Which MultiPlus to add to existing configuration

Been running a single Victron MultiPlus 24/3000/70 in my shepherd's hut setup for about eighteen months now, paired with a modest Fogstar lithium bank, and it's been utterly solid. But I'm starting to hit its limits whenever I run the workshop heater and charge the van simultaneously — the kind of scenario I didn't anticipate when I first specced everything out.

So naturally my mind has wandered toward adding a second unit in parallel.

What's giving me pause is the planning side. My current battery bank could support the extra load, but I'd want to properly model the discharge rates before committing. Has anyone here gone through the process of adding a second MultiPlus to an existing single-unit install? Particularly curious whether the configuration faff in VE.Configure is as involved as some older forum threads suggest, or whether Victron have smoothed that process out in recent firmware.

I'm also wondering if it's worth the leap to a 48V system entirely rather than bolting onto what I already have. Longer term thinking, but relevant if anyone's done a phased migration.

A few things I'm genuinely uncertain about:

• Does the second MultiPlus need to be identical firmware out of the box, or do you update both before pairing?
• Any gotchas with the MK3-USB interface when working with parallel units?
• UK suppliers worth using for the second unit — I've had good experiences with Bimble Solar but open to suggestions

Would love to hear from anyone who's scaled up an existing Victron setup rather than starting fresh. The incremental approach feels sensible but I suspect there are pitfalls I'm not seeing yet.

@IslandOffGrid what's your actual load profile looking like? On my boat I found myself in exactly this situation and nearly bought a second identical unit before realising my peak loads were only brief spikes — a second 3000 wouldn't have been worth it.

Are you planning to parallel two units or use split-phase? Because if it's just paralleling for extra capacity, Victron require matched firmware and ideally the same hardware revision — worth checking before you buy second-hand.

What's the Fogstar bank sized at? If the bottleneck is actually charge current rather than inverter output, you might get more value upgrading the battery side first. What's driving the upgrade — running out of inverter capacity, or wanting more AC charging grunt from shore power?

@IslandOffGrid been down this exact rabbit hole with my van build. Before you commit to paralleling another 24/3000, worth asking whether you actually need the extra continuous watts or just the occasional burst capacity.

If it's peaks you're chasing — a heat gun, angle grinder, that sort of thing — the MultiPlus 48/5000/70 might serve you better as a replacement than stacking two smaller units. Parallel setups are brilliant but the configuration overhead through VE.Config isn't trivial, and both units need identical firmware before you even start.

That said, if your Fogstar bank is 24V and well-specified, keeping the voltage and just adding a second 3000 keeps your existing wiring intact. Ran parallel Victrons on my emergency backup system for two years without a hiccup once the ESS settings were sorted properly.

What's triggering the bottleneck — sustained draw or startup surge?

@IslandOffGrid one thing nobody's mentioned yet: before paralleling, verify your existing 24/3000/70 has the correct VE.Bus firmware for multi-unit operation. Older units shipped with configurations that require a factory reset and re-flash before they'll handshake properly with a second unit. Victron's VE.Configure tool will tell you immediately.

Also worth noting — paralleled MultiPlus units must be identical firmware versions, ideally identical hardware revisions. Mixing -old and newer -P suffix units has caused endless grief on other threads here.

If your shepherd's hut load is predominantly resistive (heating elements, lighting), a second 24/3000 is logical. If you're running high-inrush motor loads, consider whether stepping up to a single 24/5000 might actually be cleaner long-term — fewer VE.Bus comms points of failure.

@IslandOffGrid worth considering whether your use case actually calls for more inverter capacity or more charging grunt. The 70A charger in the 24/3000 is decent, but if you're finding your Fogstar bank is sitting partially charged for extended periods rather than struggling with peak loads, a second unit purely for parallel output might not solve your real problem.

Have you looked at the MultiPlus 24/5000/120 as a straight swap instead? You'd get significantly more charge current in a single unit, avoid the paralleling complexity @Paddy mentions, and shepherd's huts rarely need the redundancy that makes paralleling genuinely worthwhile on something like @VanJulie's boat setup.

What's triggering the upgrade - running out of capacity on big loads, slow charging from shore power, or something else? That'll point you toward the right solution pretty quickly.