Sizing a battery bank for a 7kW EV charger off-grid — is it even realistic?

Been deep in the rabbit hole on this one. My cabin sits on 6kWh of Fogstar Drift LiFePO4 (two 3kWh 48V units in parallel) with 2.4kW of solar across six 400W panels and a Victron Multiplus-II 5kVA as the inverter-charger. Works brilliantly for the cabin loads — fridge, lighting, laptops, occasional power tools. But I've been seriously considering adding a 7kW Type 2 EV charger for when I drive up in the Leaf, and the numbers are making my head hurt.

The obvious problem: 7kW continuous draw would flatten my 6kWh bank in under an hour, even ignoring the Multiplus-II's 5kVA ceiling (so realistically capped at ~4.8kW AC output anyway). Solar can offset maybe 2–2.4kW during a decent summer afternoon, but that still leaves a massive gap. I've been looking at scaling up to 30kWh+ — something like five or six Fogstar Drift 100Ah 48V units — but the cost is steep and I'm not sure the Multiplus-II could sustain that load without thermal issues over a 4–5 hour charge session.

Has anyone actually built a system that handles Level 2 EV charging reliably off-grid? I'm wondering if the smarter play is to just drop the charge rate to 3.7kW (single-phase, 16A) and let the solar contribution make more of a dent, with a larger battery buffer underneath. The Leaf's onboard charger will accept anything from about 6A upward so there's flexibility there. Curious whether anyone's paired a Victron setup with an EVSE that does proper dynamic load balancing against available PV.

@QIH_Electric the brutal maths here is your inverter is already the ceiling — Multiplus-II 5kVA won't push 7kW anyway, so that's your first constraint before you even touch battery sizing.

Realistically, a Type 2 AC charger throttled to 3.6kW is where I landed with my own setup. Even then, you're burning through your 6kWh bank in under two hours at that draw, so you're entirely dependent on whether solar is actively generating whilst you're charging.

The strategy that actually works:

• Charge the EV midday only, solar-to-EV direct
• Set Victron ESS to limit discharge depth during charging cycles
• Accept you're adding 20–30 miles per sunny afternoon, not filling the car

It's not a fast charger — it's a slow, free charger. Different mindset entirely. Took me a season to stop fighting that reality.

@QIH_Electric that 5kVA Multiplus-II is already your first ceiling — it'll peak around 4kW continuous output, so a 7kW charger simply won't see its rated speed regardless of what's behind it.

Speaking from the motorhome world, I ran a similar calculation when considering an EV charger for my static caravan setup. The maths gets brutal fast.

Your 6kWh bank sounds healthy until you realise a 7kW charger would drain it in under an hour — at 100% efficiency, which never happens. You'd need at least 20-25kWh of usable storage to make a meaningful charge session worthwhile, plus the solar to recover it overnight.

The honest answer is: realistic only if you downgrade to a 3.6kW Type 2 charger and accept slow overnight charging during summer months. Winter becomes a generator conversation.

@QIH_Electric even if you swapped to a Multiplus-II 10kVA and stacked batteries to, say, 30kWh, you'd drain the whole bank in under 30 minutes at 7kW — your 2.4kW solar is basically just watching that happen whilst sipping tea. Realistically, a Type 2 AC charger dialled down to 3.6kW (16A single-phase) is the sweet spot for off-grid EV charging; pair it with a Victron ESS setup and let solar top the car up slowly during daylight. Fogstar Drift cells are perfectly capable, you just need far more of them — I'd be thinking 20kWh minimum before even entertaining it. The honest answer is 7kW off-grid is a rich man's problem that requires a very big battery shed and a diesel genny as backup, full stop.

Really good points from @Drift_Geek, @RetiredNurse58, and @MarineGeoff already covering the hard limits. One angle worth adding: have you looked at the Zappi or similar EV chargers that support solar divert mode? Running a 1.4kW granny charger overnight from your batteries is far more realistic for cabin use — yes it's slow, but if the car sits there for 12+ hours you're still adding meaningful range. Also consider what you actually need from the car versus what it arrives with. If you're topping up rather than charging flat-to-full, your existing setup might genuinely be workable with modest expectation-setting. The 7kW dream is probably a road too far without serious infrastructure investment.

@QIH_Electric worth thinking about this differently — rather than trying to match the charger's rated speed, could you throttle it right down? Most Type 2 chargers let you dial back to 6A or so via the pilot signal, which on single phase gives you roughly 1.4kW. That's well within what your Multiplus-II can handle, and on a sunny day your solar could almost cover it entirely. Yes, charging would be glacially slow — maybe 10-15 miles of range per hour — but for a cabin scenario where the car sits overnight, that might actually be perfectly adequate. A small dedicated EVSE with adjustable amperage like an Easee or Zappi would give you that control. Completely different conversation to fast charging, but arguably more sensible off-grid than throwing money at a battery bank the size of a small house.

6kWh trying to feed a 7kW charger is basically asking a hamster to tow a caravan — my motorhome setup taught me that lesson the hard way when I tried running a 2kW kettle off a system that looked great on paper.

The maths just doesn't care about your feelings: at 7kW continuous draw you'd flatten those Fogstar units in under an hour, assuming your Multiplus-II 5kVA would even attempt it (spoiler: it won't, it'll fold its arms and sulk).

@VoltDoug's throttling idea is genuinely the only sane path here — most EVs will accept 6A via Type 2, which gets you down to roughly 1.4kW. That your system can actually have a conversation with.

Really love @PanelRob's hamster analogy — brutal but accurate! One thing nobody's mentioned yet: have you looked at the Zappi charger's eco modes? It can throttle right down to 1.4kW and dynamically adjust based on available solar surplus. Paired with your Victron's ESS assistant, you could potentially set charge thresholds so the EV only draws when your battery is above, say, 80% SoC and solar is actively producing. Not glamorous speeds, but it keeps everything sensible. Might be worth a chat on the Myenergi forum too — loads of off-grid Zappi users over there with real-world data.

Really good thread this. @QIH_Electric one angle worth considering is staged charging — drop the charger to 3.7kW (roughly 16A on Type 2) rather than pushing the full 7kW. Your Multiplus-II can handle that comfortably, and you're actually extracting usable range from whatever's in the bank rather than tripping on low voltage cutoff halfway through. Pair that with a decent solar forecast app and plug in midday when panels are doing the heavy lifting. Not glamorous, but it works. What's the EV — some accept lower AC rates better than others?

@QIH_Electric worth flagging that your Multiplus-II 5kVA is already your bottleneck before you even consider the battery capacity issue. That inverter's continuous output sits around 4kW realistically, so a 7kW charger will either trip it or throttle back automatically anyway. You'd essentially be getting a 3-4kW charge rate maximum regardless. Building on @RobHenderson's staged charging idea — if you set charge current low enough to work with your solar input rather than against it, midday top-ups become genuinely viable. What's your EV and typical daily mileage? That changes the whole conversation considerably.

@Taffy55 makes a solid point about the Multiplus being the ceiling here, but even if you upgraded the inverter the maths is brutal.

6kWh usable (say 80%) = 4.8kWh before you're stressing the cells. A 7kW charger would flatten that in under 45 minutes — assuming solar's doing nothing useful at the time.

Honestly? I run a similar Fogstar setup and the EV is the one thing I wouldn't attempt off a bank that size. You're looking at 30-40kWh minimum for any kind of relaxed overnight charging without hammering your DoD daily.