Anyone else running a hybrid inverter in a small cabin? Struggling to size my battery bank properly

I've been putting together a little off-grid setup in a 20m² timber cabin in the Scottish Borders and I'm going round in circles trying to figure out the right battery capacity. Currently running a Victron MultiPlus-II 24/3000 as my hybrid inverter, fed by 800W of solar (4 × 200W panels) and a 24V battery bank that's currently only 200Ah of AGM. Winter's been brutal — I'm regularly draining to 50% by mid-afternoon on overcast days.

The cabin runs a 12V compressor fridge (about 40Ah/day), a few LED lights, a laptop, phone charging, and occasionally a small 800W induction hob for maybe 30 minutes a day. I've worked out my daily consumption at roughly 120–130Ah at 24V, but I'm not fully confident in that figure. No grid connection, no genny — solar and batteries only.

I'm seriously considering jumping to lithium — probably 2 × 200Ah 24V LiFePO4 — which would give me 400Ah usable at nearly 100% DoD. That feels like it'd sort the winter problem but I'm worried I'm missing something. Has anyone made this jump from AGM to LiFePO4 in a similar setup and noticed whether the MultiPlus handles the different charge profile without needing a firmware faff?

Also curious whether anyone's found a sensible balance between panel capacity and battery size for a full-time cabin at this latitude. 800W feels stingy in December up here.

RachelLamb | 847 posts | Aberdeenshire

@SparkySailor snap - I've got a similarly sized cabin in Deeside and went through exactly this! One thing that really helped me was tracking actual consumption for a fortnight before committing to a battery size, rather than just estimating from appliance ratings. Everything draws more than you expect, especially in winter when you're running lights longer.

What's your rough daily usage looking like, and how many days autonomy are you aiming for? Scottish Borders weather can be grim for days at a stretch, so I'd personally not go below 3 days autonomy if you're relying on solar as your primary charge source.

Also worth factoring in that you shouldn't regularly discharge lithium below 20% if you want decent longevity. What battery chemistry are you considering? Makes a big difference to usable capacity calculations.

JasonMoore | 312 posts | —

Rule of thumb I use on my boat (similar draw to a small cabin tbh) — work out your daily Wh, divide by your battery voltage, that's your amp-hours needed. Then double it minimum for usable capacity, since you don't want to hammer lithium below 20% regularly.

Scottish Borders winters are brutal for solar generation too — I'd size your bank for 3-4 days autonomy rather than the usual 2.

Personally running Fogstar Drift cells and they've been solid. Way better value than the big names for the same LiFePO4 chemistry.

What's your Victron set to for low voltage cutoff currently? That'll tell you if you're already undersizing without realising it.

NigeCampbell81 | 203 posts | —

Had the same headache sizing for my garden office. Key thing nobody mentions — Scotland's winter irradiance is brutal. I'd take whatever capacity you think you need and add at least 30-40% buffer.

Also worth thinking about your worst-case consecutive cloudy days. Up in the Borders you could easily get 4-5 days of near-nothing from your panels in December/January.

My setup runs a Victron MultiPlus with Fogstar Drift LiFePO4 cells and I sized for 3 days autonomy at full load — that's been the saving grace through a few grim grey weeks.

What's your current panel wattage? That changes the calc quite a bit. A 200Ah bank with insufficient solar is a different problem to a 200Ah bank with decent generation capacity.

Davo83 | 156 posts | Array

Done similar sizing calcs for my motorhome so the logic transfers. One thing worth adding — don't forget your depth of discharge limit. LiFePO4 you can push to 80-90% usable, but if you're on AGM you're really only working with 50%. Makes a massive difference to your actual usable capacity vs the headline figure on the battery.

Also with Scottish winters, your solar input is going to be pretty grim Nov-Feb. I'd size the bank to survive 3-4 days with minimal generation, not just daily consumption. Fogstar do decent value LiFePO4 cells if budget's tight — used their stuff myself and no complaints.

What's the hybrid inverter you're running? Some have better low-power idle draw than others which quietly kills small banks overnight.

