What's everyone's minimum viable battery reserve for riding out a 3-day grid outage in winter?

Been thinking about this a lot lately since we had a nasty storm up here in the Highlands last month — three days without grid power, temperatures dropping to -4°C overnight. My shepherd's hut is fully off-grid anyway, running a 400Ah 12V LiFePO4 bank (Fogstar Drift cells, self-built), so I was fine, but it got me wondering what the absolute floor should be for someone planning for emergencies rather than living off-grid full-time.

For context, my hut draws roughly 800Wh/day in winter — LED lighting, a 12V diesel heater fan, phone/laptop charging, and occasional inverter use for the kettle. The 400Ah bank at 80% usable gives me around 3.8kWh, which covers me comfortably for 4+ days with no solar input whatsoever. But I'm 100% off-grid, so I've sized for that. For someone retrofitting a backup system onto a grid-tied house, what's the realistic minimum — 200Ah at 48V? More?

Curious whether anyone's actually stress-tested their setup through a real multi-day outage rather than just running the numbers. Also interested in whether people are using dedicated backup banks separate from their day-to-day solar storage, or just one unified system. The Victron Multiplus-II with an ESS config seems like the obvious route for grid-tied folk wanting seamless switchover, but the cost is substantial — wondering if there are leaner approaches people have actually deployed.

EwanGreen | 847 posts | Orkney

@HighlandExplorer that storm was brutal wasn't it — felt it down here on the islands too.

My rule of thumb for a Scottish winter three-dayer: I won't feel comfortable below 15kWh usable capacity, and that's assuming I'm being disciplined about it. The trap people fall into is forgetting that heating draws are continuous rather than spiky — a small oil-filled radiator running overnight absolutely murders a battery bank you thought was adequately sized.

What's your current setup in the hut? The shepherd's hut aspect is interesting because the thermal mass is practically zero, so you're fighting heat loss constantly rather than just maintaining temperature.

I'd also factor in that January solar input up in the Highlands is essentially decorative — you genuinely cannot count on meaningful recharge during a prolonged storm event, so your reserve needs to be truly standalone.

PanelRoger | 1,203 posts | Array

My shepherd's hut survived a similar 3-day blackout last February on 200Ah of Fogstar lithium — but I'll be honest, by day three I was rationing the kettle like it was wartime and the electric blanket had been unceremoniously retired to the cupboard. Rule of thumb I've landed on: whatever you think you need, double it, because -4°C and "I'll just put another jumper on" are two very different conversations at 2am. Now running 400Ah and sleeping considerably better. @EwanGreen Orkney winters are basically a Victron stress test that nobody signed up for.

MountainChild | 412 posts | Array

Living aboard a narrowboat through last winter gave me a proper education in this. Three days at -4°C with the diesel stove running, I was burning through roughly 80Ah daily just on the Victron Cerbo, bilge pump cycling, and a small 12V fan circulating heat.

Ended up concluding 300Ah usable is my absolute floor — and that's usable, so you're looking at 400Ah of Fogstar lithium minimum, not lead-acid figures people sometimes quote.

The thing nobody mentions: it's not just capacity, it's discharge rate. @PanelRoger's 200Ah setup would've had me nervous on night two when the inverter kicks in for the kettle and those overnight loads compound.

My rule of thumb — whatever you think you need, add 40% for the "oh it's actually colder than forecast" reality of a British winter.

Zoe1993 | 312 posts | South Wales

Great thread @HighlandExplorer — those Highland winters are no joke. We're milder down here but I still size for worst case.

My rule of thumb is roughly 400Ah at 12V (so ~5kWh usable assuming LiFePO4) for a 3-day winter stretch with minimal luxuries. That covers a small inverter for cooking, LED lighting, phone charging, and crucially a 12V blanket overnight when it's really bitter.

The thing people underestimate is fridge consumption — your fridge actually works harder in winter if it's in an unheated outbuilding, which catches folks out.

@PanelRoger curious whether your 200Ah setup included any solar top-up during those 3 days? In a Highland December you're getting virtually nothing useful, so it's basically running pure battery capacity the whole time.

SussexBoater | 847 posts | Array

After three winters on the boat I landed on 400Ah LiFePO4 (Fogstar Drift cells) as my "sleep soundly" number — anything less and you're basically playing Russian roulette with your morning cuppa. @MountainChild will know the particular joy of waking at 2am to check the Victron app "just in case." The real killer isn't lights or the router — it's that sneaky diesel heater blower running all night that quietly murders your reserves while you're dreaming of warmer anchorages.

IanPearce56 | 203 posts | Array

Static caravan here, so similar situation to some of you — not a permanent structure, so insulation is the weak point in winter.

Ended up settling on 300Ah LiFePO4 (Victron SmartLithium) after doing a proper load audit. The heating is the killer — I'm running a diesel heater which doesn't pull much, but factor in lighting, phone charging, a small inverter for the kettle and you're looking at 60-80Ah/day in cold weather if you're careful.

300Ah at 80% usable DoD gives me roughly 3.5 days of minimal comfort without any solar input. That buffer matters when January cloud cover means the panels are basically decorative.

@SussexBoater the Fogstar cells are tempting — how are yours holding up through temperature swings? I've heard LiFePO4 can be funny about charging below 5°C.

JohnMason | 156 posts | Derbyshire Peak District

Good timing on this thread — we had a similar three-day outage back in February. I'd argue the honest answer depends heavily on what you're not willing to compromise on. Heating is the killer load that most people underestimate. I run 15kWh usable LiFePO4 and it felt tight with an oil-filled radiator cycling through the nights.

@HighlandExplorer at -4°C I'd honestly say you want to factor in at least 50% more than your summer calculations suggest — thermal losses change everything. @SussexBoater's 400Ah is probably fine on a boat where you're not fighting a building envelope.

What's your primary heating source when the grid drops?