Struggling to size a battery bank for a shepherd's hut — where do I even start?

I'm trying to plan the off-grid electrics for a shepherd's hut build and the battery sizing is doing my head in a bit. It's a static setup on my land in Cornwall, so no weight restrictions like a van build, but I still want to be sensible about cost vs capacity. I've been looking at Fogstar's lithium cells and the Victron ecosystem for the management side, but I genuinely don't know how many kWh I actually need.

My rough loads are: LED lighting, a 12V compressor fridge (maybe 40–50Ah/day?), laptop charging, phone charging, and occasionally a small inverter for a kettle or hair dryer. I've tried the usual "add up your Ah per day" method but I keep second-guessing myself, especially around what autonomy days to plan for — Cornwall can be cloudy for a week solid in winter, as I know too well from the boat.

Has anyone done a similar hut build and actually tracked their real-world consumption? I'm wondering whether 200Ah LiFePO4 is laughably small or actually workable for a part-time weekend setup. What did you size yours at, and do you wish you'd gone bigger?

@CornishBoater the static bit is your biggest advantage — you can genuinely go big without worrying about payload.

Start by listing every load in watt-hours per day. Lights, fridge, phone charging, whatever you're running. Then multiply that figure by 2 or 3 to get your usable bank capacity — you want headroom, especially through cloudy Cornish winters.

My narrowboat taught me this the hard way. I ran undersized LiFePO4 for a season and was constantly chasing state of charge.

For a shepherd's hut I'd be looking at 200–300Ah of LiFePO4 minimum. Fogstar do solid budget cells if you fancy DIY. Victron kit for the BMS and charge controller — their ecosystem genuinely makes monitoring straightforward.

What loads are you planning? That'll make the sizing conversation much more concrete.

@CornishBoater great project! Start by listing everything you want to power and noting each item's wattage plus how many hours per day you'd realistically use it. Multiply those together to get your daily watt-hours — that's your foundation for everything else.

Once you've got that figure, a common rule of thumb is to size your battery bank for 2-3 days of autonomy without charging, to cover Cornwall's rather generous helping of grey days! Also factor in that you typically shouldn't discharge lithium batteries below 20% usable capacity.

What's your intended charging source — solar, a small genny, or a mix of both? That'll influence the sizing conversation quite a bit. Cornwall gets decent solar hours in summer but winters can be lean, so knowing your worst-case month really shapes the numbers.

@CornishBoater great project! The best starting point is to list every appliance you plan to run and note the wattage plus how many hours per day you'd realistically use it. Multiply watts by hours to get watt-hours, add them all up, and that's your daily consumption estimate.

From there, the general rule of thumb is to size your bank so that daily figure only uses around 50% of capacity (for lead-acid) or up to 80% for lithium — that way you're not hammering the batteries and they'll last far longer.

Being in Cornwall also means accounting for those grey winter weeks where solar input can be pretty dismal for days on end, so I'd factor in at least 2-3 days of autonomy if budget allows. What appliances are you thinking of running?

@CornishBoater fellow Cornish off-gridder here, so I know exactly the kind of weather you're dealing with! Once you've got your appliance list sorted as @Steve1978 suggests, don't forget to factor in those grim January weeks where you might get very little solar gain. I'd strongly recommend sizing for 3-4 days of autonomy at minimum — Cornwall can be properly overcast for extended spells in winter. Also think about what you need versus what's just convenient; lighting and phone charging are very different loads to a kettle or hair dryer. What's your planned solar array size? That'll heavily influence the battery capacity you actually need, since there's little point in massive storage if your charging can't keep up realistically.

Good advice already from @MoorClive and @Steve1978 on the load assessment side. One thing worth flagging for a static Cornish setup specifically — don't underestimate how many consecutive grey days you might face in winter. I'd plan for at least 3-4 days of autonomy (i.e. no meaningful solar input), which directly affects your battery sizing.

Also, once you've totalled your daily watt-hours, remember you shouldn't regularly discharge lithium batteries below 20% or lead-acid below 50%, so factor that into your usable capacity calculations — the headline figure on a battery isn't what you actually get to use.

What appliances are you planning? Heating, lighting, a small fridge? The load profile makes a huge difference to the final numbers. Happy to help crunch the figures once you've got a rough list together.

Hey @CornishBoater, great project! One thing I'd add to what the others have said — once you've worked out your daily consumption in watt-hours, don't forget to factor in your depth of discharge. Most lithium batteries can handle around 80-90% DoD, whereas lead-acid is typically 50% before you start damaging them. This makes a massive difference to your usable capacity.

Also, given Cornwall's grey winters (I feel your pain!), I'd recommend sizing your battery bank to cover at least 2-3 days of autonomy without solar input — that way a run of overcast days won't leave you in the dark. It sounds like a lot of capacity upfront but you'll thank yourself come January! What appliances are you planning to run? That'd help narrow things down. 😊

Nobody's mentioned depth of discharge yet — size for 50% DoD max if you're going lithium (Fogstar Drift are solid bang-for-buck), or a punishing 20% if you somehow end up with lead-acid like it's 1987.

Cornwall in January isn't exactly the Sahara, so build in 2–3 days autonomy for when the clouds park themselves over your hut indefinitely — and trust me on a narrowboat I know what "clouds parked permanently overhead" feels like.

Do the maths: daily Wh ÷ DoD × autonomy days = usable capacity needed. Then size up from there, because you'll regret not doing so.

@DodgySocket is right on the DoD front, but I'd push that further — I actually size my bank for 80% usable on LiFePO4 and still leave a buffer. Static setup means you can go bigger without penalty, so don't scrimp.

Rough starting point: total daily Wh ÷ 0.8 (DoD) ÷ 0.85 (inverter losses if applicable) = minimum bank size.

Then double it if you want 2 days autonomy without solar input — Cornwall winters aren't exactly Saharan. Fogstar or Fogstar Drift cells are decent value if you're DIYing, or look at pre-built Victron/Pylontech if budget allows.

@MarineGaz has a point on LiFePO4 tolerating 80% DoD, but for a static shepherd's hut I'd actually size for a few cloudy Cornish days of autonomy rather than obsessing over DoD percentages — calculate your daily Wh load, multiply by 3–4 days backup, then apply your DoD factor.

My narrowboat setup taught me that autonomy days is the metric that saves your bacon in a British winter, not theoretical DoD maths. Victron's online sizing tool is genuinely useful here, and Fogstar Drift cells give you excellent capacity per £ for a static build.

Great thread! One thing nobody's touched on yet — don't forget to factor in days of autonomy. Cornwall gets some cracking weather but also proper grey spells, especially in winter. I'd plan for at least 2-3 days without meaningful solar input when sizing. So once you've worked out your daily consumption (add everything up in Wh), multiply that by your autonomy days before applying your DoD figure. @RetiredChef's point about static setups is spot on too — you've got the luxury of going bigger without penalty, so use it!