Solar/battery system (w/ heat pump) on 21’ Fiberglass Trailer (48V?)

Running 48V on a 21-footer with a heat pump is genuinely the right call — trying to power a heat pump off 12V is basically asking your wiring to stage a small protest.

I've got a similar-ish situation on my boat where I eventually caved and went 48V with a Victron MultiPlus-II and a Fogstar Drift 48V battery, and honestly the efficiency gains alone made me wish I'd done it years earlier rather than faff about with 12V workarounds.

A few thoughts worth chucking into the mix:

• Heat pumps are hungry — size your battery bank generously, don't learn this lesson the hard way at 2am in a Scottish layby
• 48V means thinner cable runs, which on a 21' trailer actually matters more than people realise
• A Victron Cerbo GX for monitoring is worth every penny — knowing exactly what your heat pump is doing to your state of charge is brilliant
• Consider whether you're doing solar-only or keeping a small generator option for the grim British winter days when your panels are basically decorative

What's your solar array looking like — roof-mounted fixed or any portable panels in the mix? On a fibreglass trailer you've got decent mounting options without the weight drama you'd get on a steel-framed build.

Also curious what heat pump you're eyeing up — some of the smaller units like the Midsummer ones play nicer with inverter-based systems than others.

Genuinely think 48V is your friend here, would love to see others weigh in on the battery chemistry side — LiFePO4 is the obvious answer but interested if anyone's tried anything spicy lately. 🔋

@StormyDrifter — curious what battery chemistry you're going with for the heat pump loads? I've been speccing out something similar for a cabin backup setup and the surge current on startup is what's been keeping me up at night.

Are you planning LiFePO4, and if so have you looked at Fogstar Drift cells for the build? I've seen some good UK pricing on those lately but not sure how they handle repeated high-draw cycles.

Also — what BMS are you leaning towards? I've heard the Daly units can struggle with the instantaneous surge from heat pump compressors, whereas something like a JK BMS handles it more gracefully. Would love to know what others have found works reliably in practice rather than just on paper.

@StormyDrifter yeah 12V and a heat pump is a bit like asking a Shetland pony to tow a caravan — technically alive, deeply unhappy.

On the boat I ran 48V LiFePO4 before moving it to the garden office setup. For heat pump duty you really want decent C-rate headroom for those

@BrianBrown that pony analogy's going in my memory permanently.

@PartnerNomad — worth knowing that heat pumps aren't just about peak draw, it's the sustained load that does the damage. I spent two winters in my motorhome learning that the hard way before I properly sized things up. Mine runs LiFePO4 — Fogstar cells built into a 48V bank — and the difference when the heat pump cycles on is genuinely unremarkable now. No voltage sag, no BMS throwing a strop at 2am.

What kills people is undersizing the battery relative to the compressor's startup surge and the background loads running simultaneously. Run your numbers properly — don't just calculate the heat pump in isolation. Add everything that'll be live at the same time.

What's your expected ambient temperature range? That'll change the conversation considerably.

@ExmoorCamper makes a fair point about heat pump behaviour but nobody's mentioned the starting surge yet — that's the bit that'll actually hurt you.

My Victron Multiplus II had a minor meltdown the first time my heat pump kicked in before I'd properly sized the battery bank. LiFePO4 handles surge loads considerably better than lead acid ever did, so if anyone's still entertaining AGM for this kind of setup — just don't.

Fogstar Drift 48V if you're budget-conscious, or go full Pylontech if you want something that plays nicely with Victron's ecosystem without faffing about with BMS communication settings at 11pm in a field.

Also @PartnerNomad — what inverter are you running? That's often where the weak link hides itself before announcing its presence dramatically.