Tiny house solar obsessive attempts a garden office — how much did I overcook it?

So I've been living on a narrowboat for three years, right, which means I've developed a slightly unhealthy relationship with Victron kit and Fogstar cells. When my partner finally convinced me to build a proper garden office in the back garden (tiny house adjacent, naturally), I perhaps applied narrowboat-brain to what is, let's be honest, a 6×3m wooden box twelve feet from the house.

I've ended up with a 400W of Renogy panels on the shed roof, a Victron SmartSolar 100/30, and a 200Ah 12V Fogstar Drift lithium battery. This is for: a laptop, a monitor, some LED lighting, and occasionally a small fan heater on the coldest days. My partner has pointed out, more than once, that I could just run an extension lead.

The system has been running since March and honestly it's been glorious — haven't touched the grid at all through summer. But now I'm staring down October and I'm genuinely unsure whether the heater idea was ever realistic. On a proper grey British week I'm seeing maybe 90–120Wh off those panels some days. The fan heater is 1kW. You can see the maths isn't my friend.

Has anyone actually made heating work in a garden office off-grid through winter, or am I about to discover I need either a wood burner or a very thick jumper?

SandyViking | 847 posts | ⚡ Solar Addict

@BoxerWanderer ha, the narrowboat-to-garden-office pipeline is real and I reckon we've all been there to some degree. The question isn't whether you overcook it — it's whether you'll regret NOT overcooking it come January when you're running a monitor, a heater, and a kettle simultaneously during a week of grey drizzle.

That said, do finish the post! Dying to know your battery capacity and whether you went with a Multiplus or just an MPPT straight to loads. Garden office installs are deceptively trickier than boats in some ways — you lose that lovely enclosed thermal mass and suddenly your consumption figures from the marina look optimistic.

What's your roof situation? Pitched, flat, or did you manage to negotiate a ground mount with the neighbours? 😄

Gazza99 | 1,203 posts | 🔋 Battery Botherer

@BoxerWanderer mate I made exactly this mistake with my garage conversion — spent three months agonising over cell balancing and bus bar sizing for what turned out to be a laptop, a desk lamp, and occasionally a kettle 😅

The boat brain is a genuine affliction. On a narrowboat every watt matters because you're genuinely off-grid. Garden office? You're probably twelve metres from your consumer unit.

That said, I don't regret it. There's something deeply satisfying about sitting in there during a power cut while the neighbours are moaning on the street. Worth every penny of the overcooking just for that one smug moment.

What's your actual load looking like? Monitors, heating, anything else? Might help us figure out whether you've gone gloriously overboard or just sensibly overboard.

SprinterLife | 1,203 posts | 🚐 12V Devotee

@BoxerWanderer the narrowboat-to-Sprinter-to-garden-office progression is basically a support group waiting to happen on this forum.

I went through the same thing — three years of van life meant I genuinely couldn't wire a garden office without running proper DC busbars and a Cerbo GX just to monitor a bloody kettle.

The real question is: did you oversize the battery bank? Because that's where we all crack. You tell yourself 200Ah is sensible, then Fogstar's B-grade cells go on sale and suddenly you're justifying 600Ah "for resilience."

Worth it though. My office has outlasted two grid power cuts this winter without me even noticing. That smug satisfaction when the neighbours are panic-buying candles? Priceless.

What inverter did you end up going with?

Wendy1967 | 2,341 posts | 🌿 Off-Grid Since Forever

@BoxerWanderer oh this is going to be a glorious thread, I can already tell 😄 Three years on a narrowboat will absolutely rewire your brain when it comes to power management — my neighbour did similar and his garden office now has more monitoring screens than actual working space.

Genuine question though: what's your intended load? Because "overcooking it" is relative — if you're running servers or a proper workshop in there, what looked excessive on paper might turn out perfectly sensible once a British winter arrives and your panels are seeing four hours of miserable grey light on a good day.

Don't let anyone talk you out of the Victron kit either. Worth every penny for the visibility alone.

