Split Battery/Microinverter day/night power generation/storage

Ran this exact setup in my cabin last summer — Victron MultiPlus handling the overnight storage side whilst a cheap Enphase microinverter on the south-facing panels feeds straight into the AC circuit during the day, completely bypassing the batteries.

The logic being: why cycle your expensive Fogstar lithium cells unnecessarily when the sun's cracking it and you can just consume directly off the microinverter output?

Where it gets interesting is the split — essentially you end up with two "virtual" systems:

• Daytime: microinverter handles loads directly, surplus spills into batteries via the Victron
• Night: Victron draws down stored capacity, microinverter is obviously sat doing nothing

Efficiency gains are real but honestly the wiring topology is where most people come unstuck. Getting the AC coupling right so the Victron doesn't throw a wobbly when it sees the microinverter frequency-shifting took me an afternoon I'll never get back.

Anyone running a similar split arrangement in a van context? I've been mulling whether the weight penalty of carrying both an inverter/charger and a dedicated microinverter is actually worth it for a mobile setup, or whether you'd just be better off with a proper MPPT and call it a day.

Curious what topologies others have landed on — especially if you've cracked AC coupling without losing the will to live. 🔌

@Lefty72 the sneaky genius of this is the microinverter essentially becoming free daytime power whilst your MultiPlus barely breathes — batteries stay fat for night duty and your Fogstar cells love not being hammered with constant charge cycles 🔋

Worth flagging though: G98/G99 compliance is the uninvited guest at this party — your DNO needs to know about grid-tied microinverters even on a cabin setup, otherwise you're technically running an unnotified installation which gets awkward fast.

Also check your MultiPlus firmware is handling the AC coupling correctly — Victron's frequency-shift power control needs to babysit the microinverter output when batteries hit absorption, otherwise you get that lovely moment where everything argues and trips out simultaneously.

Had similar shenanigans in my van conversion before I sorted the settings properly. Victron's ESS documentation is your best mate here.

@Lefty72 @BoatPaddy — worth flagging the bit nobody mentions: your MultiPlus will still be "online" during the day and its AC input sensing will see the microinverter output as grid-like power. Make absolutely certain you've configured the AC input current limit correctly in VEConfigure, otherwise the MultiPlus may attempt to draw additional current from that microinverter leg and you'll get some very unhappy waveform behaviour.

Also — Enphase units grid-tie and therefore require a live AC reference to operate. If your MultiPlus goes into inverter mode (say, overnight battery protection kicks in unexpectedly), the microinverter will island-detect and shut down. That handoff sequence matters enormously.

On my narrowboat I run a similar split arrangement and the PowerControl settings in VEConfigure saved me considerable grief. Worth reading Victron's ESS documentation thoroughly before assuming it'll self-manage.

Really interested in this setup @Lefty72 — one thing worth getting your head around is the grid-forming side of things. Your MultiPlus needs to be the one establishing the AC bus for the microinverter to lock onto, so if the MultiPlus ever trips or goes into passthrough, the Enphase loses its reference and shuts down pretty sharpish. Seen a few folk catch out by this when their battery hits low voltage cutoff mid-afternoon. Worth setting your MultiPlus low battery threshold conservatively so it doesn't disconnect before the sun's done its job. Also check your Enphase monitoring — it'll likely be logging that AC source as "grid" which looks daft on the stats but functionally it's fine.

Running something similar on my static caravan — been wondering about this exact split approach actually.

@SilverSpanner what's the real-world efficiency hit when the MultiPlus is doing that grid-forming work all day even with minimal load? Feels like the idle draw could quietly eat into whatever the microinverter is supposedly "saving" you.

Also — has anyone metered the actual consumption on the MultiPlus standby vs just letting it handle everything conventionally? I keep meaning to chuck a power monitor on mine but never get round to it.

And is there a minimum battery bank size where this setup stops making sense? Wondering if a smaller Fogstar DIY pack changes the maths compared to something beefier.

Not saying it's a bad idea, just want to see actual numbers before I start rewiring anything.