Marine battery bank sizing for continuous cruising

Looking at options for a proper battery setup in my motorhome conversion and keen to understand the marine perspective since we're essentially living aboard.

Currently running a 100Ah LiFePO4 system with Victron MPPT and a 3000W inverter, which gets me through most days. However, I'm planning a three-month continuous cruising stint across Europe next summer and want to understand if my existing capacity is realistic or if I need to upsize significantly.

The challenge is I've got two different power profiles depending on what I'm doing:

Daily cruising days (8-10 hours moving, minimal other loads):

• Fridge/freezer: ~30Ah
• Heating (diesel heater, minimal): ~5Ah
• Navigation/comms: ~3Ah
• Charging phones, laptop: ~5Ah
• Total: roughly 43Ah daily

Stationary days (engine off, working from the van):

• Same fridge, but heating potentially higher: ~15Ah
• Office equipment: ~15Ah
• Heating: ~15Ah
• Total: roughly 45-50Ah daily

I'm assuming the engine alternator can recover 30-40Ah on cruising days, and I'll have decent solar (800W panel array) for stationary periods. But I'm wondering if 100Ah LiFePO4 is genuinely enough headroom, or whether I should look at 200Ah for peace of mind and longer periods without cruising.

What's the real-world experience on battery oversizing for continuous cruising? Does it pay dividends, or am I overthinking this?

100Ah is a decent starter, but cruising continuously will have you rationing showers faster than you can say "grey water tank." The marine lot size everything for extended autonomy — you're looking at needing roughly 3-5 days' worth of your actual daily draw at anchor without sun or wind.

Run a proper audit first: measure everything for a week (fridge, heating, lighting, water pump), then multiply by 1.5 for winter. I've got 400Ah LiFePO4 spread across two banks in my setup and honestly still get antsy on cloudy weeks.

Victron's battery monitor is essential — you'll quickly realise where the phantom loads hide. And whatever capacity you pick now, you'll want 50% more in two years. It's a proper curse.

The marine angle does change things a bit. With continuous cruising you're not just topping up at a campsite every few days—you're genuinely off-grid for weeks at a time.

What's your typical daily consumption looking like? That's the real question. I've found with van conversion setups it's easy to underestimate—things like heating, water pumps, and fridge all drain faster than you'd think. 100Ah works if you're disciplined about it, but you'll likely be charging every single day from either solar or engine.

Are you considering splitting the battery bank? Some folks run a separate leisure battery for essentials and keep the main bank topped up. Also worth thinking about charge sources—how much solar are you fitting? Decent panel coverage makes a massive difference when you're stationary for longer periods.

What's driving the continuous cruising plan—travelling or anchoring in one spot?

The marine context is genuinely different from static off-grid living. You're not sat on a known solar resource or near a wind turbine—you're moving, which means your generation changes weekly.

100Ah is tight for continuous cruising unless you're running minimal loads. I'd target 200-300Ah depending on your consumption. More crucially, sort your charging strategy: engine alternator alone won't cut it, and you can't rely on shore power when you're underway.

Consider a hybrid approach—stack your LiFePO4 with decent solar (even 400W flexible panels make a difference on deck) and an undersized wind gen if you're coastal. A Victron MPPT will help you squeeze every amp from inconsistent conditions.

What's your typical daily draw? That'll tell you whether you need to upsize or if better load management sorts it instead. Shore power backup at marinas is your safety net, but don't bank on it for continuous cruising.

The marine context does shift your calculations quite a bit, @AndyRobinson. With continuous cruising, you're dealing with variable charging conditions—engine alternators aren't reliable income like a static solar array, and you can't simply wait for better weather before moving on.

100Ah LiFePO4 gives you roughly 5kWh usable capacity (accounting for the 50% DoD sweet spot most marine installers prefer). That's genuinely tight for continuous living unless you're disciplined about consumption. The real issue is charging dependency. You'll need a robust alternator setup—ideally 150A+ with a split charge relay or Victron Orion intelligent charger to manage the incoming power properly. Many motorhome converters underestimate their daily draw; factor in fridge, heating, water pump, and you're looking at 40-60Ah daily minimum in winter months.

Consider whether 200Ah is actually feasible for your payload and space. The cost difference between 100Ah and 200Ah LiFePO4 is substantial, but so is the freedom it buys. I'd also budget for a decent

@AndyRobinson, the 100Ah LiFePO4 is reasonable as a starting point, but you'll want to think differently about your charging sources than static installations.

The critical bit most people miss: your charge rate vs. consumption balance. On a boat, you're rarely stationary long enough to rely purely on solar—cloud cover, heading, latitude all conspire against you. If you're running an engine anyway, that's actually your primary charging source on a marine setup, unlike most off-grid homes.

I'd seriously consider sizing your alternator output relative to your daily draw. If you're pulling 20-30Ah daily from leisure systems (fridge, heating, water pump), a decent 100A+ alternator running a couple of hours daily will comfortably cover that with a Victron DC-DC charger managing the LiFePO4 safely.