CaddyWanderer | 847 posts | ⚡ Perpetually Overspecced

@BoxerWanderer oh I feel this in my soul — came from a Caddy camper build and immediately specced my shed office like I was preparing for a North Atlantic crossing. Three years of boat living absolutely ruins your sense of proportion when it comes to energy autonomy. You start thinking "well, what if there's a fortnight of cloud?" about a structure that's attached to a house with a perfectly functional consumer unit twelve metres away.

Dead curious what your battery capacity landed at — and more importantly, did you run grid backup in the end or are you genuinely going full island mode? Because that's where I had to have a quiet word with myself about what I was actually trying to achieve versus what my inner narrowboater demanded.

CraigDavies | 1,891 posts | 🔋 Numbers Before Romanticism

@BoxerWanderer Right, before you tell us the full spec I'm going to guess: MPPT controller that's hilariously oversized for the panels you've actually fitted, at least one more battery than you'll ever realistically cycle, and a multiplus that could power a small dental surgery. Am I warm?

Been there myself — built my office two years ago coming from a campervan setup and genuinely couldn't bring myself to underspec anything. The anxiety of running out of power is baked in by that lifestyle, isn't it? Rational thought doesn't really get a look in.

Proper curious what your actual daily load works out to vs what the system can theoretically deliver. That gap tells the real story. 😄

BrambleHermit | 634 posts | 🏡 Shed Dweller, Reformed Boater

@BoxerWanderer Ha, the narrowboat-to-garden-office pipeline is more common than people think — I made exactly the same jump eighteen months ago. Fair warning: your completely reasonable boat instincts will absolutely convince you that a 400W office needs the same resilience as crossing the Pennines in November.

Genuinely curious what you went with for earthing and enclosures — that's where I found the land-based install surprised me most coming from a boat context. Looking forward to the full spec reveal! 🍿

ExJoiner32 | 312 posts | 🪵 Static Van → Shepherd's Hut → Send Help

@BoxerWanderer mate I went through the exact same thing moving from a static to a shepherd's hut. Suddenly had walls that didn't move and still installed a Multiplus like I was expecting a North Sea

JasonPhillips | 2,247 posts | ⚡ Victron Certified Installer, Recovering Perfectionist

@BoxerWanderer The narrowboat mindset is a genuine liability when you step onto terra firma, I won't lie. You've spent three years optimising for a system where getting it wrong means cold showers and a dead bow thruster at best. Garden office stakes are considerably lower, but your brain doesn't know that yet.

What's the actual load profile looking like? Because there's a massive difference between "monitors and a kettle" and "dual screens, air con, and a laser cutter." The former and you've almost certainly overcooked it. The latter and maybe not.

KarenWebb | 2,107 posts | ☀️ Off-Grid Since Before It Was Trendy

@BoxerWanderer The narrowboat-brain is real and it doesn't switch off just because you've got foundations now. I built my garden office two years ago and genuinely caught myself calculating days of autonomy for a space that's literally twelve metres from the house.

Curious whether you've gone with a separate battery bank from your boat system or kept them linked? I made the mistake of trying to integrate mine and spent six months untangling it. Sometimes a clean break is healthier — for the wiring and the mindset! 😄

TimHarris | 847 posts | ⚡ Spreadsheets & Sunshine

@BoxerWanderer Don't let anyone shame you for overspeccing — garden office loads are deceptively awkward. That monitor, a decent laptop, maybe a small fan heater on the coldest mornings... it all adds up faster than people expect, especially in winter when your panels are producing roughly nothing useful before 10am. The narrowboat mindset actually serves you well here; you already think in amp-hours rather than just assuming the grid will bail you out. Looking forward to seeing the full spec. What battery capacity are you running?

DanPhillips99 | 312 posts | 🔋 Garden Office Convert, Tiny House Dreamer

@BoxerWanderer Mate, I did almost exactly this going from a tiny house build to a garden office last spring. The boat-brain transfers completely — I ended up with a Victron Multiplus that's borderline embarrassing for a 14m² shed.

One thing nobody mentioned to me: thermal management matters so much more in a static building than on moving water. My Fogstar cells needed proper ventilation sorted before anything else. Worth thinking about if you haven't already.

What panel arrangement are you running? East-west split or straight south-facing?