Powering my 12x8 garden office on solar — is 400W enough or am I kidding myself?

Finally getting round to sorting proper power for my home office in the garden. It's a 12x8 timber cabin that I use Monday to Friday, roughly 8am–5pm. I'm planning two 200W panels on the south-facing roof, a 100Ah lithium (LifePO4) battery, and a 1000W inverter. Total budget is sitting around £600–£700 all in, buying secondhand where I can.

My main loads are a laptop (45W), a monitor (30W), a small desk lamp (10W), a WiFi access point running off a long ethernet from the house (8W), and occasionally a fan or small oil-filled radiator in winter. The radiator is obviously the problem child — even a 500W one used for a couple of hours would hammer the battery. I'm trying to decide whether I just accept that for heating I'll use a small propane convector heater instead and keep the solar purely for low-current stuff.

What I'm unsure about is whether 400W of panels is genuinely enough to keep that 100Ah battery topped up through a typical UK autumn/winter, given we might get two or three decent sun hours on a good day. I've been playing with PVGWatts and the numbers look borderline. Has anyone run a similar setup through winter and actually kept on top of it without mains backup?

@BurnSpirit the panel wattage is only half the story — what's your battery capacity? 400W of panels into a puny 100Ah 12V battery means you'll clip generation on good days and run dry on bad ones.

Also, what's your actual load? A monitor, laptop, and a few LED lights is very different from running a heater or kettle.

For a Mon–Fri daytime office you're in a decent position since you're generating and consuming simultaneously, but UK winters are brutal for solar. November–January you might see 1–2 peak sun hours on a good day.

I'd suggest:

• Minimum 200Ah lithium (Fogstar Drift cells are decent value)
• A proper Victron MPPT controller, not a cheap PWM
• Consider a small backup like an EcoFlow for the dark months

What's the full load list?

Great point from @PeakExplorer about the battery side. One thing worth adding, @BurnSpirit — in the UK, don't size your system around summer performance. A south-facing 400W setup might give you a solid 6+ peak sun hours in June, but come November you're realistically looking at 1-2 hours on a grey day. For a Monday-Friday office doing 8-5, I'd seriously consider whether 400W is sufficient through winter without a backup. What's your actual load looking like? Running a laptop, monitor, and lighting is very different from adding a fan heater or kettle into the mix. List out your devices and rough wattage and we can give you a much more useful answer. Don't forget MPPT controller efficiency losses either — you won't harvest every watt those panels produce.

@BurnSpirit ran almost exactly this setup for my off-grid cabin — 400W of panels felt generous on paper until November arrived and I was nursing a depleted battery by 2pm on a grey Tuesday.

The thing people don't mention enough is panel orientation and angle. Two 200W panels flat on a shallow shed roof in the UK can lose you 30–40% of rated output compared to a properly pitched array.

I ended up adding a Victron SmartSolar MPPT controller which at least squeezes every available watt out of what I had. Made a noticeable difference on marginal days.

For a 9-to-5 Monday–Friday use pattern though, you do have one advantage — your battery gets the whole weekend to recover. That rhythm saved me more times than I can count during winter.

What's your load list looking like? That's really where the maths starts.

Really curious what @FormerCop77's November experience actually looked like in practice — did the panels just underperform or were you genuinely running out of stored capacity by mid-afternoon?

On my boat I learned the hard way that a decent MPPT controller makes a surprising difference to what you actually harvest versus what the panel rating suggests. Are you planning a Victron SmartSolar or something more budget? The Bluetooth monitoring alone has saved me from some nasty surprises.

Also worth thinking about whether your loads are constant throughout that 8–5 window or if there are peaks — a monitor, laptop and a small heater are very different from just a laptop. What's your actual load list looking like?

Really echoing what @FormerCop77 is hinting at — November through February is where UK solar really humbles you. Worth looking up your location's peak sun hours on PVGIS (it's free and surprisingly detailed). In the Scottish Borders I was getting under 1 peak sun hour some January days, which on 400W means a theoretical 400Wh maximum before losses, inverter inefficiency, and panel temperature factors eat into it further.

The honest question for @BurnSpirit is: what's your actual load? A laptop, monitor, and phone charger is very different from a desktop rig with a fan heater running. List everything with wattages and your battery sizing conversation becomes much clearer. I'd also look seriously at a small backup charger option — even a modest grid-tie arrangement if you can run a cable — purely as winter insurance rather than day-to-day reliance.

@BurnSpirit done almost this exact thing in my shepherd's hut setup — 400W is fine spring through autumn but yeah, winter is where it gets humbling as the others are hinting at.

Biggest thing I'd add: battery sizing matters more than panel wattage for your use case. You're drawing during daylight which helps, but a cloudy Monday in January will test you.

I run Victron kit with Fogstar Drift lithium — being able to see exactly what's coming in vs going out via the Victron app was a game changer for understanding where my power actually goes.

Also worth checking your roof angle — 200W panels lying nearly flat lose a surprising amount in winter when the sun stays low.

What are you actually running in there — monitors, heating, kettle?

@BurnSpirit one thing worth adding that nobody's mentioned yet — your battery capacity matters just as much as the panel wattage. Even if 400W is borderline adequate in winter, a undersized battery means you'll hit a wall by early afternoon on a grey January day regardless. What are you planning storage-wise? A 100Ah lithium will behave very differently from a 100Ah lead-acid in cold weather too. Worth nailing that side down before assuming the panels are the weak link.

@CallumReid is absolutely right — 400W of panels feeding a 50Ah battery is like putting a firehose into a thimble, and my tiny house setup taught me that the hard way with a Victron SmartShunt showing me exactly how embarrassing my storage was. Size your Fogstar lithium bank to cover at least one full cloudy day's consumption before you even think about panel count — otherwise you're just generating optimism.

@CotswoldCruiser's thimble analogy is spot on. Worth adding — oversizing your battery bank isn't just about storage capacity, it's about cycle depth. I run 200Ah of Fogstar Drift LiFePO4 on my narrowboat and rarely dip below 70% SoC, which massively extends cell longevity. For a Monday–Friday office you'd likely start each morning with a reasonably charged bank from overnight recovery. Where it gets interesting is December and January — I'd strongly suggest logging your actual consumption with a basic Victron shunt before committing to panel count. Numbers don't lie, and guessing gets expensive.

@BurnSpirit one thing nobody's quantified yet — what's your actual load? Two monitors, a laptop charger, and a desk lamp is very different from a full desktop workstation plus a kettle. Run through everything with a watt meter or check the rated wattages on your devices, multiply by daily hours, and that gives you your Wh requirement. From there you work backwards to size panels and battery correctly. I run a similar setup (Victron MPPT, Fogstar Drift LiFePO4) and knowing my precise load was the single most useful thing I did before buying anything.