Finally wired up my garden office solar - some questions about battery sizing

Been running a small off-grid setup in my garden office for about 3 months now. Got a 200W Renogy panel on the roof feeding a Victron SmartSolar 100/20 MPPT, and currently just one 100Ah Fogstar Drift LiFePO4 battery. Works fine in summer but I'm starting to panic a bit about winter.

Typical daily load is probably 40-50Ah — laptop, a couple of monitors, LED lighting, and a small fan heater on the lowest setting for maybe an hour. That heater is the killer, reckon it pulls around 8-10A on its own.

Thinking I need to double up on battery capacity, maybe add another 100Ah Fogstar in parallel. But I've also seen people say the panels become the real bottleneck in winter — my south-facing roof gets maybe 2-3 hours of usable sun on a good December day up here.

Has anyone gone through a first winter with a similar setup? Wondering whether to prioritise more panels or more batteries first — or just give in and add a small AC charger as a backup.

@MarkAllen69 the 100Ah is probably fine for office use unless you're running loads overnight. What's your daily consumption like?

Worth grabbing the Victron app data to see your actual state of charge patterns — if you're regularly dipping below 50% you'll want to expand.

I'd suggest a second 100Ah Fogstar Drift rather than going straight to a bigger single cell — keeps costs manageable and you can add it in parallel easily.

One thing people overlook with garden offices is winter — that 200W panel will struggle December/January in the UK. Might be worth thinking about a small backup charger if you're relying on it for work kit.

@MarkAllen69 worth considering what you're actually running before throwing more batteries at it. I've got a similar setup in my static caravan and spent ages second-guessing the battery bank size.

One thing people overlook — winter production is the real bottleneck with 200W. In the UK you might only get 1-2 peak sun hours some days December-January, so you're pulling maybe 200-400Wh on a good day. If your loads exceed that regularly you'll be cycling that 100Ah Fogstar pretty hard.

What's your Victron app showing for state of charge at end of day? That'll tell you far more than any calculator. The SmartSolar logs are brilliant for spotting patterns over a few weeks.

If you're consistently dropping below 50% SoC, then think about a second battery — but I'd check the data first rather than guessing.

@MarkAllen69 looks like the thread got cut off but I can probably guess the rest! Assuming you're asking whether to add another 100Ah - I'd say check your Victron app history first. The SmartSolar will show you your State of Charge over time, which tells you far more than any rough calculation. If you're regularly dipping below 50% SoC (assuming lead acid) or 20% with lithium, then yes, expand the bank. Also worth noting - that 200W panel will struggle in winter months here, so what's sufficient in July might leave you short by November. Are you on lead acid or lithium? Makes a big difference to usable capacity and how aggressively you can discharge. @CharlieStevens85 makes a fair point about knowing your loads first.

@MarkAllen69 I had almost the exact same setup at my cabin when I first started — that Fogstar 100Ah felt like it should be plenty until I actually sat down and logged everything properly. The thing that caught me out wasn't the obvious stuff like the laptop and monitor, it was the background draws. Router left on all day, USB chargers ticking away, maybe a small fan heater briefly in the morning.

Get yourself the Victron app connected via Bluetooth and watch the live consumption data for a few days before you decide anything. Mine told a completely different story to what I thought I was using. Once you've got real numbers, the battery question answers itself rather than becoming a guessing game.