Garden office solar sizing — am I massively undershooting this?

Finally pulling the trigger on a proper off-grid setup for my garden office. Currently running a 200W panel into a Victron SmartSolar 100/20 MPPT and a single 100Ah Fogstar Drift LiFePO4. Powers a laptop, a couple of monitors, and a small desk lamp. Works fine in summer but come October it's a different story.

Winter loads are roughly 150–200Wh/day but with the rubbish UK sun hours (sometimes 1–1.5 peak hours here) I'm worried the 200W panel just isn't cutting it. Did the maths and on a bad day I'm only pulling in maybe 200–300Wh before any losses. Feels marginal at best.

Thinking about adding another 200W panel in parallel to bring it to 400W total, and possibly a second 100Ah Fogstar for storage headroom. But is doubling the panel capacity actually worth it in winter, or does the panel spec become almost irrelevant when the sky is just grey soup for days on end? Would a bigger battery bank just buy me more buffer through the dark spells rather than the extra panel making much difference?

Has anyone actually measured their real-world winter yields from a garden office setup in the UK? Curious what numbers people are actually seeing vs what the calculators suggest.

@Nobby yes, massively undershooting it. 100Ah LiFePO4 gives you ~1.2kWh usable. A laptop, monitor, and basic lighting will chew through that by early afternoon on a decent day — and in November you'll be lucky to pull 40% of rated output from that 200W panel.

I run a shepherd's hut office in the north of England. Minimum I'd suggest for year-round reliable use:

• 400-600W panels (east/west split if roof space is awkward)
• 200Ah+ LiFePO4 — Fogstar do a 200Ah Drift that stacks neatly
• Victron MultiPlus if you want any grid fallback

The MPPT you've got is actually fine for now — 100/20 handles up to ~290W on 24V.

What's your actual daily load in Wh? Without that figure nobody can size this properly.

@Nobby the panel is your bigger bottleneck IMO. 200W in a UK garden office — factor in shading, low winter sun angles, and a south-facing pitch that's probably not optimal, and you're realistically pulling 60-80W average on a decent day. That barely keeps pace with moderate usage, let alone builds reserve.

I run a similar setup at my cabin — bumped to 400W across two panels and it transformed reliability from October through March. The SmartSolar 100/20 has headroom to handle more panels, so you're not throwing money at new kit.

Also worth logging your actual consumption for a week before adding battery capacity — most people overestimate how much they need once they see real numbers. Victron's VRM portal is brilliant for this if you've got the GX device or a Cerbo.

Hey @Nobby, both @SolarJunkie and @Inverter_Pro have nailed the main issues, so I won't repeat them. One thing worth adding though — what's your panel orientation and roof pitch? I've got a south-facing setup at around 35° and even then December/January can be genuinely grim for generation. If your office roof faces anything other than south, I'd be even more pessimistic about winter performance than @Inverter_Pro suggests.

Also worth thinking about whether you need this fully off-grid year-round or just spring through autumn. Plenty of folk on here (myself included) run a small grid backup charger for the dark months rather than massively oversizing the whole system. Keeps the initial outlay sensible. What's your actual daily usage looking like in kWh? That'd help give you more concrete numbers.

@Nobby one thing nobody's mentioned yet — have a look at your actual load profile before you start throwing money at more panels and batteries. Grab a plug-in energy monitor (Owl or similar) and run your office kit for a full working day, then you've got real numbers to size from rather than guesstimates.

Also worth considering: a small immersion diverter or dump load isn't relevant here, but scheduling your heavier tasks (video calls, rendering, whatever) around midday when generation peaks absolutely is. Behaviour changes can stretch that existing battery surprisingly far in spring/autumn.

The Fogstar Drift is a solid battery mind you, good choice for the base of an expanded bank when you do upscale. Keep the Victron — it'll handle more capacity no bother.

@IslandDweller replied:

@Nobby good shout from @JakeHall76 on the load profiling — that's where I'd start too. One thing I'd add: think carefully about battery depth of discharge across consecutive cloudy days, not just daily balance. A single 100Ah LiFePO4 sounds decent on paper but a run of grey December days in the UK will chew through usable capacity faster than you'd expect. Even a second 100Ah Fogstar in parallel gives you a meaningful buffer without jumping to a much larger system. The Drift cells handle parallel configurations well in my experience. Also worth checking your MPPT's charging profile is correctly set for LiFePO4 — absorption voltage especially. Small thing but easily overlooked when swapping from AGM. What's your typical worst-case winter working day look like hours-wise?