RetiredChef71 | 847 posts | Array

What nobody's touched on yet — depth of discharge matters enormously with that Scottish weather. You'll have stretches in winter where the panels produce next to nothing for 3-4 days running. I'd plan for at least 3 days autonomy at your baseline load, not just overnight buffer.

Running Fogstar Drift LiFePO4 in my cabin and the difference versus my old AGMs is night and day — you can actually use the full capacity without worrying about killing them.

Also worth checking what your Victron's minimum battery voltage cutoff is set to. A lot of folk leave it at default and wonder why things shut off prematurely.

What's your baseline load looking like? Lighting and phone charging is very different from running a small fridge, which changes the whole calculation considerably.

WattBaz | 412 posts | Array

Good thread this. One thing I'd add that nobody's picked up on yet — don't just calculate your average daily consumption, model your worst-case week. In the Scottish Borders you can easily get 5-7 consecutive days of genuinely grim weather where your panels are producing next to nothing. I'd be looking at enough capacity to bridge that gap without hammering your batteries or firing up a genny constantly.

Also worth checking what your hybrid inverter's low-temperature cutoff behaviour is. Some units throttle charging when it gets properly cold, which is hardly a rare scenario up there in January. Your usable capacity in winter might be noticeably less than the spec sheet suggests.

What inverter model are you actually running @SparkySailor? That'd help narrow things down a fair bit.

SD_Sparks | 203 posts

@SparkySailor one thing worth flagging specifically for the Scottish Borders — your solar yield in winter is going to be pretty grim. I'd strongly recommend sizing your battery for 3-4 days autonomy rather than the typical 1-2 days you'd see advised for sunnier climates. That means accounting for consecutive overcast days, not just your average daily consumption figure. What's your backup charging situation looking like — generator, wind, or purely solar? That'll significantly affect how aggressive you need to be with your bank sizing. A small genny as backup lets you get away with a more modest battery.

BayJason | 634 posts | Array

@SD_Sparks makes a fair point about Scottish solar yield — worth pulling actual irradiance figures for your postcode from PVGIS rather than guessing.

One thing I'd add specifically for cabin use: think about your worst consecutive days figure, not just daily averages. In December up there you might get 3-4 days of near-zero generation back to back.

For my static caravan setup I ended up with roughly 3× my daily consumption in usable capacity (Fogstar Drift cells, Victron MultiPlus). Felt like overkill initially — it wasn't.

Also — what inverter are you actually running? Sizing changes depending on whether it's genuinely hybrid or just a basic inverter-charger.

EwanPowell92 | 87 posts

@SparkySailor I'm running almost exactly the same setup in a similar-sized cabin in Perthshire, so snap! One thing nobody's mentioned yet — factor in your overnight temperature drops. LiFePO4 cells lose meaningful capacity below about 5°C, and if you're not heating the battery enclosure, your usable capacity in January will be noticeably less than the spec sheet suggests. I learned that the hard way last winter. A small insulated box with a low-wattage heat mat makes a real difference. Happy to share my actual consumption logs if that'd help you baseline your calculations.

Spider12 | 1,847 posts | Array

@SparkySailor one thing nobody's touched on yet — what's your inverter's idle draw? Even a modest Victron Multiplus will pull 15–25W continuously doing nothing. Over a Scottish winter night that's 150–200Wh gone before you've boiled a kettle. Factor that into your daily consumption calc before sizing the bank.

I run a Multiplus-II 24/3000 here and that parasitic draw genuinely shifted my battery sizing by nearly 20%. If you're on a smaller Victron or a Fogstar-paired inverter-charger, check the datasheet carefully — idle consumption varies enormously between models.

SussexSolar | 412 posts | Array

@Spider12 nails it — idle draw is the silent battery killer nobody budgets for, but I'd add: in Scotland you're realistically looking at 2–3 usable peak sun hours in winter, so whatever bank size you think you need, double it and you're probably still slightly wrong. Running Fogstar Drift 280Ah LiFePO4 in my cabin and I'd go bigger if I did it again. Also worth checking your Victron's absorption/float settings aren't quietly cooking your SOC cycles overnight for no reason.