MultiPlus-II GX with solar panels - what's on "AC-out-1" in "No feed-in" mode with 220V on "AC-in"?

Been wrestling with this exact scenario on my van build — though I've since scaled it up to a proper home backup system, so it applies there too.

Picture this: MultiPlus-II GX sitting there, grid tied on AC-in, battery at 100%, solar panels absolutely cracking off in the August sun, and "no feed-in" mode enabled because your DNO agreement won't allow export. What actually happens to that surplus solar energy?

From what I've tested and read through the Victron docs, the MultiPlus will prioritise powering AC-out-1 from AC-in (the grid) passthrough whilst the solar handles the DC side and keeps the battery topped. But here's where it gets murky — if solar is genuinely over-producing beyond what the battery can absorb, does the inverter start blending sources on AC-out-1, or does it just throttle the MPPT to prevent the battery going over voltage?

In my setup with a Victron SmartSolar 150/35 and a Fogstar 200Ah lithium, I've noticed the MPPT just gets curtailed rather than the surplus somehow magically appearing on the AC output. Grid passthrough seems to dominate AC-out-1 completely when AC-in is live.

My suspicion is that with no feed-in enabled, you'll never see "free solar power" on AC-out-1 whilst the grid is connected — the passthrough just wins. You'd need ESS with self-consumption mode configured properly to actually displace grid usage.

Anyone running a similar configuration who's dug into the VRM data on this? Genuinely curious whether there's a setting I'm missing or if this is just a fundamental architecture limitation of how the MultiPlus handles source priority.

@SussexVanLifer — AC-out-1 on a MultiPlus-II GX in no-feed-in mode with grid present is essentially a passthrough with conditioning. The grid feeds through to AC-out-1 directly, but critically the unit is still actively monitoring and can switch to inverter seamlessly if grid drops. It's not raw passthrough like a basic transfer switch — the MultiPlus buffers it slightly.

What catches people out: your solar MPPT (if AC-coupled via a Fronius or similar) still feeds into that AC-out-1 bus. The MultiPlus regulates excess generation by frequency shifting to throttle the inverter rather than exporting. That's your no-feed-in enforcement mechanism.

Worth checking your ESS Assistant configuration — specifically the "feed-in excess solar charger power" toggle. Even with no-feed-in selected at grid level, that setting independently controls whether DC-coupled MPPT power gets pushed back.

What MPPT arrangement are you running?

@QJ_Builds has the passthrough side covered, so I'll add the nuance that matters practically:

With grid on AC-in and solar coming via MPPT, the GX will prioritise solar to cover AC-out-1 loads first, then top up from grid if solar's insufficient. Any solar surplus goes to battery (up to absorption/float limits) rather than grid, because you're in no-feed-in.

What this means in real terms — your AC-out-1 loads effectively run "free" during good solar days without exporting a single watt. Done this on my cabin setup with a Victron SmartSolar 150/35 and it works exactly as advertised.

One gotcha: if your battery hits full and solar still exceeds load, the MPPT will throttle itself. You don't waste power, but you don't export it either. That's the trade-off you're accepting in no-feed-in mode.

So the bit nobody's mentioned yet — what actually prioritises between solar and grid on AC-out-1 in that scenario?

From my static caravan setup running a similar config: when solar (via MPPT) is covering the loads, does the MultiPlus draw down supplementary power from grid seamlessly, or is there a threshold/lag you notice on heavier loads like a kettle kicking in?

Asking because I'm planning a near-identical arrangement for my cabin and I want to know whether anything sensitive on AC-out-1 would even notice the transition — or whether it's genuinely invisible to whatever's plugged in. Fogstar cells feeding a Victron MPPT into the MultiPlus-II GX if that changes anything.

@LisaStewart71 good question — the prioritisation logic is handled by the MPPT charge controller feeding the DC bus, not AC-out-1 directly. In ESS mode with "No feed-in" enabled, the MultiPlus-II GX will preferentially consume solar via the DC side to offset AC loads, only drawing from AC-in when solar plus battery can't meet demand. The "optimise self-consumption" setting in VictronConnect is what drives this. AC-out-1 itself just sees clean 230V regardless of source — the source switching happens invisibly at the inverter/charger stage. On my own setup I monitored this through VRM portal and the transitions are seamless. Worth noting the minimum SOC threshold in ESS settings also affects when grid kicks in as backup.