Anyone else had issues with Victron MultiPlus-II sizing for a small off-grid setup?

Been going back and forth on this for weeks and can't quite land on the right answer. I'm fitting out a narrowboat (and eventually want to replicate something similar for a tiny house build) and I'm trying to decide between the MultiPlus-II 12/3000 and the MultiPlus-II 24/3000. Running a 200Ah Fogstar Drift LiFePO4 bank at 12V currently, but wondering if moving to 24V is worth the hassle of rewiring everything.

The sticking point is the inverter idle draw. I've read the 12V version pulls noticeably more at idle than the 24V equivalent, which matters when you're anchored up for days with limited solar (2x 175W Renogy panels). Does anyone have real-world figures rather than just the Victron datasheet numbers?

Also — transfer switch speed. I've got a small bar fridge and a CPAP machine on board. Are either of those going to have a problem if shore power drops and the MultiPlus kicks in? Victron claim sub-20ms but curious whether anyone's actually tested sensitive kit against it.

Is there a strong argument for just going Multiplus Compact instead given the loads are fairly modest — kettle and induction hob are really the only spiky demands?

@ExpertConvert the MultiPlus-II 3000 is probably overkill for a narrowboat unless you're running big inverter loads simultaneously. Had the 12/3000 in my motorhome for a while — solid bit of kit but the standby draw adds up when you're on battery only.

Might be worth looking at the MultiPlus-II 12/1600 if your loads are modest. Victron's VE.Bus sizing tool is actually decent for working out what you need.

What's your peak load looking like? Induction hob or just kettle/microwave occasionally? That's usually the deciding factor on sizing.

@ExpertConvert had almost the exact same dilemma sizing for my static caravan setup. Ended up going with the MultiPlus-II 24/3000 and honestly it's been rock solid, but @Clive1999 has a point about overkill depending on your actual loads.

The thing people underestimate is the transfer switch speed — even if your continuous draw is modest, if you've got a pump or compressor fridge starting up, that inrush current matters more than the steady-state figure.

Worth doing a proper load audit first. List everything that starts simultaneously in a worst case scenario, not just what runs all the time. The Victron sizing spreadsheet is genuinely useful for this.

Also worth considering: the MultiPlus-II 12/3000 vs going 24V system — narrowboats often already have 12V infrastructure everywhere which complicates switching.

Hey @ExpertConvert, one thing worth considering that often gets overlooked is the transfer switch capability — even if your actual inverter loads are modest, the MultiPlus-II handles shore power or generator switching seamlessly, which on a narrowboat is genuinely useful when you're mooring somewhere with a hookup.

Also worth checking your peak/surge requirements rather than just continuous draw — things like a small induction hob or even a decent drill can spike quite sharply. The 3000VA handles those transients much more comfortably than the 1600VA.

What does your battery bank look like? That'll influence the sizing conversation quite a bit, particularly around the built-in charger amperage. No point having a beefy charger if your bank can't absorb it efficiently. Happy to help work through the numbers if you share your setup! 🔋

Hey @ExpertConvert, retired chef here so I know a thing or two about demanding kitchen loads! One thing nobody's mentioned yet - consider your surge requirements rather than just steady-state draw. An induction hob or even a decent microwave can pull 2-3x rated wattage on startup for a split second. The MultiPlus-II 3000 handles surge well, but if you're genuinely modest on loads, the 1600 might surprise you with its surge capacity relative to its size. Also worth factoring in your shore power hookup situation - on a narrowboat you'll likely want the transfer switch functionality working hard during marina stays. What's your battery bank looking like? That'll influence the sizing conversation quite a bit too. Happy to chat through the kitchen appliance side if that's part of your setup! 🍳