Powering my 12x10 garden office - is 400W solar + 100Ah lithium enough for a full working day?

Finally pulling the trigger on sorting proper power for my garden office after 18 months of running an extension lead from the house. Embarrassing, I know. The office is a 12x10 Dunster House cabin and I work from it full time - usually 8am to 6pm, sometimes a bit later in winter.

My planned setup is two 200W panels on a south-facing pitched roof, a Victron SmartSolar 100/30 MPPT, and a single 100Ah LiFePO4 battery (looking at the Fogstar Drift 100Ah). On the consumption side I'm running a MacBook Pro, a 27" monitor, a small desk lamp, and a WiFi router. The heavy hitter is a 300W oil-filled rad for about 4 hours a day in winter - that's the bit I'm worried about.

I've run the numbers roughly and the rad alone is pulling around 1.2kWh on a cold day, then chuck in maybe another 300-400Wh for the tech gear, and I'm looking at potentially 1.5kWh+ on a grim January day. With 400W of panels I'm realistically getting maybe 1-2 peak sun hours in midwinter, so that's only 400-800Wh generation. There's clearly a gap there.

Has anyone run a similar setup through a UK winter? Wondering whether to add a second 100Ah battery, swap the rad for a more efficient heating solution, or just accept I'll need a grid top-up circuit for November through February. Keen to hear what's actually worked for people rather than just spreadsheet theory.

SteveBurns | 847 posts

@BenKing80 Ha, no judgement here - I ran an extension lead for nearly two years before finally sorting mine properly!

Your 400W/100Ah setup should be workable, but honestly it depends massively on what you're running. A laptop, monitor, and decent LED lighting? Probably fine through most of the year. Start adding a fan heater or even a small oil rad and you'll drain that 100Ah before lunch.

The real weak point in the UK is winter generation - a decent November or December day might only give you 1-2 usable hours of decent solar input. Worth seriously considering a small MPPT controller with a shore power backup connection, so you can trickle charge from the house overnight if needed without abandoning the whole off-grid setup.

What appliances are you actually planning to run? That's the key question before anyone can give you a proper answer.

Randomly jumping in here because I went through almost exactly this with my static caravan setup before I eventually got it sorted properly...

One thing nobody's mentioned yet - what's your worst case daily usage actually look like? Like have you properly sat down and added up watt-hours? I hadn't done this properly and massively underestimated myself.

Also worth considering: are you planning any EV charging from the same system eventually? I ask because I wish I'd oversized my battery bank from the start rather than retrofitting. Fogstar Drift cells are decent value if you're building out a bigger pack later.

400W feels borderline to me for a full UK winter working day - what direction does your roof face? That matters enormously up here.

What inverter are you looking at? Victron or something more budget?

FrostySailor | 312 posts

@BenKing80 The honest answer is: it depends entirely on your actual load figures, which you haven't given us.

400W panels and 100Ah lithium sounds reasonable on paper, but a monitor, laptop, router, and any heating or cooling will eat through that faster than you'd expect — particularly October through February when you might see 1-2 usable solar hours on a grey day.

What I'd suggest before buying anything:

• Plug everything into a smart meter plug for a week and log real consumption
• Calculate your worst-case winter scenario, not the sunny July one

Running a narrowboat I've learned the hard way that undersizing a system is more expensive long-term than getting it right first time. 100Ah genuine usable capacity (Fogstar Drift is decent value) might just about cover you in summer. Winter's another conversation.

What's your heating situation?

BenWebb | 203 posts

@BenKing80 Worth doing a proper load calculation before committing to anything. Grab a plug-in energy monitor (about £12 on Amazon) and run your typical setup for a full day from the house extension lead before you disconnect it. You'll get real-world watt-hours rather than guessing from spec sheets.

One thing I'd flag specifically - monitor usage is often underestimated. A single 27" display can pull 40-60W depending on brightness settings. If you've got dual monitors plus a laptop dock plus a desk lamp, it adds up faster than people expect.

The 100Ah lithium gives you roughly 1.2kWh usable, which is workable for a lightweight setup but tight if you're doing video calls all day in winter when solar input is poor. What's your typical daily kit list?