The LiFePO4 chemistry is spot-on for marine use—better than AGM for cycling and temperature stability. But honestly, 100Ah might feel tight if you're adding heating or constant fridge/nav equipment. Have you calculated your actual daily consumption? That

You've got the right chemistry with LiFePO4, but have you considered your actual daily consumption pattern whilst cruising? That's where most folk trip up.

The issue I've found with boat setups versus our shepherds hut is you can't predict your solar input day-to-day—cloud cover changes rapidly, you're moving locations with different angles, sometimes you're moored up shaded. So you need either more battery capacity than the static calculations suggest, or a robust backup charging strategy.

With 100Ah, what's your shore power access like? If you're genuinely continuous cruising and unable to hook up regularly, you might find yourself relying on engine charging more than expected, which defeats the efficiency gains. I'd look at whether 150-200Ah would be more realistic for your actual usage without constant generator running.

Also worth asking—are you running 12V or 48V? At 12V with significant loads, you lose efficiency in cabling. Marine setups often benefit from stepping up to 48V even on smaller systems.

What does your daily consumption actually run to, and how often are you expecting to be at a mooring with

The difference between stationary and marine setups is really about your charge sources becoming unreliable. On a boat, you're dependent on engine hours, solar angle (often poor in UK waters), and wind—none guaranteed daily.

Your 100Ah gets you maybe 5kWh usable if you're sensible with depth of discharge. That's tight for continuous cruising unless you're disciplined. I'd actually be looking at 200-300Ah as a minimum, especially if you're running heating or cooking off the battery.

The real consideration is charge acceptance. LiFePO4 handles high-current charging better than lead, which matters when you've only got 2-3 hours of decent solar or need to charge heavily off the engine. A quality MPPT like Victron's SmartSolar will help you capture what you can, but you're still fighting the odds in November.

What's your actual daily draw? That'll determine whether you need expansion now or whether you're genuinely happy at 100Ah. If you're mostly at anchor with minimal propulsion, it's workable. If you're moving regularly, the engine running to charge

@AndyRoberson raises a solid point about the marine angle. Where stationary setups rely on predictable solar orientation and mains backup, you're looking at variable anchor points and irregular engine running. That's where your 100Ah becomes either adequate or undersized depending on your consumption profile.

The critical difference is your charge sources become genuinely unreliable. Unlike a static caravan where you can optimise panel angle, a motorhome's solar gain varies wildly with mooring position and weather. You'll burn through reserves faster when engine charging becomes your fallback during cloudy spells.

I'd suggest logging a full week's consumption before expanding. Track your fridge duty cycle, water pump usage, and inverter draws—these vary dramatically under way versus at anchor. If you're regularly hitting below 30% state of charge, you're undersized. If you're consistently topping out, 100Ah might suffice with disciplined power management.

Consider your engine's alternator output and charge controller compatibility too. Many motorhome conversions use inadequate chargers that can't properly bulk-stage a LiFePO4 bank under marine conditions. A

The 100Ah is workable for weekends, but continuous cruising's a different beast. I'd be targeting minimum 200-300Ah for genuine liveaboard peace of mind.

Key difference from stationary: your charge sources are genuinely compromised. Cloudy week on the Kennet & Avon? Your solar output drops 70%. Engine hours become your safety net, which means running costs spike. Victron's MPPT controllers help squeeze maximum from marginal light, but they can't create electrons that aren't there.

Consider also your alternator capacity—most conversions underspec this. A quality 100A unit (Balmar or similar) combined with smart battery management gives you realistic midweek recovery between proper solar windows.

@ForestBoater's spot on about unreliability. @BatteryPaddy—daily consumption is crucial, but so is peak draw. A kettle or shower pump hitting your system simultaneously changes everything when you're relying on finite stored energy rather than grid backup.

Suggest logging a fortnight's actual usage before expanding. Some find 150Ah sufficient with disciplined management;

Right, here's the thing nobody mentions until you're sat in the pouring rain with a flat battery at 2am on the Trent & Mersey — your 100Ah feels like a generous cup of tea until you're actually living on it.

I learned this the hard way. Solar works brilliantly when the sun decides to show up, which in the UK

@ExSquaddie49's right about the step-up needed. Question though — are you factoring in how much you're actually drawing daily? I've got 200Ah in my van conversion and thought it'd be bulletproof, but rainy weeks where solar's rubbish showed me the real picture. What's your typical consumption looking like? That'll determine if you need more capacity or just better generation backup.

You lot are dancing round the real question. I went from 100Ah to 300Ah on the narrowboat and it still felt tight until I actually logged what I was pulling — kettle, inverter, fridge running 24/7. That's the bit that catches you. What's your actual daily draw, Andy? That'll tell you whether you need 200 or 500.