ESS connection of a Multiplus to the grid

Been thinking about this a lot lately after seeing a few discussions elsewhere about ESS setups with the Multiplus II.

My current setup on the boat is fairly straightforward — Victron Multiplus-II with a Fogstar lithium bank, no grid connection obviously. But I've been seriously considering a shore power integration for the times we're moored long-term, and it's got me down a rabbit hole of how ESS actually behaves when the grid is present.

The bit I keep coming back to is the AC-coupled solar question. If your PV inverter is feeding directly into the consumer unit on the grid side (rather than into the Multiplus output), how does the ESS actually "see" that generation? Presumably it relies entirely on the CT clamp / energy meter reading to understand what's being produced and consumed?

I'm thinking specifically about setups where:

• Grid is present and metered
• Solar AC-coupled on the grid side of the Multiplus
• Battery charges/discharges based on the meter readings

Does the Multiplus then purely manage the battery charge/discharge based on net import/export figures? And what happens during a grid outage — does the AC-coupled solar on that side just island off completely, leaving you reliant on the battery alone?

Feels like there are some real gotchas here for anyone retrofitting ESS into an existing domestic setup where the solar was already grid-connected before the Multiplus arrived.

Anyone running something like this in a residential or static tiny house context? Curious whether the ET340 meter or similar is doing the heavy lifting in terms of keeping the logic sensible.

Would love to hear real-world experiences rather than just the Victron docs, which get a bit dense.

@BoatMel interesting one — I've been looking at ESS for a home backup setup rather than marine, but a lot of the same questions apply.

One thing I keep getting stuck on is the G98/G99 notification requirements when connecting to the grid here in the UK. Did you have to go through your DNO for any of this, or are you running the Multiplus-II in a mode that keeps it isolated from export?

Also wondering whether the ESS assistant in VE.Configure causes any headaches with the shore power switching on a boat — I'd imagine there are scenarios where it gets confused about which "grid" connection is valid.

My use case is purely backup so I'm less fussed about export, but understanding how the grid interaction works seems crucial before I commit to anything.

@BoatMel ESS on a boat — so when the grid goes down you're doubly off-grid 😄 Love it.

One thing worth flagging that nobody's mentioned yet: if you're connecting to shore power via ESS, make sure your grid code settings match what the marina is actually providing. Got caught out myself when the

ESS on a Multiplus-II requires a grid code set correctly for your region — UK is EN 50549-1, not something you want to bodge or the DNO will have words with you.

Also worth knowing: ESS needs the VE.Bus BMS or a compatible BMS talking to the Cerbo/CCGX, otherwise it'll refuse to export — spent a weekend in a layby once debugging exactly that on my motorhome setup before realising the GX device firmware was three versions behind.

@BoatMel marine install adds another wrinkle — shore power polarity detection on the Multiplus-II is your friend, don't disable it thinking it's clever.

Key bits you'll likely need:

• Cerbo GX or Venus GX
• Keep batteries charged vs Optimised mode — pick carefully
• MFD integration if you're feeling fancy on the boat

@FormerCop makes a really important point about the grid code — worth adding that you'll also need G98 or G99 approval depending on your export capacity. Under 3.68kVA you're typically looking at G98 (just notify your DNO), but above that G99 requires prior approval which can take a while.

@BoatMel — one thing specific to your boat situation: is your shore power connection via a proper marina pedestal? ESS can get a bit confused distinguishing between a weak grid supply (long cable runs to a marina pedestal can cause voltage fluctuations) and an actual grid fault. Worth tweaking the AC input current limit and voltage thresholds in VE.Configure accordingly. Some marina supplies are surprisingly wobbly!

Also check whether your marina's electricity is sub-metered — exporting back could cause complications depending on your agreement with them.