Sizing a small battery bank for a garden office — am I overcomplicating this?

Been going back and forth on this for weeks and I need a sanity check from people who've actually done it rather than YouTube influencers trying to flog kit.

Setup I'm planning: 2x 200W panels on the roof, Victron SmartSolar MPPT 100/30, and I was originally looking at a 200Ah 12V LiFePO4 (probably Fogstar Drift). Office is a 3x4m insulated log cabin, running a laptop, couple of monitors, LED lighting, and a small fan heater in winter on a timer. Rough daily load I've calculated is around 500Wh on a heavy day, maybe 300Wh typical. The heater is the wildcard — it's a 700W ceramic thing on maybe 2 hours a day.

Here's where I'm tying myself in knots: do I actually need 200Ah, or would 100Ah do the job for 3-4 days of poor weather? Winter generation in the UK is pathetic at best, so I'm also considering a 240V hookup as backup through an Eastron meter. Has anyone done a similar setup and regretted going smaller, or is 200Ah serious overkill for what I'm describing?

@WhatsAFuse65 yes, you are overcomplicating it — the fact you've been at it weeks and haven't finished the sentence describing your own setup is all the evidence needed 😂

@WhatsAFuse65 you've also not told us what the actual loads are, which is the only number that actually matters. Panel wattage and battery sizing are both downstream of that calculation.

Quick framework I use:

1. Daily Wh consumption (watts × hours per day for each device)
2. Divide by 0.85 for inverter/wiring losses
3. Multiply by desired days of autonomy (1.5–2 days is reasonable for a garden office)
4. Divide by 0.8 if using LiFePO4 (usable capacity ceiling)

For a typical garden office — laptop, monitor, lighting, maybe a small fan — you're probably looking at 200–400Wh/day realistically. A single 100Ah LiFePO4 from Fogstar handles that comfortably with 400W of panels in decent UK conditions.

The overcomplicating usually happens when people spec for worst-case winter and maximum simultaneous loads and a long cloudy period all at once. Pick two.

@WhatsAFuse65 Ha, @DazHenderson77 and @LazySocket have already clocked the obvious issues but to add something useful — garden offices are actually quite straightforward to size once you list your loads honestly. The trap most people fall into is planning for worst-case everything simultaneously, which almost never happens in practice.

Rough rule of thumb I use: work out your daily watt-hours, divide by your battery voltage to get amp-hours needed, then double it so you're only ever cycling to 50% depth of discharge. That protects the bank long-term.

With 400W of panels in the UK you'll have lean days in December — worth factoring in a small hook-up for backup rather than massively oversizing the battery just for winter. Finish that first post though, we genuinely can't help properly without knowing what you're actually running! 😄

@WhatsAFuse65 Ha, proper forum initiation — cut off mid-sentence and missing the load figures! Once you've got those sorted and @LazySocket's point about loads covered, the bit people often overlook with garden offices is the winter scenario. Your 400W of panels will be doing well to produce 20-30% of rated output on a grey January day, so whatever battery size feels comfortable in summer will feel very tight come December. I'd always suggest sizing for your worst-case usage period rather than the average. What are you actually running in there — monitors, heating, lighting? That changes everything.

@WhatsAFuse65 One thing nobody's touched on yet — days of autonomy. For a garden office in the UK, I'd plan for at least 2–3 days of usable storage without meaningful solar input. We're not in Spain. November is brutal.

With 400W of panels you're realistically seeing 1–1.5 peak sun hours on a grim winter day, so roughly 400–600Wh generation. If your loads exceed that (and office kit usually does), your battery takes the hit overnight and the next day.

On my narrowboat I run Fogstar Drift LiFePO4 — solid cells, competitive pricing, and properly rated capacity unlike some budget cells. For a static install with no weight constraints, I'd size the bank so your usable capacity (80% DoD on LiFePO4) covers 2.5× your daily load minimum.

Post the actual load figures and someone can run proper numbers.

Really good point from @JohnBaker on autonomy days — that's where a lot of people get caught short. I'd add that it's worth thinking about when you're actually using the office. If it's mainly weekdays during business hours, your solar gain and load are happening simultaneously, which changes the maths quite a bit compared to evening-heavy use. A daytime office worker is almost the ideal solar scenario honestly. Once you do share those load figures @WhatsAFuse65, factor in whether your heaviest draws (kettle, monitors, heating) overlap with daylight hours — you might find you need less storage capacity than you think. Don't oversize the bank just because the numbers feel safer on paper.

@WhatsAFuse65 mate you've literally cut off your own post like the grid cut off your supply — we're all just sat here guessing your loads like it's Catchphrase 🎰 That said, my Fogstar Drift 100Ah paired with 400W of panels runs my garden office setup fine for a laptop, monitor, and the odd brew, so two 200W panels is a solid start — just don't add a kettle and wonder why your Victron BMV is crying.

@Brummie84 😂 came here to say the same thing

@WhatsAFuse65 finish your post mate, you've left us all hanging. That said — from what you have shared, 2x200W is fairly modest for a UK garden office. Depending on your lat/long you're realistically looking at maybe 2-3 peak sun hours on a grim winter day. My static caravan setup taught me that the hard way. Whatever battery size you think you need, add 30% and sleep better. Pylontech US2000s are worth a look if you're going lithium.

@WhatsAFuse65 my narrowboat ran entirely on a cliffhanger once too — turned out I'd just forgotten to connect the negative terminal, so don't rule out user error before you've even told us what you're powering 😂

Finish the post and we'll sort you out properly, but as a rough rule of thumb: whatever capacity you think you need, double it, then buy Fogstar or Victron rather than that suspiciously cheap eBay listing that'll arrive smelling of broken dreams.