught me out when I added a second inverter to my garden office setup; the comms kept dropping until I sorted that.

Also @EdHamilton, worth considering whether both inverters actually need to communicate. If they're on separate AC circuits (e.g. one for the office, one for the caravan loads) standalone might honestly be simpler. Less to go wrong.

One thing nobody's mentioned — the Victron Multiplus-II side of this matters too. Are you feeding both Fronius units from the same AC output? If so, check your Multiplus isn't undersized for the combined inverter capacity during grid-parallel operation. The 3000VA limit bites faster than you'd think.