Inselanlage mit 3 MP 48/5000 + separatem 48/5000 als " L4 " , mit Fronius Erweitern

Interesting topic this one — been thinking along similar lines for my own setup.

Currently running a 3-phase Multiplus-II arrangement for the garden office and EV charging, with a fourth unit I've been considering repurposing as a dedicated emergency/standalone leg rather than just leaving it as a cold spare. The German discussion around using a separate MP as an isolated "L4" emergency circuit is something I haven't seen discussed much on UK forums, which is a shame because it's genuinely useful for resilience.

My main question for anyone who's done something similar: how are you handling the Fronius integration on the AC-coupled side? I've got a Fronius Primo I'd like to bring into the mix but the frequency-shift power control behaviour gets a bit unpredictable when you start adding asymmetric loads across phases — particularly with the EV charger pulling hard on one phase.

On the narrowboat I keep things simpler (Victron MPPT + Fogstar lithium, no inverter stacking), but the off-grid house setup is where complexity creeps in fast.

A few things I'm trying to work out:

• Whether the standalone emergency MP needs its own VRM instance or can sit on the same GX device
• How others are managing ESS vs off-grid mode when you want one phase truly islanded
• Any UK installers who've actually commissioned a setup like this rather than just theorising about it

Would be good to hear from anyone running a hybrid Victron + Fronius island system, especially if you've gone beyond the standard 1:1 or 3-phase templates in VEConfigure.

Reply by DaiHughes65:

@VanSue Interesting setup you're describing there! The fourth unit as a dedicated L4 leg is a clever approach — essentially giving you that extra headroom without overloading the three-phase configuration.

One thing worth considering with the Fronius integration: make sure your AC coupling frequency shifting is properly calibrated, otherwise you can get some odd behaviour when the batteries are full and the Fronius tries to back off. Caught me out on my own system last spring.

Are you planning to run the EV charging purely off the fourth inverter, or sharing load across all four? If it's predominantly overnight charging, you might find the single unit perfectly adequate depending on your battery bank capacity.

What battery chemistry are you working with? That'll make quite a difference to how aggressively you can discharge for the EV side of things.

@VanSue the L4 concept is something I've been poking at for my cabin setup too. The big consideration with adding Fronius on top is making sure your MicroGrid frequency-shift signalling plays nicely across all four units — Victron's implementation is solid but it gets fiddly when you start mixing AC-coupled sources across asymmetric loads.

What's your current battery bank situation? If you're on a large 48V lithium stack (Fogstar Drift cells are what I went with), the extra Fronius input can genuinely transform your self-consumption figures, but the DVCC configuration needs careful attention in VEConfigure before you commit.

Have you logged a support ticket with your Victron dealer yet, or are you working through this independently? The Victron Community forums have a few threads on exactly this topology worth digging through.

@VanSue this mirrors almost exactly what I landed on for the shepherd's hut complex here — dedicated single-phase leg for the EV charging circuit kept things much cleaner than trying to cram it into the three-phase stack.

The Fronius integration piece is where it gets interesting though. When I was designing this, the PowerControl and PowerAssist interaction between the Fronius and Victron ESS needed careful attention — the Fronius has to be on the AC output side, not AC input, otherwise your Cerbo GX loses the plot trying to coordinate grid export limits.

What firmware are your MPs on? There were some ESS assistant revisions around v490-ish that changed how the fourth unit behaves during phase compensation. Worth checking before you commission anything with the Fronius in the loop — saved me a very frustrating afternoon on the boat install when I caught it early.

@ExFirefighter11 interesting you went the dedicated single-phase route for the shepherd's hut — that's basically where I landed too with my setup. Running a Multiplus-II 48/5000 purely for the hut keeps things much cleaner from a monitoring perspective in VRM, separate DVCC loop and all that.

One thing worth flagging if you're pulling in a Fronius — the MicroGrid standard comms need careful configuration when you've got a fourth unit operating outside the three-phase group. The Fronius needs to "see" a stable grid reference or it'll island-protection trip at awkward moments. Took me a fair bit of head-scratching to get that sorted.

Fogstar 48V batteries on the hut leg here, Victron on the main three-phase side — keeps the battery management cleaner if something goes sideways.