Output of 1 Multiplus feeding another Multiplus

Been thinking about this setup lately and wondered if anyone's had hands-on experience with it.

I'm looking at running a Victron Multiplus-II in my cabin to handle the main battery bank and solar input, then potentially feeding a second smaller Multiplus in a separate outbuilding — basically using the AC output of the first unit as the "grid" input for the second.

The question is around how well the second unit handles being fed by an inverter source rather than true mains. Victron's AC coupling documentation touches on this but I've not seen many real-world accounts of a Multiplus feeding another Multiplus directly, especially in a pure off-grid scenario.

A few things I'm mulling over:

• Frequency shifting — if the first unit tries to curtail solar via frequency, does the second unit get confused?
• Load spikes — will the first unit cope if the second suddenly demands a chunk of power for something like a pump startup?
• UVA settings — presumably the second unit needs its input sensitivity loosened right off so it doesn't reject the "soft" inverter output?

I've seen people run a generator into a Multiplus without issues, but a generator at least has some inertia. Another inverter as the source feels like it could get trickier.

Anyone actually running something like this in the UK — maybe a main house system feeding a workshop or separate annex? Curious whether VE.Bus networking between the two units changes anything, or if they'd need to be kept completely independent.

Would love to hear how others have approached it before I start throwing money at cables and mounting plates! 😄

TerryLewis | Posts: 847

@DODQueen interesting idea but I'd steer well clear of daisy-chaining Multiplus units like that. The downstream inverter will see a synthetic AC waveform from the first unit and that can cause all sorts of grief with synchronisation - you'd essentially be asking one inverter to run off another's output as though it were grid power, which it really isn't.

If you're after more capacity, paralleling or three-phase stacking via the proper Victron configuration is the supported route. They talk to each other via the VE.Bus cable and actually coordinate properly.

What's your actual load requirement and battery bank size? There might be a more straightforward solution depending on what you're trying to achieve at the cabin. Sometimes a single larger unit sorts it without the complexity.

@TerryLewis what's the actual failure mode you're worried about though? Curious whether you've seen this go wrong in practice or if it's more of a theoretical concern.

@DODQueen I've been mulling something similar for my static caravan — one Multiplus handling shore power/solar charging, and wondering whether a second unit downstream makes any sense for certain loads.

Is the goal here to create some kind of isolation for sensitive equipment, or more about expanding capacity? That changes things quite a bit I'd imagine.

Also, does the second unit in this scenario even "see" a clean enough AC input to function properly, or does it start behaving erratically trying to regulate against an already-inverted source?

@JennyCole the core issue is feedback impedance. The upstream Multiplus sees the downstream unit's AC input as a load, but when the downstream inverter backfeeds during a grid/generator dropout event, the upstream unit can interpret that as a legitimate AC source and attempt to sync to it — you can end up with two units fighting over phase reference. I've seen this cause nuisance tripping and in one nasty case, a damaged transfer relay.

That said, there are legitimate cascade scenarios — Victron actually document using a Multiplus to power a second unit's AC input for certain island mode configurations, but it requires careful programming of the assistants on both units so each knows its role explicitly.

What's your actual goal here @DODQueen? If it's load splitting or capacity expansion, there are cleaner paths — parallel units on a shared busbar, or a proper Multi/Quattro with dual AC inputs.

RetiredPlumber | Posts: 2,341

Done something adjacent to this in my static caravan setup — not intentional daisy-chaining, more a legacy wiring headache I inherited.

@RustySpanner has the right end of it. The practical consequence I'd add is that the upstream unit's transfer switch timing can create brief gaps that the downstream unit interprets as a grid fault. You end up with nuisance trips, particularly under variable loads.

Victron's own documentation steers you toward ESS with parallel or three-phase configurations if you genuinely need expanded capacity. Two Multiplus-II units in parallel on the same bus is the supported path — they communicate via VE.Bus and handle load sharing properly.

What's the actual problem you're trying to solve @DODQueen? Capacity, redundancy, or separating circuits? The answer might be simpler than stacking inverters.

@RustySpanner that impedance point is the crux of it, and it bit me in a roundabout way on my motorhome build.

I had a Multiplus 12/3000 feeding a small inverter-charger downstream — not two Multiplus units, but close enough. The upstream unit kept throwing a high AC ripple alarm because it couldn't reconcile what it was "seeing" on the output versus what it expected. Took me weeks to diagnose it.

The thing people overlook is that Victron's transfer switch logic gets deeply confused when the AC source it's connected to isn't a "dumb" generator or mains feed — it's looking for specific voltage stability characteristics that another inverter simply doesn't produce consistently.

If you genuinely need this kind of redundancy in a cabin setup, @DODQueen, I'd seriously look at a proper parallel or ESS configuration instead. Two units working together rather than one feeding the other.

Tried something vaguely like this when I was wiring my garden office — accidentally fed my little 500W inverter from the output of my Multiplus-II and spent three days wondering why everything kept browning out before my mate pointed at the wiring diagram and laughed at me 🤦

The bit nobody mentions: Victron's own VE.Bus comms gets very confused when two units aren't properly paired, throws phantom fault codes that'll send you down a rabbit hole of firmware updates for absolutely no reason — ask me how I know.

If you genuinely need the capacity, just parallel two units properly with the VE.Bus cable rather than daisy-chaining — Fogstar batteries handle the extra draw fine in my experience, and you won't want to hurl your laptop into a hedge.