Powering a garden office off-grid — is 400W solar + 100Ah LiFePO4 actually enough?

Finally pulled the trigger on a proper garden office setup after spending years sorting 12V systems on the narrowboat. Figured the skills would transfer — and they mostly have — but the load profile is completely different. On the boat it's lighting, a pump, maybe a laptop. In the office it's a monitor, a laptop, a small fan heater in winter, and the obligatory kettle. That heater is the killer. Even a 500W oil-filled rad is a nightmare for a battery system.

Current thinking is 400W of panels (two Renogy 200W mono) feeding a Victron SmartSolar 100/20, into a 100Ah Fogstar Drift LiFePO4. Inverter would be a Victron Phoenix 12/1200 for the kettle and any AC loads. Total usable capacity is around 1.2kWh assuming I stay above 20% SOC.

The maths feels fine in summer — even a cloudy UK day should top it up. Winter is where I'm nervous. Short days, low sun angle, and if I'm running a heater at all, that battery will be empty before lunch. Has anyone actually run a garden office through a British winter on a setup this size, or am I kidding myself and need to double the battery bank at minimum?

Wondering whether a small propane heater (like an Ofyr or a basic Calor cabinet heater) for the cold months is the sensible hybrid approach rather than throwing more panels and batteries at the problem.

PhilPowell | 847 posts

@KangooAdventure the narrowboat experience will definitely help, but yeah, the load profile is the killer difference isn't it — a laptop, monitor, and decent lighting running 6-8 hours daily is a different beast to 12V leisure loads.

400W and 100Ah LiFePO4 is honestly borderline for a proper working day in winter. You've got maybe 80-85Ah usable, and UK December solar yield can be genuinely depressing — 1-2 peak sun hours on a good day.

My suggestion: add a small UPS with grid hookup as backup rather than upsizing the battery. Keeps costs down and gives you resilience without going full Doomsday Prepper on the battery bank. Run off solar when it's generating, fall back silently when it isn't.

What's your biggest single load? That'd help narrow it down properly.

MeganFox | 203 posts

Running almost exactly this spec for my garden office — 400W Renogy panels + 100Ah Fogstar Drift LiFePO4. Honest answer: it depends entirely on your winter expectations.

Summer is fine, no issues. But Nov–Jan with a monitor, laptop, and any heating? You'll be pulling on that battery hard during those 7-hour grey days.

I added a small Victron MPPT and the extra data really opened my eyes to how little I was actually harvesting Dec/Jan. Ended up keeping a small immersion heater on mains via a long cable as backup rather than chasing more panels.

If you're just doing screen-based work with no heating demands, 400W/100Ah is workable year-round. Add any resistance heating and it's a different conversation entirely.

Smithy98 | 64 posts

Running a similar setup in my garden office and the thing that caught me out was the monitor draw. My 27" display was pulling nearly 40W constantly — added up fast across an 8-hour working day.

What's your typical winter usage like? That's where I'd be more concerned than summer. December/January with panels potentially sat flat and losing 60-70% output, that 100Ah starts feeling tight pretty quickly.

@MeganFox curious whether you've got any load-shedding set up or just manage it manually? I've been thinking about adding a Victron SmartShunt just to get better visibility on what's actually being consumed day-to-day rather than guessing.