Sizing a solar setup for a shepherd's hut on wheels — where do I even start?

So I'm converting a trailer-mounted shepherd's hut (approx 4.5m x 2.2m) and I want to run it fully off-grid. It'll mostly be static but I need to be able to tow it between sites a few times a year. Trying to figure out whether to treat this more like a motorhome build or a static cabin setup — feels like it falls awkwardly between the two.

Current thinking is 400W of panels on the roof (probably Renogy mono, as the curved roof limits my options slightly), feeding into a Victron SmartSolar MPPT 100/30, and then 200Ah of lithium — looking at Fogstar Drift as they seem decent value. That should handle LED lighting, a small 12V compressor fridge, phone/laptop charging, and maybe a small inverter for the odd power tool. No mains hookup planned at all.

The bit I'm unsure about is whether 200Ah is enough for 3–4 cloudy UK winter days without a backup source. I've read some people run a small B2B charger off the tow vehicle alternator, which would at least top things up on moving days. Has anyone done something similar with a hut or van build in the UK? Wondering if a Victron Orion-Tr Smart is the right shout for the B2B side of things.

@SophieHobbs74 Shepherd's hut on wheels is just a motorhome that's embarrassed about it — start by listing every appliance, multiplying watts × hours = Wh per day, double it for cloudy days, then that's your battery size, and panels are roughly battery capacity ÷ 2 for decent UK recharge times... Fogstar for the LiFePO4, Victron for the brains, Renogy if budget's tighter than your tow hitch.

@SophieHobbs74 What's your expected daily usage like? That's the bit that trips everyone up at the start.

Also worth clarifying — will it have mains hookup as a backup option when you're on site, or genuinely 100% off-grid? Changes the battery sizing conversation quite a bit.

I'm mid-way through a van build myself and the thing I wish I'd done earlier was run everything for two weeks before committing to panel/battery size — just track what you actually use rather than what you think you'll use. Especially if you're planning any heating (a diesel heater draws surprisingly little, but electric heating is a different story entirely).

What's your rough budget looking like? Victron kit is the gold standard but Fogstar batteries have become pretty popular for keeping costs down without sacrificing too much.

@SophieHobbs74 Slightly different angle — what's your roof situation? Shepherd's huts often have that lovely curved or ridged tin roof which makes panel mounting a real headache. Worth figuring out early whether you're going flat rigid panels, flexible adhesive ones, or a small ground-mount array you deploy when static.

I'm on a boat so same kind of constraints around awkward mounting surfaces — ended up going flexible panels but honestly wish I'd thought harder about it before committing.

Also, being mostly static is actually a big advantage for sizing — you're not losing harvest to driving shade or dodgy angles like a moving motorhome would. What region are you typically siting it? UK solar yield varies more than people realise between say Cornwall and Scotland.

BitsAndBobs9 | 47 posts

@SophieHobbs74 One thing nobody's mentioned yet — because you're towing it, weight and vibration matter more than in a fixed install. I'd strongly suggest AGM or lithium over flooded lead-acid; flooded batteries really don't thank you for being bounced down country lanes a few times a year, and the maintenance headache isn't worth it.

Also factor in your tow vehicle — if it has a decent alternator you could wire in a B2B charger to top up your batteries on the move between sites, which gives you a nice head start on arrival rather than relying solely on solar after a grey February transit. Small thing but it adds up.

@MuddyTinker raises a good point about the roof — curved rooflines can complicate tilting mounts too, so worth thinking about early.

RhysGrant | 312 posts

Great project @SophieHobbs74! To add to what @TorChild is asking — once you've got your rough daily kWh figure, a decent rule of thumb for the UK is to size your panels assuming maybe 2–3 peak sun hours in winter rather than the summer figures you'll see on most calculators. We don't exactly live in the Algarve unfortunately!

A 400–600W panel array with a 200Ah lithium battery is a reasonable starting point for a small shepherd's hut with modest loads, but your actual usage really does dictate everything. Are you planning any heating through the solar setup, or keeping that separate? That's often the make-or-break question for winter viability up here. Most people end up with a wood burner handling heat and solar covering lights, phone charging, a 12V pump and maybe a small fridge.

StuCampbell | 1,847 posts

@BitsAndBobs9 raises a genuinely important point about vibration — worth expanding on.

On my narrowboat I learned the hard way that lithium cells in a moving application need proper compression and rigid mounting. Don't just set a Fogstar or EG4 battery on a shelf and call it done. Cell-level movement causes micro-fractures over time.

Also, for a static-mostly-but-sometimes-towed setup, I'd seriously consider a Victron SmartShunt from day one rather than retrofitting. Knowing your actual consumption patterns over the first few weeks is invaluable for deciding whether your initial battery bank sizing was right.

On the solar side — that curved roof will likely limit you to 2–3 panels max without going portrait orientation, which then creates partial shading issues. A Victron MPPT with individual string optimisation handles that far better than a cheap PWM controller ever will.