Shed office solar setup — worth going bigger on the battery from the start?

Finally getting round to sorting proper power for my 6x4m garden office. I've been running an extension lead from the house for two years which is embarrassing, frankly. Plan is to go fully off-grid for the office so I stop adding to the house bills and get rid of the cable running across the lawn.

I'm looking at a 400W panel setup (two 200W monos) feeding into a Victron 100/30 MPPT, with a 12V 200Ah LiFePO4 battery — probably a Fogstar Drift or similar. The office runs a monitor, laptop, a small fan heater on eco mode, LED lighting, and occasionally a laser printer. I've done the rough sums and reckon I'm pulling maybe 600-700Wh on a typical working day, which should be fine most of the year. Winter is the obvious worry.

My mate reckons I should just go straight to 400Ah battery from the off, arguing it costs less in the long run than buying a second battery later and potentially messing around with the wiring. I can see his logic but it's a fair chunk more money upfront and I'm not sure the two panels would charge it adequately in December anyway. Would adding a third panel make more sense than doubling the battery capacity?

Has anyone here actually done a similar setup and lived with it through a UK winter? Especially curious whether people find a small immersion or panel heater genuinely workable off-grid or whether that's always going to be a losing battle with solar in January.

CliveCrane | 847 posts

@BigAl31 short answer — yes, absolutely go bigger on the battery if budget allows. The panels are the easy bit to add later (just wire in more), but retrofitting battery capacity is a proper faff, especially if you've gone with a lithium system where the BMS needs to match.

I did exactly what you're describing three years ago and started with 100Ah thinking it'd be plenty. Within six months I was wishing I'd gone 200Ah from the off, particularly through the winter when charging hours are limited and you're running a monitor, laptop, and a fan heater on low.

What's your expected daily usage looking like? And are you heating the office electrically? That changes the whole equation dramatically — resistive heating will eat a battery bank for breakfast.

Chopper72 | 1,204 posts

@BigAl31 One thing worth adding to what @CliveCrane said — think about your winter usage specifically. A battery that feels generous in July can leave you short come November when you're getting maybe 2-3 hours of usable generation on a grey day. I'd work out your worst-case daily load first (heating, monitors, lighting, kettle if you're treating yourself), then size the battery to cover at least two days of that without any solar input. That buffer saves you constantly rationing power during gloomy spells. Also worth considering LiFePO4 over lead-acid if you haven't already — the usable capacity difference alone often justifies the price gap for a permanent office setup. Don't scrimp and regret it six months in!

JimKelly | 312 posts

@BigAl31 went through exactly this two years ago with my 5x3 office. Skimped on battery initially and regretted it within the first November.

Ended up with a Fogstar Drift 200Ah which has been solid, but honestly I'd have gone 300Ah if I'd thought it through properly.

One thing nobody's mentioned — don't forget the inverter sizing. Kettle, monitors, and a laptop charger all kicking in together caught me off guard. Make sure whatever Victron setup you go with can handle the surge.

Also worth checking whether your shed gets decent south-facing exposure year-round — trees that seem fine in summer can properly kill your harvest come December once leaves drop. Learnt that the hard way too. 😅

Compo55 | 2,156 posts

@BigAl31 One angle nobody's mentioned yet — factor in what you're actually running simultaneously, not just total daily consumption. A modest battery can look fine on paper but if you're firing up a monitor, laptop, desk lamp, and a small heater all at once, your inverter and battery need to handle that peak draw comfortably.

Also worth thinking about expandability — some lithium systems let you add battery modules later, which softens the upfront hit if budget's tight right now. Though as @CliveCrane says, going bigger from the off is usually the smarter move once you've added up the faff and cost of upgrading later. What kit are you planning to run in there day-to-day?

Gazza22 | 847 posts

@BigAl31 One thing I'd flag that nobody's touched on yet — consider your battery chemistry carefully before just going bigger. A smaller LiFePO4 bank will likely serve you better long-term than a larger lead-acid setup. The usable capacity difference is significant (roughly 80-90% vs 50%), plus LiFePO4 handles partial states of charge much better, which matters enormously in a garden office scenario where you're not always running it to zero and back up. They're pricier upfront but the cycle life is in a different league. I upgraded mid-way through my own setup and wished I'd just started there. So yes, go bigger on capacity, but make sure you're also going smarter on the chemistry itself. False economy to do one without the other.