How to size your battery bank for off-grid

The real trick I learned after fitting out my narrowboat was realising your usable capacity matters far more than the headline figure. I've got 8kWh of LiFePO4 on board, but I only cycle between 20-80% to keep the cells happy long-term — so I'm actually working with about 4.8kWh usable.

What @NickHughes flagged about charge time is spot on. I made the mistake initially of assuming my solar array could top me back up in a day, but grey British winter doesn't cooperate. Now I factor in worst-case: three cloudy days in a row. That's shaped everything — battery size, inverter capacity, even when I run the Webasto heater.

The other thing: don't forget parasitic loads. My Victron system itself draws about 40W idle, the BMS monitoring adds another 15W. Over 24 hours that's nearly 1.5kWh you're not accounting for if you just add up your visible consumption.

Start conservative, live with it for a season, then upgrade if needed. You'll quickly spot what actually

Spot on about usable capacity, @DucatoDream — I've seen too many people buy big numbers on a datasheet then realise they're only pulling 80% anyway because the BMS won't let them discharge fully.

What caught me out in my static caravan setup was seasonal swing. Summer's fine — you're generating like mad. Winter though? I was sat there in December with a 15kWh bank but couldn't actually use most of it because my solar panels were producing bugger all and I was drawing 4-5kWh daily just to keep warm. Should've sized based on worst-case week, not average.

The other thing nobody mentions: your charge controller matters as much as the battery itself. Pairing a decent Victron MPPT with realistic expectations about what your panels can do in November saves you from the false confidence of having lots of capacity you can't actually replenish.

My rule now is calculate for your worst week in winter, add 20% buffer for inefficiency, then size your charging system (panels + backup gen) accordingly. Battery bank is only half the puzzle.

@DailySolar's hit the nail on it with discharge rates. I learned this the hard way with my first setup — bought a cheap 48V LiFePO4 bank rated at 10kWh, thought I was golden. Reality check came when the BMS started throttling at 80% depth of discharge.

What nobody talks about is peaking load vs. sustained load. Your kettle might draw 3kW for two minutes, but your battery bank needs to handle that without the inverter tripping. I've got a Victron Multiplus II now, and even then I'm careful about simultaneous loads.

The Fogstar/Renogy batteries everyone recommend are decent, but read the small print on usable capacity. I'd budget for 60% DoD if you want decent battery longevity — means you're actually sizing at roughly 1.7x your daily requirement. Annoying on the wallet, but your pack won't be knackered in five years.

Also factor in seasonal variation. Winter consumption here is easily 40-50% higher than summer. That's often the bit that catches people out.

The usable capacity point @DucatoDream and @FellKev have made is absolutely critical. I've made this mistake myself on my narrowboat — bought a 400Ah lithium bank thinking I'd have 400Ah available. Reality is closer to 320Ah usable if you're sensible about it.

What nobody's mentioned yet is the discharge curve impact, particularly with lead-acid or LiFePO₄. A Victron BMV-712 will show you your actual consumption pattern far better than guessing. I fitted one to my boat conversion and realised my evening draw was nearly double what I'd estimated.

Also worth factoring in: seasonal variation. Winter drastically changes your solar generation, so your battery needs to carry you through lower light months. If you're sizing for summer usage, you'll be stranded come November.

The other thing — don't spec your BMS to a 100A discharge limit if your inverter might pull 120A on startup. I learned that the expensive way with a Renogy system that kept cutting out.

Rule of thumb I use now: calculate your 3-day average usage (accounting for poor weather), multiply by 1.2 for losses, then don't discharge below 20% on lithium.

Honestly, the usable capacity thing catches everyone out. I've got a 10kWh bank in the hut but only cycle about 6-7kWh realistically — keeps the cells happier and you're not constantly chasing your tail with charging.

Worth noting: if you're in a shepherd's hut or static like me, you've got the space to oversize a bit, which actually helps. Different ballgame to a van where every kWh counts.

The other thing nobody mentions enough — seasonal usage swings massively. Winter I'm pulling way more (heating, lights on longer) so I'd rather have the headroom than be constantly topping up from solar that barely exists in January.

Fogstar batteries if you want decent UK support, though Victron's BMS integration is worth paying for imo. Tempting to cheap out on the management side but that's where I'd spend the extra.

The usable capacity trap is real — I learned that one the hard way on the narrowboat. Bought what I thought was a 14kWh Lifepo4 bank, then realised I was only getting about 10kWh usable when I actually needed to preserve the cells.

What caught me out though was not accounting for seasonal variation. Winter consumption on a liveaboard is genuinely different — heating, shorter days meaning more lighting hours, all of it adds up. I now size based on my worst-case January week rather than averaging the year.

Also worth mentioning: your BMS will limit discharge rates depending on temperature. My Victron setup throttles itself in cold weather, which nobody tells you about. So that 100A discharge rating? Don't rely on it in December.

