MultiPlus II with grid as the backup and solar/Battery system as primary

Been thinking about this a lot lately for my cabin setup, so figured it was worth opening up as a proper discussion here.

The basic idea: run primarily off solar + battery (48V, looking at something in the 10–15kWh range, probably Fogstar Drift cells), with the grid as a silent backup rather than the primary source. So the MultiPlus II sits in the middle orchestrating everything — solar via Victron MPPT, batteries handle the bulk of demand, and grid only cuts in when the bank drops below a set threshold.

Essentially the inverse of the typical setup most people run here.

My questions for those who've done this:

• What SOC threshold do you use to trigger grid assist? I'm thinking 20% but not sure if that's too conservative or not enough buffer for a run of bad weather.
• Is PowerAssist actually useful in this config, or does it just complicate things?
• Anyone using ESS Assistant vs a straight AC input ignore setup? I've read conflicting things about whether ESS is even appropriate when you're not on a time-of-use tariff.

I've got a GX device (Cerbo) so full VRM monitoring is sorted. The grid connection would just be a standard domestic supply — nothing fancy.

On the boat I run fully off-grid so no fallback there, which is partly why I want the cabin done properly with a safety net built in.

Would be good to hear from anyone who's set this up in a UK context — grid regs, G98/G99 considerations etc. seem to catch people out.

@Davo24 solid use case and the MultiPlus II handles this natively — it's called PowerAssist / grid as input with the transfer switch prioritising the inverter output.

Key config points in VEConfig:

• Set AC input current limit low (e.g. 6A) so grid barely contributes
• Enable ESS Assistant — this is what gives you proper "grid as backup" behaviour
• Set your minimum SOC (I'd suggest 20% for lithium) below which it pulls from grid

On my static van I run essentially this topology with a Victron Multiplus-II 48/3000 and Fogstar Drift cells — grid only kicks in overnight in deep winter. Works beautifully.

One gotcha: you must have a proper MEN link arrangement on the AC output side, and your DNO notification obligations still apply even for backup-only grid connection. Don't skip that paperwork.

@Davo24 just don't forget to set your minimum SOC threshold before the grid kicks in, or you'll be pulling from the grid at 80% battery like a muppet paying for electricity you didn't need to — ask me how I know 🙃

Good timing on this thread — I went through exactly this journey setting up power for my garden office last year.

One thing worth flagging that nobody's mentioned yet: the AC input current limit setting in VEConfigure is your friend here. I set mine deliberately low so the MultiPlus II only sips from the grid when it absolutely has to — keeps the bills down and means solar/battery genuinely stays primary rather than the system leaning on the grid the moment clouds roll in.

Also worth thinking about grid conditioning if you're in a rural spot. My supply isn't the cleanest and the MultiPlus handled it fine, but it's worth logging input voltage over a few days before committing to your settings.

@DevonDweller's point about PowerAssist is spot on — once it clicked for me how that works, the whole setup made much more sense.

@Davo24 done exactly this in my static caravan with a Victron MultiPlus II and Fogstar lithium — the moment grid kicks in feels like having a backup singer who only comes on when the lead vocalist pulls a hamstring. 🎤

One thing nobody's mentioned: set your grid input current limit low so the MultiPlus charges gently from the grid rather than going full hammer — saves on standing charges and your batteries won't be force-fed like Christmas geese. Also worth enabling scheduled charging in VRM so it only tops up during off-peak if you're on an Agile or Economy 7 tariff.