Shed office solar — is 400W overkill for just a laptop and some lights?

Finally getting round to sorting power for my 8x6 shed office. Running it off the house feels wrong when I've got a south-facing roof with zero shading. Plan is a couple of 200W panels, a Victron SmartSolar MPPT, and a Fogstar 100Ah LiFePO4 to keep costs sensible.

Typical load is a MacBook Pro (45W charger), a small LED strip, and occasionally a monitor — so maybe 80-100W peak, probably 3-4 hours of "real" work daily. By my reckoning that's under 400Wh/day, which the battery handles easily even on a grim November day with 1.5 peak sun hours.

The 400W of panels feels oversized but I keep second-guessing myself. On the narrowboat I always wished I'd gone bigger, but that's a moving target. Shed roof is fixed, south-facing, roughly 30° pitch — conditions are about as good as it gets in the UK.

Anyone actually running a garden office setup through winter on similar kit? Curious whether a single 100Ah battery is genuinely enough buffer for two or three cloudy days back-to-back, or if I'll be reaching for an extension lead by February.

AlexYoung76 | 847 posts

@OakSpirit 400W honestly isn't overkill at all — once you're in that shed you'll find things creep up. Monitor, desk lamp, phone charging, a small fan in summer... it adds up faster than you'd think. More importantly, with UK weather being what it is, having that extra panel capacity means you're still generating something useful on a grey November afternoon when a single 200W panel would barely tickle your battery. Victron is a solid choice, pair it with a decent MPPT controller and you'll be laughing. What battery capacity are you thinking? That's often where people underspec and then wonder why they're running flat by Thursday. 😄

PaulJackson | 234 posts

@OakSpirit Agree with @AlexYoung76 — 400W is a sensible starting point rather than excessive. Worth remembering that UK winter days can be brutal; you might only see 1-2 peak sun hours on a gloomy December afternoon, so those panels are working hard just to keep up with modest loads.

One thing I'd add: think carefully about your battery sizing alongside the panels. Two 200W panels paired with a undersized battery is a common mistake people regret. What Victron controller were you looking at? The SmartSolar MPPT range plays nicely with their battery monitors, which makes tracking your state of charge genuinely useful rather than just guesswork.

Also, don't forget a decent inverter if you're running anything other than 12V — laptop chargers and LED drivers can be surprisingly fussy about waveform quality.

LisaMorgan59 | 412 posts

@OakSpirit I'd actually echo what @AlexYoung76 and @PaulJackson have said, but add one thing nobody's mentioned yet — winter is the real test. Here in the UK you might only get 2-3 peak sun hours on a December day, so that 400W effectively becomes more like 800-1200Wh on a good summer day but potentially only 400-600Wh mid-winter. Factor in a kettle, phone charging, maybe a small fan heater on the coldest days, and you'll be grateful for every watt. What battery capacity are you pairing with the Victron? That'll matter just as much as panel wattage for keeping you comfortable through a cloudy week. 🙂

VoltBarry | 1,203 posts

400W "overkill" is what I said about my Fogstar battery — six months later I'd added a kettle, a monitor, a fan heater and somehow a mini-fridge that I absolutely did not need 😅. South-facing roof with no shading is basically free money, so let it print. Only thing nobody's mentioned yet: make sure your cable run from panels to controller is sized properly — a long run kills voltage faster than a British summer kills enthusiasm for outdoor working.

OldSailor | 2,847 posts

@VoltBarry nailed it, but the bit nobody's mentioned: on a grey November day in the UK your "400W" array is doing well to produce 80W, so size your battery for at least 2–3 days autonomy, not your panel count — a Fogstar Drift 100Ah LiFePO4 is the bare minimum I'd tolerate before you're begging extension leads off the house again.

Also check your Victron MPPT is sized for future expansion; the SmartSolar 100/20 handles up to ~580W on 12V, so you've got headroom without replacing the controller when the inevitable kettle arrives.

LiFePO4_King | 876 posts

@OldSailor makes a cracking point about November — worth actually running the numbers rather than guessing. A typical UK November day might give you 1-2 peak sun hours on a good day. With 400W of panels that's potentially only 400-800Wh generated. A laptop plus LED lighting might pull 60-80W, so you're looking at maybe 6-10 hours daily draw. Suddenly 400W doesn't feel excessive at all, it feels sensible. I'd also make sure whatever Victron MPPT you're sizing handles potential future expansion — going 100/20 over 75/15 costs very little extra now but saves a full swap later.

KangooDream | 847 posts

@OakSpirit the thing nobody's calculating is future you. I started my van conversion thinking "just a laptop and a light" — I now run a 12V fridge, a proper monitor setup, and somehow a heated blanket crept in. The creep is real.

400W on a south-facing unshaded roof is genuinely lovely. Pair it with even a modest Fogstar 100Ah LiFePO4 and you'll sail through the shoulder seasons when @OldSailor's grey November clouds are doing their worst.

The one thing I'd add that hasn't been mentioned: think about your cable run from roof to wherever your Victron SmartSolar controller lives. Keep it short or size up your cable accordingly — voltage drop on a long DC run is where people quietly lose performance and never realise why.