Sizing a off-grid system for a garden office — where did I go wrong first time round?

Fitted out my shepherd's hut last year with what I thought was a decent little system: 2x 200W Renogy panels, a 40A Victron SmartSolar MPPT, and a single 100Ah Fogstar Drift LiFePO4. Ran fine through summer but come November I was regularly hitting low-voltage cutoff by mid-afternoon. Classic mistake — I'd sized for peak generation, not for a proper UK winter where you're lucky to see 2 peak sun hours on a good day.

Rebuilt the bank over winter. Went to 2x 100Ah Fogstar in parallel (200Ah total) and added a third panel. Made a massive difference but I'm now looking at doing something similar for a mate's garden office build — proper insulated cabin, about 3x4m, he wants to run a laptop, monitor, a couple of LED circuits, and a small fan heater occasionally. That heater is the killer — even a 1kW unit on a thermostat will absolutely murder a modest battery bank if it's cycling all day in January.

My current thinking is to skip resistive heating entirely and push him toward a 12V or 24V diesel/paraffin heater (Webasto or a decent Chinese unit) for the thermal load, then keep the electrical system focused on lighting and low-draw devices. 400–600Ah at 24V with 600W of panels feels more realistic than trying to run a heater off solar in winter.

Has anyone actually done the maths on a garden office running an electric heater off-grid through a UK winter? Curious whether anyone's made it work without going absolutely massive on the battery bank, or whether the diesel heater approach is the only sensible answer here.

@Bomber78 the classic summer trap — systems that sail through June/July then hit a wall come November when you're getting maybe 2-3 peak sun hours on a good day and the heating load kicks in.

I've got a similar setup in my garden office and made exactly the same mistake initially. The panels are rarely the bottleneck — it's always battery capacity in winter. Your 100Ah is really only 80Ah usable, and if you're running a monitor, laptop, and any heating that's gone before lunch.

Worth looking at what your actual daily consumption is in winter specifically. Fogstar Drift cells are decent so no issues there, just not enough of them. A second 100Ah would make a noticeable difference without touching the rest of your kit.

What are you running in there typically?

@Bomber78 Classic summer trap that one. I did almost the exact same thing on my first van build — sized everything on glorious July figures then wondered why I was sitting in the dark come November.

The real killer with shepherd's huts and garden offices is that your usage often peaks in winter (heating, lighting, longer working hours) while your generation absolutely tanks. I'm talking maybe 20% of your summer yield on a grim December day in the UK.

What saved me was running a whole month on the Victron VRM portal data before committing to battery size. Turned out I needed roughly 3x what my "back of envelope" maths suggested.

What does your actual consumption look like? Kettle? Monitor? Are you running any heating off it?