Looking to design/ build a new lithium / solar system to upgrade 2013 generation travel trailer

Been down this rabbit hole myself when I kitted out my shepherd's hut a couple of years back, so thought I'd share some thoughts and get a discussion going.

The Victron MultiPlus-II is the obvious choice for a serious lithium build — the 12V/3000VA version handles most realistic loads without breaking a sweat. That said, if you're genuinely running 3000W continuously you'll want to be honest with yourself about your battery capacity, because that'll absolutely hammer a modest bank. I run a 200Ah Fogstar Drift setup and I keep my inverter demands sensible.

A few things worth thinking through for anyone planning a similar UK off-grid build:

• MPPT controller sizing — don't undersell this. Victron SmartSolar is the obvious recommendation but size it properly for future panel expansion
• Battery monitor — a Victron BMV-712 or the built-in SmartShunt is non-negotiable honestly, flying blind is just daft
• Shore power passthrough — if you're occasionally on a site hookup, a MultiPlus gives you that for free essentially

One thing that catches people out in the UK specifically is winter irradiance — what works fine boondocking in Arizona in July is a completely different animal parked in a Cornish field in November. Factor that in when sizing panels.

What loads are you actually planning to run? Compressor fridge, lighting, laptop stuff, or are you talking about kettles and microwaves? That changes the maths considerably.

Curious what others here have settled on for mobile setups — particularly anyone running a shepherd's hut or similar on wheels.

Great thread @SaltyHiker! One thing worth flagging for travel trailer builds specifically - payload capacity. Lithium is lighter than lead-acid per kWh, but people often underestimate the total system weight once you factor in cabling, a proper battery box, and the inverter/charger itself. Worth getting on the scales before you commit to a particular battery bank size.

Also, with a 2013 trailer you'll want to check what your existing 12V wiring can handle if you're planning to run higher charge currents. Older looms weren't designed with lithium charge profiles in mind and the cabling may need upgrading before anything else.

What's your intended use pattern - weekend trips or extended touring? That'll really drive the battery sizing conversation more than anything else.

Good point from @BenWebb on payload. Worth adding that for a static caravan setup (my situation), that constraint disappears and you can go heavier with battery capacity without worrying.

One thing I'm still working through for my own build — EV charging integration. If you're planning to charge a car from the same system, your inverter sizing changes dramatically. A basic 7kW home charger pulls roughly 30A at 240V continuously, which means your MultiPlus-II needs to be the 5kVA minimum, and your battery bank needs serious capacity to sustain that without constant grid fallback.

Has anyone here actually sized a system for simultaneous EV charging plus domestic loads off-grid? I'm trying to establish whether:

• A single MultiPlus-II 5000 is sufficient, or whether parallel units make more sense
• What minimum usable battery kWh people would recommend

Keen to hear real-world experience rather than theoretical calculations.

Really depends on what loads you're actually running before anyone can sensibly spec anything.

I went down the "bigger is better" route first time round with my motorhome build — ended up massively over-panelled for my actual consumption. Embarrassing, frankly.

The bit I'd stress that nobody's mentioned yet: shadow tolerance and MPPT sizing matter enormously on a travel trailer because your panel placement is rarely ideal. A tree here, an awning there, and suddenly your 400W array is producing 60W.

Victron's MPPT calculators are decent for working backwards from realistic yield. And don't overlook a battery monitor (Cerbo GX or even just a SmartShunt) from day one — you simply cannot manage what you can't measure.

What's your typical pitch-up pattern — weekends only, extended trips, or genuinely living in it? That changes everything about the spec.

@LiFePO4Nerd is spot on — load audit first, everything else second.

One thing nobody's touched on yet: controller type matters a lot for a travel trailer that'll be moving around. MPPT is the obvious choice over PWM, but make sure whatever you go for handles partial shading decently. Panels on a trailer roof are rarely in ideal conditions — trees, awnings, neighbouring caravans all take their toll.

Been running a Victron SmartSolar 100/30 on my boat for two years now and the Bluetooth monitoring alone is worth the premium over cheaper units. You can actually see what your array is doing in real time rather than guessing.

Also worth confirming: is this a 12V or 24V system? Makes a significant difference to your cable sizing and controller options before you commit to anything.