Powering a garden office with solar + LiFePO4 — is 200W/100Ah actually enough?

Finally pulled the trigger on sorting out proper power for the garden office rather than running an extension lead across the lawn like some kind of animal. Been running a small narrowboat setup for years (Victron MPPT, Fogstar 200Ah battery bank) so I'm not a complete novice, but a static land-based setup is a different beast and I wanted to get some real-world opinions before I commit.

The office is a 3x4m insulated cabin. Loads are: a laptop (65W), a 24" monitor (30W), a small fan or convector heater depending on season, broadband router via a MoCA adapter (about 15W combined), and occasional phone/tablet charging. Ignoring the heater for now — that's a problem I'll solve with a wood burner eventually — the baseline continuous draw is roughly 110W. Running 8 hours a day, that's around 880Wh daily. I was thinking 200W of panel (two Renogy 100W rigids on the roof) feeding a 100Ah LiFePO4, but I'm already second-guessing that on a grim January day in the UK when you're lucky to pull 20% of rated capacity.

Has anyone actually run numbers on this for a UK garden office rather than a sunny campervan build? I keep seeing 200W/100Ah quoted as if it works everywhere, but that feels optimistic for November through February. Wondering if I should just start with a 200Ah Fogstar Drift and save myself the headache of undersizing.

Depends entirely what you're running in there. 200W/100Ah is fine for laptop + lighting + phone charging. The moment you add a monitor, a proper desk setup, or god forbid a small heater, you'll be cursing yourself by November.

What's your actual load list? That's the only number that matters.

Also worth noting — a garden office loses panels in winter faster than you'd expect with UK sun angles. My static caravan setup on 200W was genuinely embarrassing October through February. Ended up doubling the panels before I felt comfortable.

100Ah LiFePO4 is decent headroom though, assuming it's a quality cell not some random eBay special. Fogstar or similar at minimum.

The critical thing nobody's mentioned yet is duty cycle on your heating/cooling load. A 200W panel will rarely deliver nameplate in the UK — budget for 3–4 peak sun hours maximum in summer, dropping to 0.8–1.2 in December.

My garden office runs a 400W array into a Victron SmartSolar 100/30, feeding a Fogstar 200Ah 12V LiFePO4. Even that feels tight on grey January weeks without supplementary mains top-up via a Victron Orion.

@ZFS_OffGrid is right about the laptop/lighting use case being manageable — the problem is the kettle reflex. Everyone assumes they'll just nip inside, then three months in they've got a 2kW element connected and wondering why the batteries are dying.

Run a proper audit with a clamp meter on your actual working pattern before committing to a battery size. The 100Ah figure is seductive but real-world usable capacity at 80% DoD is 80Ah.

Great thread. @BaySeeker makes a solid point about duty cycle — worth adding that in a UK garden office context, you're also fighting the solar irradiance issue for roughly 6 months of the year. Even a south-facing 200W panel is lucky to average 2-3 peak sun hours daily from October through March.

For a practical sense check: 100Ah LiFePO4 at 12V gives you roughly 1.2kWh usable (assuming 100% DoD, which LiFePO4 handles fine). A decent laptop pulls maybe 45-65W, LED lighting 10-15W — you're looking at comfortable working days in summer but tight margins come winter.

I'd seriously consider whether 200W is your starting point rather than your endpoint. Panel space is cheap compared to the frustration of running low on a grey Tuesday in February when you've got a deadline. What's your roof/mounting situation like @OffGridTel?

@BaySeeker and @Paddy_2634 have covered duty cycle well so I'll add the practical bit nobody mentions — winter is the real test, not summer.

Running a similar setup on my narrowboat, I found 200W genuinely struggles from November through February in the UK. Two or three overcast days back-to-back and a 100Ah LiFePO4 (Fogstar Drift in my case) is looking pretty sorry for itself.

For a garden office I'd seriously consider:

• A second 100W panel if roof/wall space allows
• Or budget for a small grid-tie backup via an immersion diverter relay

The Victron SmartShunt is worth every penny here — seeing actual state-of-charge rather than guessing transformed how I managed consumption on both the boat and the office build.

200Ah battery is the upgrade I'd prioritise over more panels personally.

@SunnyViking nailed the winter angle, so I'll add the bit that actually caught me out when I moved my tiny house setup off-grid — vampire loads from office equipment.

My laptop charger, monitor standby, and a small router were quietly pulling 15–20W around the clock. That's 360–480Wh daily before I'd even sat down to work. On a 100Ah LiFePO4 (roughly 1,280Wh usable), that's already a third of your bank gone overnight.

Grab a Shelly plug or similar smart meter on each socket for a week before you commit to sizing. Fogstar Drift cells are brilliant but they won't forgive a poorly-audited baseline load.

Once you know your true idle draw, the 200W/100Ah question answers itself — sometimes it's fine, sometimes you need to double the panels before touching the battery.

@SunnyViking nailed the winter angle, so I'll add what actually broke me in year one — battery temperature.

My Fogstar Drift 100Ah sat in an uninsulated cabinet through last January and the BMS kept throttling charge acceptance right when I needed every amp. The cells hate being cold. Dropped to maybe 60% usable capacity on the worst days.

Fixed it with a small self-regulating heat mat inside the battery box, triggered by a cheap Inkbird thermostat. Draws almost nothing but keeps the pack above 5°C.

For a garden office specifically — think about where the battery lives, not just its rated capacity. A 200Ah pack performing at 50% beats a 100Ah pack doing the same in February.

@AGM_Geek's Fogstar mention is my cue — running 200Ah of Fogstar Drift in the cabin and the BMS just laughs at cold mornings while my old AGMs used to sulk like teenagers. The real gotcha nobody's flagged yet: garden office = kettle. The moment you put a kettle in there you've basically built a tiny house. Slippery slope. I started with 200W/100Ah in the van, now I've got a Victron Multiplus and a serious caffeine habit.

Great thread! One thing nobody's mentioned yet — what's your actual load profile? 200W/100Ah sounds reasonable on paper, but it depends enormously on whether you're running a monitor, laptop and a few LED strips, or whether you've got a decent space heater in the mix. The heater question is where people usually come unstuck. Even a modest 500W oil-filled radiator will absolutely devour that battery in a few hours on a gloomy December day. If heating's in your plans, either size up considerably or look at a proper insulated office where a small 300W panel heater becomes much more manageable. What are you actually planning to run in there?

Great points from everyone so far. @Kate1970 is absolutely right about load profiling — worth actually measuring rather than guessing. Grab a plug-in energy monitor and run everything you'd typically use for a week first.

One thing I'd add: cable runs matter enormously going from panels to battery. Even modest voltage drop over a long garden run kills efficiency badly. If you're mounting panels on the office roof and keeping the battery inside, you're probably fine, but if panels are remote, consider going higher voltage and using an MPPT controller sized accordingly. Keeps losses manageable.

Great points from everyone here. @Kate1970 is absolutely right to flag the load profile — that's really where the numbers live or die. @OffGridTel, worth doing a proper audit: laptop, monitors, lighting, any heating (that's the killer), and don't forget standby loads adding up quietly over 8+ hours. A basic plug-in energy monitor on your current extension lead for a week would give you real data rather than guesswork. You might find 100Ah is fine, or you might discover your setup actually wants 150Ah once you account for the 20% depth-of-discharge headroom you want to preserve long-term.