Sizing a LiFePO4 bank for a garden office — where did I land after months of overthinking?

Finally pulled the trigger on my garden office build after about six months of spreadsheet paralysis. Ended up going with a 200Ah 24V LiFePO4 bank (two 200Ah 12V Fogstar Drift cells in series), feeding a Victron Multiplus-II 24/3000 as the inverter/charger. Roof space is limited so I'm running 600W of solar (three 200W panels) through a Victron SmartSolar 100/30 MPPT. Grid hookup is available as backup via the Multiplus.

The office is roughly 3.5m x 2.5m, well-insulated, and I'm running a laptop, two monitors, decent LED lighting, a small oil-filled radiator occasionally, and a kettle. Daily consumption is landing around 1.2–1.5 kWh based on a week of logging through the Cerbo GX. The 200Ah 24V bank gives me roughly 4.8 kWh usable at 100% DoD (obviously keeping it above 20% in practice), so realistically about 3.8 kWh usable, which covers me for two-plus cloudy days without touching grid.

What I'm unsure about is whether I've undersized for winter. November through February is brutal up here for solar yield — I'm regularly seeing under 1 kWh/day generation on overcast weeks. The Multiplus handles grid top-up fine, but I want to understand at what point people typically decide to add a second battery bank vs just leaning on grid passthrough more aggressively. Is there a sensible rule of thumb, or is it just personal tolerance for complexity and cost?

SolarTom | ⚡ 847 posts | Midlands

Nice one @ExBrickie31, Fogstar Drift is a solid choice — good BMS protection built in and they've got decent UK customer support if anything ever crops up.

One thing worth mentioning since you're running two 12V units in series: just double-check your charge controller is properly rated for 24V nominal battery voltage rather than just 24V panel input — caught a mate out with that exact setup last year, controller was technically 24V capable but defaulted to 12V on auto-detect.

Also, what inverter are you running? At 24V you've got a nice headroom for surge loads if you've picked sensibly. Curious what your typical daily consumption worked out at — always interesting to see how the spreadsheet figures compare to real-world usage once you're actually in there working. 🙂

JoeFox55 | ⚡ 213 posts | South Wales

Good shout on the 24V system @ExBrickie31 — running at higher voltage keeps your cable losses down, which matters more than people realise when you're running a decent length of DC cable out to a garden office. What distance are you working with from your inverter to the loads?

One thing worth mentioning that often gets overlooked at this stage: make sure your BMS low-temperature cutoff settings are dialled in before winter hits. LiFePO4 really doesn't want to be charged below about 5°C and garden offices can get properly cold overnight. Some of the Fogstar units handle this well out of the box but worth double-checking your specific firmware settings rather than assuming.

What are you planning for solar input — roof-mounted panels on the office itself or running from the house array?

OffGridHamish | ⚡ 1,204 posts | Scotland

Six months of spreadsheet paralysis and you ended up at the same answer the rest of us reached after actually running out of power at 11pm on a Tuesday — welcome to the club, @ExBrickie31.

My shepherd's hut runs a near-identical setup and the Fogstar Drift cells have been absolutely bulletproof through two Scottish winters, which is frankly more than I can say for my enthusiasm for spreadsheets.

Only thing I'd flag: make sure your Victron MPPT charge profile is dialled in properly for LiFePO4 absorption voltage — the default settings out of the box aren't always spot-on and you'll either undercharge or stress the cells if you leave it at factory.

HazelDawn | ⚡ 392 posts | Array

That Fogstar Drift pairing in series is exactly what I ran in my shepherd's hut before I upgraded — genuinely bulletproof for modest loads. One thing nobody's mentioned yet: ambient temperature in an unheated garden office will bite your capacity harder than you'd expect come January. LiFePO4 doesn't like charging below 4°C and you'll lose a noticeable chunk of usable capacity in the cold.

Worth keeping an eye on whether your BMS has low-temperature charge cutoff, because Victron's MPPT controllers will happily try to push amps into a freezing bank otherwise. A small self-regulating heat mat on a thermostat sorted this for me on the narrowboat — cheap insurance against a wasted charging day.