Tiny house solar spilling into the garden office — worth running one system or keeping them separate?

So here's my situation. The tiny house is already sorted — 400W of Renogy panels feeding a Victron MPPT 100/30, dumping into a 200Ah Fogstar Drift LiFePO4. Works a treat. But I've just plonked a 6x4m garden office next to it (planning permission is a whole other saga involving a very suspicious neighbour and a tape measure) and now I'm wondering whether to tap into the existing setup or just build a second independent system.

The office needs maybe 300-400Wh a day — laptop, a couple of LED strips, a small fan heater for taking the edge off in winter. Nothing dramatic. The temptation is to just cable across from the existing battery with a decent run of 6mm² twin and earth and be done with it. But something in my narrowboat brain is twitching — on the boat I always hated having everything on one bank when something inevitably went wrong at 11pm on a Tuesday.

Has anyone actually run a shared system across two separate structures? I'm particularly curious about the cable run losses over roughly 8 metres, and whether anyone's stuck a secondary Victron BMV or SmartShunt on a sub-circuit to keep tabs on what the office is pulling independently. Feels like the kind of thing that's elegant until it isn't.

@BoxerWanderer separate systems every time — nothing worse than your garden office Zoom call going dark because the tiny house kettle had other ideas 🔌

@PanelRoger makes a fair point but I'd push back slightly — have you looked at just running a decent cable from the tiny house with a secondary Victron SmartShunt monitoring the office load separately? Keeps one battery bank but gives you visibility on what's being munched where.

That said, I've got a similar dilemma on my narrowboat vs. the van — and keeping them separate has saved me grief more than once. At least if one system does something daft, you're not dragging the other down with it.

Main question for @BoxerWanderer though — what's the actual distance between the two buildings? Because if it's more than 10 metres you'll be losing meaningful voltage over the cable unless you go chunky (and expensive). Might make the maths swing firmly toward a standalone setup anyway.

@HollyGaz is onto something — if the cable run isn't too long it's genuinely the simpler option. I've got a similar setup on mine and just ran a decent 16mm² cable between the two. No second MPPT, no second battery bank to manage.

Main thing I'd check is your existing Fogstar's state of charge patterns day-to-day. If you're consistently sitting at 80%+ through the afternoon, the office load is basically free energy you're currently wasting anyway.

That said, what's the office pulling? If it's just lighting and a laptop it's nothing. If you're running a monitor, heaters, kettle — that changes the maths pretty quickly. Might actually justify bumping to 600W on the array rather than splitting everything into two separate headaches.

@PanelMatt the cable run question is exactly the right one to be asking. I ran a 10mm² twin-and-earth about 14 metres from my Vivaro to a small workshop and voltage drop was negligible at the loads I was pulling. Key thing people overlook though — if you're running a shared DC bus, your Victron MPPT 100/30 will see the combined load and you need to check it isn't throttling solar harvest trying to service both buildings simultaneously.

The cleaner solution depending on @BoxerWanderer's office loads: a second small MPPT dedicated to the office, both feeding the same Fogstar bank. Victron's BatteryProtect on each consumer circuit means neither space cannibalises the other during low-state-of-charge. The Drift handles parallel charging sources absolutely fine — it's one of the reasons I spec Fogstar for exactly this kind of expandable setup.

What's the anticipated office load in watts?

@VivaroAdventure's 14m run is useful data, but the voltage drop calculation really depends on what load current you're expecting to push through it. On 12V systems especially, even short runs at moderate current can cost you meaningfully.

Worth flagging: if you're running separate Victron MPPTs, you can still tie the battery banks together via a proper DC busbar and let both charge controllers feed one combined bank — no separate cabling debate needed. I've got my garden workshop connected this way. The Victron smart networking via VE.Direct or VE.Smart lets both MPPTs communicate and share charging duties intelligently rather than fighting each other.

One combined 400Ah+ bank handles morning and evening loads across both buildings far better than two smaller separate banks that individually flatline in bad weather.

@BoxerWanderer what's the actual load profile in the garden office going to be like? That's what I'd want nailing down before deciding anything. If it's just a laptop, a few LED strips and phone charging, the tiny house bank can probably swallow that without blinking — Fogstar Drift cells are pretty capable and 200Ah gives you decent headroom. But if you're running a monitor, a proper desk setup, maybe a small heater in winter, you're eating into your existing system's reserves on the very days when solar input is lowest. Worth doing a rough Wh/day estimate for both buildings separately first, then see whether that 200Ah actually stacks up across both loads through a typical overcast December week. That's where a lot of these combined system plans fall apart in practice.

Really worth nailing down whether you want the two systems to feel genuinely independent, @BoxerWanderer. If the tiny house battery ever gets pulled low by a big evening load, you don't necessarily want that dragging the office supply down with it. Keeping them separate means a fault in one doesn't cascade into the other — and honestly a small dedicated setup for the office (even just 100-200W of panels and a modest LiFePO4) might cost less than the cabling and protection gear needed to properly tie them together safely.

@BoxerWanderer one thing worth considering before deciding is whether you'll ever want to sell either structure independently. Keeping separate systems preserves that flexibility. That said, if they're definitely staying together long-term, a shared DC bus with the Victron kit you've already got makes real sense — the SmartSolar ecosystem handles multiple charge sources brilliantly and you'd get consolidated monitoring through VRM.

@Brummie92 makes a fair point about load profiling though. If the office is mostly a laptop and a few LED circuits, your existing 200Ah might absorb it comfortably. If you're planning a proper monitor setup or any heating, the maths changes considerably.

@BoxerWanderer I ran a similar dilemma when my motorhome and timber cabin shared a yard in Harrogate. Ended up keeping them separate — and I'm genuinely glad. The moment you tie the systems together, any fault on one side starts hunting the other. A Victron SmartShunt on each keeps the data clean and honest too. That said, a simple Victron Cerbo GX can let both systems talk without actually being bridged — you get visibility across the whole picture without the dependency headache.