What's the real-world difference between 100Ah and 200Ah LiFePO4 for a weekend van build?

Been quietly putting together a small van conversion over the past few months — nothing fancy, just a reliable weekend and occasional longer trip setup. Got a Fogstar Drift 100Ah 12V sitting in a box waiting to be wired in, but I keep second-guessing whether I should've gone straight to 200Ah before I committed.

My rough daily load works out to about 40–50Ah: a small 12V compressor fridge (Alpicool 15L, pulls maybe 20–25Ah/day in British summer temps), phone and laptop charging, a few LED lights. On paper, the 100Ah gives me a usable ~90Ah with LiFePO4, so I'm not in disaster territory. But I've got a single 175W Renogy panel on the roof, and I'm nervous about two overcast days back-to-back.

Has anyone run a similar load profile on a 100Ah LiFePO4 through a grey UK weekend and actually struggled, or does the anxiety not match reality? Wondering if a Victron SmartSolar 100/20 MPPT and good panel placement does enough heavy lifting to compensate for the smaller bank.

JoeFisher | 312 posts

@PaddyDavies the honest answer is it depends almost entirely on what you're running. A 100Ah LiFePO4 gives you roughly 100Ah usable (brilliant versus lead acid), so map out your actual loads first. Fridge compressors are the killer — a decent 12V fridge might pull 30-40Ah per day depending on ambient temperature. Add phone charging, lighting, maybe a laptop, and you can see how quickly a weekend trip starts pushing limits, especially if you're not getting reliable solar top-up.

The 200Ah obviously doubles your headroom and lets you relax a bit — less obsessing over the battery monitor. For strictly weekends with moderate usage the 100Ah is probably fine, but if you're eyeing up longer trips later, buying bigger now saves you the faff of upgrading. What's your solar situation looking like?

CallumHamilton83 | 847 posts

@PaddyDavies for weekend use the 100Ah will likely serve you fine, but the 200Ah genuinely changes the game once you start adding a compressor fridge into the mix. That alone can chew through 30-50Ah overnight depending on ambient temperature.

The other thing worth considering — LiFePO4 gives you roughly 80% usable capacity without stressing the cells, so your 100Ah is effectively ~80Ah real-world. A 200Ah becomes ~160Ah, which is a meaningful difference once you're on a grey overcast weekend (common enough in the UK, let's be honest) and your solar isn't pulling much.

If longer trips are genuinely on the cards, I'd stretch to the 200Ah now rather than regret it later. Retrofitting is a right faff once everything's built out.

WattGemma | 1,204 posts

@PaddyDavies one thing worth adding to what @JoeFisher and @CallumHamilton83 have said — with LiFePO4 you're actually using roughly 80-100% of rated capacity versus maybe 50% with lead-acid, so your 100Ah Fogstar Drift is genuinely closer to 100Ah usable. That's a decent chunk of real-world power.

The question I'd ask yourself is: are you regularly hitting that limit on current trips, or are you mostly anxious about what if scenarios? If it's the latter, focus first on nailing your charging setup — a decent B2B charger and solar panel will stretch that 100Ah considerably further than you might expect, especially over a full weekend.

Upgrade the battery later if you genuinely outgrow it. No point spending now on capacity you might never need.

ThingamyBob62 | 203 posts

Running a Fogstar Drift 100Ah myself in a small Berlingo build. Honest take — the jump to 200Ah isn't just about capacity, it's about depth of discharge. With 100Ah you're really working with ~80Ah usable before you start stressing the cells. 200Ah gives you that same buffer but with actual headroom left over.

Where I noticed it most was overnight. Fridge running + a bit of lighting and my 100Ah was down to 40% by morning. Not a disaster, but on longer trips it gets uncomfortable.

If you've already got the 100Ah fitted, I'd just run it for a few weekends and track actual usage before buying more. Plenty of people overthink this before they've even switched anything on. 📊

HarbourHamish | 412 posts

@PaddyDavies one angle nobody's mentioned yet — think about where you're going. If you're mostly on sites with hookup, 100Ah is genuinely plenty as you'll top it back up overnight anyway. But if you're drawn to more remote spots, wild camping on the west coast or similar, that 200Ah buffer becomes proper peace of mind, especially through winter when solar yield drops off significantly and you might string together several grey days in a row.

Also worth considering: a second 100Ah later is always an option if your needs grow. Easier on the wallet now, and you'll have a clearer picture of your actual usage patterns after a few trips.

The Fogstar Drift is a solid bit of kit either way — you've started well there.

CamperTel | 512 posts

@PaddyDavies one practical angle nobody's mentioned yet — consider your charging situation. With a 100Ah bank you'll realistically replenish it far quicker from your alternator or a modest solar setup, meaning you're back to full capacity sooner between trips. Double the capacity and you need roughly double the input to charge it in the same timeframe. If you're only heading out Friday evening and back Sunday, a 200Ah half-charged at departure isn't necessarily better than a 100Ah that's been sitting topped up all week. What's your current charging setup looking like? That might actually be the deciding factor here rather than the capacity figure itself.

BirchDaz | 287 posts

@PaddyDavies worth thinking about your actual overnight draw rather than just the headline capacity. Sit down and add up everything running whilst you sleep — fridge compressor cycling, any heating controls, phone charging — then multiply by hours. A lot of weekend vanners overestimate how much they actually use. If that total comes in under 50Ah, honestly the 100Ah will serve you well for two nights without breaking a sweat. The 200Ah buys you genuine peace of mind for longer stretches or cloudy weekends when solar input is poor, but it's a significant weight and cost jump for occasional use.

JennyCole | 89 posts

Had almost exactly this dilemma with my static caravan setup before I moved to a van conversion. One thing worth adding — the 200Ah option gives you much more headroom if you ever want to run a compressor fridge. My 100Ah felt fine until I added one and suddenly everything was tighter than expected. LiFePO4 usable capacity is brilliant, but a fridge cycling overnight chews through it faster than the specs suggest. If there's any chance your build evolves, the 200Ah is harder to regret later.