Victron Multiplus tripping out when microwave kicks in - is it undersized?

Had almost the exact same drama on my narrowboat last spring. The Multiplus I had at the time was a 12/1600 and every time the missus nuked her morning porridge it would throw a wobbly — that characteristic clunk and the red overload LED lighting up like a Christmas tree.

The thing with microwaves is they're sneaky. The rated wattage on the label is not the startup surge. A 700W microwave can easily pull 1.2–1.5kW for that first half-second as the magnetron fires up. If your Multiplus is already warm or the batteries are sitting low, it's got even less headroom to cope with the spike.

A few things worth checking before assuming it's undersized:

• Battery cable gauge and connection quality — a dodgy connection creates resistance, which robs the inverter of headroom exactly when it needs it most
• Battery state of charge — at 50% SOC my LiFePO4s sag noticeably under surge loads
• The PowerAssist settings in VictronConnect — worth tweaking if you're on shore power at the same time

Eventually I did upgrade to a 12/3000, which honestly solved it permanently and gave me room to grow. But the cabling fix alone bought me several months of reliable use before that.

What size is your bank, and is it lead-acid or lithium? That changes the picture quite a bit — lead tends to sag far harder under surge loads.

@Jonno yeah microwave surge is brutal, they're basically liars on the tin — a "700W" microwave pulls way more than that on startup.

Had this exact issue on my narrowboat before I upgraded. Switched to a Multiplus 12/3000 and never looked back. The headroom matters more than the rated wattage imo.

Few things worth checking first though:

• Battery cable thickness — undersized cables will cause voltage sag which trips the inverter before it even gets to surge territory
• SOC when it trips — if your bank's below ~50% it'll struggle regardless
• What batteries are you running? LiFePO4 handles surge draw way better than AGM

What's your current battery setup? Could be the inverter or the bank bottlenecking you. Fogstar Drift cells transformed my setup more than the inverter upgrade tbh.

@Jonno mate, microwaves are the drama queens of the kitchen appliance world. My 12/3000 Multiplus barely flinches now, but I distinctly remember the dark days of the 12/1600 — sitting there like a nervous parent watching their kid's first driving lesson every time someone so much as looked at the kettle.

@Jonno the elephant in the room nobody's mentioned yet — what's your battery bank doing when this happens? A 12/1600 Multiplus has a 30ms overload tolerance before it trips, and if your bank's internal resistance is pushing voltage sag during that surge window, the inverter sees it as a sustained overload rather than a spike it can ride through.

Had exactly this with my static caravan setup before I sorted my Fogstar Drift cells properly. Swapping to the 3000 without addressing the battery side would've solved nothing.

Check your BMS logs if you have them — I'd wager you're seeing a voltage dip below 11.5V on startup and the Multiplus is protecting itself perfectly correctly. It's not undersized, it's doing its job. Victron's PowerAssist might also be worth configuring if you haven't touched it.

@FormerTeacher nails it — battery internal resistance is massive here. Even if your bank looks fine, if you're running older AGMs they'll sag hard under that initial surge and the Multiplus sees low voltage and shuts down before it even breaks a sweat.

Few things worth checking:

• Battery voltage under load — chuck a meter on it while the microwave starts
• Cable sizing — undersized DC cables cause voltage drop that mimics a weak battery
• Transfer switch setting in VictronConnect — you can tweak the AC input current limit

That said, a 12/1600 is genuinely marginal for a microwave on a boat. Upgrading to a 24V system with a Multiplus 24/3000 made a night-and-day difference on my garden office setup. Halves your DC current and everything runs cooler.

@FormerTeacher and @CE_Builds have both touched on the battery side which is spot on, but worth adding — what cable runs have you got between your batteries and the Multiplus? On a narrowboat especially, people often underestimate voltage drop across undersized or lengthy cable runs. Even a well-specified inverter will trip on overload if it's seeing artificially low voltage at its terminals due to resistance in the cabling.

I'd suggest grabbing a multimeter and checking the voltage at the inverter terminals the moment the microwave kicks in, not just at the battery. If you're seeing a meaningful difference there's your culprit before you go spending on a bigger unit. Proper 70mm² cabling with short runs and quality connections made a noticeable difference on my setup before I even considered upsizing.