Help with DC excess exporting/feed-in

Been mulling this one over for a while and finally posting because I can't find a clear answer anywhere.

I've got a Fronius Primo grid-tie inverter sat in my garage doing nothing because my setup went full off-grid last year. Bit of a waste really. Got me thinking about whether there's a sensible way to use excess DC from my Victron SmartSolar 150/45 once the batteries are full, rather than just watching it get throttled back.

My tiny house setup is relatively modest — batteries are Fogstar Drift 48V LiFePO4, feeding a MultiPlus-II 3000. On a good summer day the batteries hit absorption by mid-morning and the MPPT just starts strangling the panels. Feels like leaving money on the table.

A few questions I can't quite work out:

• Is there any legitimate way to "export" excess within an off-grid island system — say, to a resistive dump load like a Immersun or similar hot water diverter?
• Has anyone used Node-RED on a Cerbo GX to trigger a dump load relay when SOC hits, say, 95%?
• Would a dedicated DC coupled diverter even make sense here, or is AC diversion from the MultiPlus output the more straightforward route?

I'm not connected to the grid at all so genuine feed-in isn't an option, just trying to squeeze more usefulness out of what's being generated rather than curtailing it.

Anyone running something similar on a garden office or small off-grid dwelling? Curious what's actually working in practice versus what looks good on paper.

@Liz1979 you've cut off mid-sentence there 😄 finish the thought!

But if you're heading where I think you're heading — Fronius Primo won't play nicely with a pure off-grid DC setup, it needs a grid reference to operate. Had a similar headache with my shepherd's hut build, ended up just flogging the grid-tie unit and grabbing a Victron MultiPlus instead.

Might be worth finishing your post first though, could be something completely different to what I'm imagining 😅

@Liz1979 a Fronius Primo doing nothing in a garage is basically the saddest thing I've read all week — that's a proper expensive doorstop you've got there.

@Liz1979 the Fronius Primo is an AC-coupled grid-tie inverter, so it won't directly interface with a DC battery bank — it needs AC on its output side to synchronise against. That said, it's absolutely usable in an off-grid or hybrid setup via AC coupling with something like a Victron Multiplus or Quattro. The Victron uses frequency shifting (the "frequency power control" / ESS approach) to throttle the Fronius when batteries are full — Fronius specifically supports this through their MG50 or MG60 firmware profiles, which Victron's documentation covers brilliantly.

Worth noting: the Primo needs to see a stable AC waveform, so your Victron must be running in inverter mode to provide that island. Done correctly it's an elegant system — I run AC-coupled solar on my narrowboat setup for exactly this reason.

What size is your Primo, and what battery chemistry are you working with?

@Liz1979 @DevonDweller is right about the AC coupling side, but worth knowing Victron's ecosystem handles this really well — a Multiplus or Quattro can AC-couple with the Fronius using frequency shifting (the "Fronius Zero Feed-In" setup via ESS). The Victron effectively throttles the Primo when batteries are full by nudging the grid frequency up.

Depending on what your full setup looks like, you might not need the Primo to gather dust at all. What battery chemistry are you running and is there an existing inverter/charger in the mix? That'll determine whether AC coupling is even feasible or worth the hassle vs just selling the Fronius and putting the money toward something more native to your system.