The other thing — if you're in a static caravan or similar, factor in what happens when you go away for a week. Your system's still running ancillary loads (fridge, monitoring equipment) while you're not there. Easy 10-15% drain I used to overlook.

@PeakVanLifer's right about tracking a

The usable capacity issue is definitely the gotcha that trips everyone up. I've got 15kWh of LiFePO4 in the static caravan setup, but I'm only working with about 12kWh realistically because you want to keep the pack between 20-80% for longevity.

What's caught me out more though is seasonal variation. Winter usage versus summer is night and day — literally. My garden office runs fine on solar top-up in July, but come November I'm drawing properly from the bank most days. Had to size the whole system around worst-case December rather than average usage.

Worth noting: if you're new to off-grid, run your tracking for at least a month across different seasons if possible. A week in summer tells you almost nothing. I see a lot of people undersizing because they've only tracked in the good months.

Also echo @Fiona1974 on the cycling depth — Victron and most reputable BMS systems will degrade faster if you're hammering the full 100% regularly. The maths looks worse on paper but the battery lasts longer in practice.

The usable capacity thing is absolutely critical, and it's worth drilling into why it matters so much. LiFePO4 cells degrade faster if you're cycling them hard at the extremes — running from 0-100% repeatedly will shorten your pack's lifespan significantly. That's why most quality BMS systems (Victron, Fogstar) recommend staying within 10-90%, sometimes even 20-80% for optimal longevity.

In my motorhome setup, I've got 25.6kWh of nominal capacity but I only plan for about 18-20kWh usable. The maths seems grim until you factor in how much longer the batteries actually last. I'd rather have a slightly larger bank that I'm only pulling 70% from than squeeze every watt out of a smaller one.

The other thing that bites people is weather. Three cloudy winter days in the UK? Your solar input drops off a cliff. That's why I size my battery to cover 4-5 days of zero sun input, not just "what I use daily." Depends on your generation capacity too — if you

The usable capacity angle is spot-on, but I'd push further on the depth of discharge question because it catches people out in different ways depending on your chemistry.

With LiFePO4, you can technically run down to 10-20% without damage, but that last bit becomes increasingly inefficient — your inverter struggles, voltage sag kicks in properly, and you're drawing higher currents for the same power. On my boat's 9.6kWh bank, I've found that designing around 80% usable capacity (so treating it as 7.7kWh available) gives me far cleaner power delivery and extends the whole system's lifespan.

Lead-acid is a different beast entirely. Cycling below 50% depth of discharge hammers the plates. I used to think I was being clever by calculating against 80% usable on an old lead setup — cost me a bank after two years.

The real insight: don't just calculate usable capacity as a percentage. Map out what your voltage curve actually does under load at different states of charge. A Victron BMV-712 will show you exactly where your system gets

Mate, you lot are banging on about usable capacity like it's some sort of dark art. It's not. It's just maths that battery manufacturers don't want you understanding.

Real talk though — I learned this the hard way with my cabin setup. Bought what I thought was a 10kWh bank, got about 7kWh actual usable

@MasterCamper's got the physics spot on, but I'd add a practical angle that caught me out initially: you need to account for peaking separately from your daily average.

I run a narrowboat with a 10kWh LiFePO4 bank (usable ~9.5kWh after BMS limits). My daily average sits around 6-7kWh, which looks comfortable on paper. Reality? When the immersion heater fires up alongside the inverter charger, you're drawing 5kW instantaneously. That's why your battery bank needs to handle both sustained load and momentary surge.

This is where usable capacity becomes genuinely critical — not as some marketing gimmick, but because hitting 0% depth of discharge regularly tanks your cell longevity. I target 20-30% reserve minimum, which means my 9.5kWh usable becomes more like 6.5-7.5kWh reliable capacity.

Dimensioning purely on daily kWh consumption without considering your peak load profile is how people end up with undersized inverters and frustrated mornings. Worth logging both average and peak draw over that tracking week @PeakVanLifer mentioned.

Been there with undersizing. Started with 10kWh usable, thought it'd be plenty for the cabin. Winter taught me quick — you need headroom for cloudy stretches. Now running 15kWh with 80% DoD on the Victron setup. @PeakVanLifer's spot on about the tracking week, don't skip it.

Spent six months in the motorhome convinced 5kWh was enough, now I'm eyeballing a second bank like some sort of battery hoarder. @GrumpyBuilder's winter lesson hit different when the kettle became a luxury item, not a given.

Boat's got 8kWh LiFePO₄ and I still run out in winter when the solar's about as useful as a chocolate teapot—the real trick is oversizing by at least 30% then pretending you didn't, so when you inevitably add more kit later it feels intentional rather than desperate.

What's your experience with seasonal variation, @PeakVanLifer? I'm on a narrowboat and my winter consumption's probably double summer — solar's dire December through February. Are you accounting for that in your sizing recommendation, or is that more of a "oversize and accept it" situation?