What's the actual minimum battery capacity needed for a serious home emergency backup system?

Been putting together a dedicated emergency backup setup for the house — not a full off-grid system, just something that'll keep the essentials running during a grid outage. We're talking fridge-freezer (~150W), a few LED circuits, phone/laptop charging, and a small gas boiler that needs ~80W for the pump and controls. Realistic worst-case scenario is 3–4 days without grid power.

Running the numbers, I'm landing somewhere around 4–6kWh of usable capacity depending on how conservative I am with the estimates. Currently looking at either a pair of Fogstar Drift 200Ah 12V lithium batteries (giving me ~4.8kWh usable at 12V) or stepping up to 24V with four of them for ~9.6kWh. The 24V option obviously runs more efficiently and opens up better inverter choices — leaning toward a Victron MultiPlus-II 24/3000 which handles the transfer switching cleanly.

What I can't find a solid answer on is whether people running similar setups find 4–5kWh genuinely sufficient for that kind of 3–4 day window, or whether real-world usage always creeps higher than the spreadsheet says. Also curious whether anyone's running a setup like this without any solar top-up — purely relying on pre-charged batteries — and how that's worked out practically.

BenChapman67 | 847 posts

@CamperDan good project to tackle. The fridge-freezer is often the sneaky one people underestimate — it's not just the running wattage, it's that compressor startup surge which can be 3-4x the rated draw.

For a serious emergency setup I'd say 10kWh usable is a realistic minimum if you want 24-48 hours of genuine comfort without rationing. That assumes a modern A++ fridge-freezer, some lighting, phone charging, and a router.

One thing worth factoring in that people overlook — what's your inverter efficiency at partial load? Running a 3kW inverter at 200W draw is genuinely wasteful. A properly sized inverter makes a meaningful difference to how far your battery capacity actually stretches.

What battery chemistry are you leaning towards? LiFePO4 is the obvious choice for longevity but it does change the sizing conversation slightly.

Nick1982 | 312 posts

Good thread. One thing worth adding — don't forget to factor in the inverter's idle draw and inefficiency losses. A 10-15% overhead on top of your calculated load is sensible minimum. Also consider what @BenChapman67 is getting at with the fridge — it's not just the running watts but the compressor startup surge which can be 3-4x the rated wattage momentarily.

For a "serious" backup I'd say 5kWh usable is the realistic floor for most households covering essentials through a 24hr outage, but honestly 10kWh gives you proper peace of mind without sweating every amp. What's your depth of discharge limit on whatever chemistry you're going with? LiFePO4 changes the maths significantly compared to lead-acid.

BorderCruiser | 1,203 posts

Great question @CamperDan. Without knowing your exact loads it's hard to pin down, but as a rough starting point I'd suggest working backwards from your daily essential consumption and then doubling it — that gives you a comfortable two-day buffer without stressing the battery bank below 50% DoD, which matters enormously for longevity if you're running lead-acid.

For a typical UK household with fridge-freezer, basic lighting, phone charging and a router, you're realistically looking at 3-5kWh minimum usable capacity. So with LiFePO4 that might be a 5kWh nominal bank; with AGM you'd want closer to 10kWh nominal.

Also worth considering whether you've got any solar topping up during the outage — changes the calculus significantly if you have even a modest array.

TracyRobinson | 564 posts

Something nobody's mentioned yet — think carefully about your usable capacity versus nameplate capacity. A 10kWh LiFePO4 battery is genuinely usable down to around 90-95% of rated capacity, whereas lead-acid you'd only want to draw 50% before you start hammering the cycle life. So if you're comparing prices between chemistries, the lead-acid option needs to be roughly twice the stated capacity to give you the same usable storage.

Also worth considering how many days you're designing for. One day of autonomy feels very different when you're actually living through a three-day winter outage, which isn't uncommon after storms here in the UK. I'd personally suggest planning for at minimum 2 days without any solar input, especially if your panels might be snow-covered or heavily overcast.

WattEd | 847 posts

@TracyRobinson makes a solid point on usable capacity — worth extending that thought into chemistry selection. For a backup system specifically, LiFePO4 gives you ~95% usable versus ~50% for AGM, so a 200Ah LiFePO4 (Fogstar Drift cells are decent value) genuinely outperforms a 400Ah AGM bank in real-world terms, at lower weight and without the sulphation risk from sitting partially discharged between outages.

That last bit matters enormously for emergency setups — your battery could sit at 80% SOC for months between actual use. AGM hates that. LiFePO4 is largely indifferent.

For a fridge-freezer plus lighting plus phone charging over 24–48 hours, I'd calculate your actual Wh demand first (clamp meter on each load for an hour), then size to cover that twice over using usable capacity figures, not nameplate.

Holly1960 | 2,341 posts

Good thread this. One thing I'd add beyond what @TracyRobinson and @WattEd have touched on — don't forget to factor in winter as your baseline scenario. Your fridge-freezer works harder in summer heat but your longest outages statistically tend to happen in winter storms, when you've also got lighting loads creeping up significantly and phones/radios getting hammered for news updates.

I sized my backup system in spring and genuinely underestimated my winter evening load by about 15%. Did a proper audit in December and it was a real eye-opener.

Run your calculations against a worst-case 48-hour winter outage rather than average conditions — if the system handles that comfortably, you'll have proper peace of mind rather than just hoping for the best. Better to slightly overspec than find yourself rationing power on a dark January night! 🙂

CE_Builds | 1,203 posts

Good points from @TracyRobinson and @WattEd on usable capacity and chemistry. One thing I'd add — don't forget inverter surge draw. Your fridge-freezer compressor kicks in at 3-4x running watts. If your inverter can't handle that peak, it'll shut down regardless of how much battery you have.

For my garden office backup I run a Victron MultiPlus 2 — handles surge without breaking a sweat. Size your inverter first, then work backwards to battery capacity.

Rough ballpark for serious home backup: 10kWh usable minimum, LiFePO4. Fogstar Drift cells if DIY appeals.

SussexVanLifer | 634 posts

Done exactly this for my house after building out my van system — the lessons transfer directly.

One thing nobody's mentioned yet: runtime assumptions are your enemy. Everyone calculates for a 12-hour outage, then Storm Éowyn turns up and you're dark for 60 hours.

My rule of thumb from van life — size for your worst realistic scenario, not your average one. For UK homes that means 3 days minimum given how our grid behaves in serious weather events.

Also factor in that a fridge-freezer draws considerably more at startup than running watts. Those inverter surge ratings matter enormously once you're doing real loads.