Multiplus / SMA3.0 configuration

So this one's been rattling around in my head ever since I started speccing out the next phase of my van-to-tiny-house transition...

I'm looking at AC-coupling a Victron MultiPlus-II with an SMA Sunny Boy 3.0 — and honestly, the configuration rabbit hole goes deep. 🕳️

The classic headache is what happens when the grid drops (or in my case, when I'm running fully off-grid): the Sunny Boy needs to see a stable AC reference to stay alive, and the MultiPlus has to do that "pretending to be the grid" trick via its internal frequency shifting to curtail PV output when the batteries are full. Getting those two to play nicely together without the Sunny Boy throwing a strop and disconnecting every five minutes is... an adventure.

Has anyone here actually run this combo in a UK off-grid setup — so no grid at all, not just backup? I'd love to know:

• What firmware versions you're running on both units
• Whether you tweaked the SMA's grid code settings away from the default UK G98/G99 parameters
• If you ended up needing a Victron GX device (CerboGX etc.) in the loop to make frequency shifting reliable

I've seen folks in other threads suggest the ESS Assistant isn't actually the right tool when you're fully off-grid — apparently the AC PV on output assistant behaves better. Can anyone confirm that from real experience rather than forum mythology? 😄

Would love to hear from anyone who's wrestled this particular beast into submission.

@KangooDream the frequency-shift thing with the SMA is the bit that catches people out — the MultiPlus-II needs to be able to raise mains frequency to signal the SMA to curtail output, which works fine if you've got the ESS assistant configured correctly and your SMA's grid management settings aren't fighting it.

On my narrowboat-to-tiny-house hybrid I ran into a similar AC-coupling headache. The key thing nobody mentions clearly is the 1:1 rule — your AC-coupled inverter's PV capacity shouldn't exceed the MultiPlus's inverter rating, otherwise when grid (or generator) drops, it can't absorb the surplus fast enough and you get instability.

What's your battery bank? LiFePO4 with a CAN-bus BMS talking to the Victron changes things considerably compared to a dumb lead setup.

Has anyone actually done this on a boat installation? Asking because I'm weighing up something similar — my setup is predominantly 12V DC at the moment but I'm wondering whether AC-coupling makes sense when you've got the added complication of shore power coming in intermittently.

Specifically curious about what happens when you disconnect from shore — does the SMA 3.0 island correctly with the MultiPlus-II handling the frequency shifting, or are there edge cases where it falls over?

Also, what's the minimum battery bank size people are running with this kind of setup? I've seen Victron's own guidance suggesting you need enough capacity to absorb the SMA's full output during a grid-loss event, but the numbers feel quite conservative for a smaller system.

@KangooDream done exactly this in my cabin — the Victron/SMA handshake is fine once you've set the ESS assistant correctly, but prepare for an afternoon of staring at VE.Config wondering which version of yourself made that decision.

@EwanCole58 done it on a narrowboat — the main gotcha is keeping your AC cable runs short enough that voltage drop doesn't confuse the frequency-shift handshake, because water and electricity already have a complicated relationship without adding communication errors to the mix.

Also worth checking your Fogstar battery BMS plays nicely with the Victron DVCC settings before you commit — had a fun afternoon discovering mine had opinions about charge current limits that nobody had bothered to document.

VRM portal is your friend once it's all talking; being able to monitor the whole lot remotely from a towpath café is genuinely the peak of off-grid living.