Anyone know if a 200Ah lithium battery is overkill for a small shepherd's hut?

Running a small garden office rather than a shepherd's hut, but the sizing question is identical so hopefully this is useful context.

I'm currently trying to work out whether a 200Ah LiFePO4 (looking at Fogstar Drift) is oversized for a low-draw setup. My office runs:

• A laptop (45W)
• LED lighting (~20W total)
• Small fan heater occasionally (but realistically I'd use a propane heater off-grid)
• Phone/tablet charging

Rough daily consumption comes out around 150-200Wh on a typical working day. Even with the standard advice of not going below 20% SoC, a 100Ah battery at 12V gives you ~960Wh usable — that's nearly a week's worth without any solar input.

So my actual question is: does the larger capacity make sense purely for longevity reasons? I've read that LiFePO4 cells last significantly longer when they're cycled shallowly rather than hammered down to 20% regularly. If a 200Ah bank means I'm only ever pulling it down to 60-70% on bad winter days, does that meaningfully extend the lifespan enough to justify the extra cost upfront?

Or am I overthinking this and a decent 100Ah with a Victron SmartSolar controller would be perfectly adequate?

Anyone who's actually been running a small structure through a UK winter would be especially helpful here — suspect the solar harvest figures I'm finding online are too optimistic for the north of England.

@TonyLee — depends entirely on your loads and whether you're grid-tied for backup or fully islanded.

For a garden office, tally up your actual consumption first:

• Laptop: ~45–65Wh/day
• Monitors: 20–80W depending on size
• LED lighting: negligible
• Router/networking: ~10–15Wh/day

A 200Ah (at 48V that's 9.6kWh, at 12V it's 2.4kWh usable) might be sized perfectly or massively over-specced depending on your voltage system and autonomy requirement.

My tiny house setup runs a Victron SmartShunt alongside Fogstar Drift cells — the shunt data genuinely shocked me how little I consumed day-to-day versus what I'd estimated on paper.

Key question: how many days' autonomy without solar input do you need? UK winters mean your panels will underperform significantly — that's usually what drives oversizing, not the loads themselves.

@TonyLee — 200Ah LiFePO4 is only "overkill" until the first cloudy week in November when you're staring at a laptop with 4% battery and a deadline.

Rough rule of thumb: add up your daily Wh consumption, divide by 0.8 (usable capacity), and that's your minimum. My shepherd's hut runs a Fogstar Drift 200Ah paired with a Victron SmartSolar, and honestly for a garden office with a monitor, laptop, and occasional kettle, 200Ah is comfortable rather than excessive — especially through a British winter where your solar input drops off a cliff.

If anything, I'd scrutinise your charge source more than the battery size — a weedy 100W panel feeding a 200Ah bank is the actual problem most people hit.

@TonyLee — I've been running a garden office on a 200Ah Fogstar Drift LiFePO4 for two winters now and honestly the sizing question answered itself once I tracked my actual loads for a week rather than guessing.

The number that matters isn't the battery capacity — it's your daily consumption versus your worst-case solar input. In December I'm sometimes pulling 80–90Ah daily (monitors, heating mat, router) whilst my panels are giving me perhaps 30Ah on a grey Suffolk day.

Without that buffer I'd be throttling the Victron MPPT and rationing kettle usage by Tuesday.

Run a proper load audit first. If your total daily draw sits under 40Ah and you've got decent south-facing panels, then maybe 100Ah serves you. But most people who've done this more than one season quietly wish they'd gone bigger from the start.

@TonyLee — one thing nobody's mentioned yet: usable capacity vs nameplate. With LiFePO4 you're realistically drawing down to maybe 80–90% DoD safely, so your 200Ah is genuinely ~170–180Ah of working headroom. That changes the maths quite a bit.

On my boat I run a 200Ah Fogstar bank and the critical question was always days of autonomy, not raw Ah. Work backwards: total daily load in Wh ÷ nominal voltage = Ah/day, then multiply by however many sunless days you want to ride out.

Garden office in winter, UK latitude — I'd want minimum 3 days autonomy built in. Pair that 200Ah with even a modest 400W of panels on a Victron MPPT and you're in decent shape. Anything smaller and @MarineGeoff's November scenario becomes very real, very fast.