MultiPlus-II : How to connect AC in a parallel topology?

Setting up a MultiPlus-II for my garden office build and trying to get my head around the parallel AC topology before I commit to wiring anything up.

My plan is:

• MultiPlus-II 48/3000 as the inverter/charger
• Fogstar Drift lithium batteries (probably 2x 200Ah)
• Renogy solar feeding via a separate MPPT into the battery bank
• Grid connection available as backup/top-up

The part I'm fuzzy on is how the AC-in and AC-out sides should be wired when running alongside the grid rather than fully off-grid. Specifically:

1. Does AC-in just connect straight to my consumer unit feed, or does there need to be some kind of changeover/isolation between the grid supply and the MultiPlus output?
2. When the MultiPlus is in "grid parallel" mode, is it essentially always passing through grid power with the inverter topping up or taking priority depending on settings in VE.Config?
3. Do I need a separate RCD/RCBO on the AC-out side, or does the MultiPlus handle that internally?

I've watched a fair few Victron webinars and read through the ESS design guide but it gets a bit dense around the transfer switch behaviour and what the regs expect for a garden office setup specifically. Not sure if Part P applies differently here compared to a main dwelling either.

Anyone run a similar setup in the UK? Keen to understand what a compliant install actually looks like before I get an electrician involved.

Hey @CE_Builds, welcome to the forum!

Just to clarify before you go further — with a single MultiPlus-II you're not really doing a "parallel topology" as such, that term usually refers to multiple units running together.

For a single unit, your AC In connects to the grid/generator supply and AC Out 1 feeds your critical loads (those get backed up and pass-through when on mains). AC Out 2 is non-essential loads that drop off when inverting on battery.

Key thing to double-check: make sure your incoming supply is properly fused and that you've got a suitable isolation switch before the unit. Victron recommend 32A max input on the 3000VA model.

What's your battery setup and are you planning grid-tied or fully off-grid? That'll affect how you configure the transfer switch behaviour in VEConfigure. 👍

Hey @CE_Builds, good thinking to plan this out before you commit to anything!

To add to what @Compo55 is saying — "parallel topology" with a single unit isn't really a thing as such. With one MultiPlus-II you've essentially got a single inverter/charger doing its job.

If you're thinking about running grid and inverter together to supplement each other, what you're actually after is the AC-coupled or ESS configuration depending on your goals. The MultiPlus-II handles grid passthrough and switching automatically via its internal transfer switch.

Worth having a read through Victron's MultiPlus-II datasheet and the ESS design guide on their documentation portal — they're genuinely well written and will clarify the topology options nicely.

What's your overall setup looking like? Solar panels, battery bank size? That'll help work out which configuration suits your garden office best.