Anyone else finding 12v vs 24v genuinely confusing when sizing a system from scratch?

Building out the electrical on my narrowboat and I keep going back and forth. Got a 200Ah Fogstar Drift LiFePO4 sitting in the bilge (12v, obvs) but now I'm reading that 24v would've been smarter for running an inverter properly. The boat's got a 2000w Victron MultiPlus and the cable runs are long — we're talking 4-5 metres to the bow thruster alone.

Voltage drop is doing my head in. Every calculator I use gives me different answers and I've already bought 70mm² cable thinking that'd be fine for 12v. Now I'm not sure.

Is there actually a clean rule of thumb for when 24v becomes the obvious choice? Or is it more like "it depends" forever and I just need to commit to something before I spend any more money on the wrong stuff?

DebbieEvans | 847 posts

@BayLisa honestly the 12v vs 24v question paralysed me for months when I did our widebeam! Here's the practical bit nobody mentions early enough: check what your biggest DC loads actually are. On a narrowboat it's usually the inverter and the bow thruster if you have one. If you're running anything over about 1500W inverter capacity, 24v halves your cable losses and lets you use significantly thinner wire runs, which matters when you're routing through a boat.

That said, you've got the 12v battery now, so don't beat yourself up. Plenty of liveaboards run perfectly happy 12v systems. Just keep your cable runs short and use decent gauge wire. What's your solar panel wattage looking like, and how long are the runs from your battery to your main consumer unit?

RayTaylor | 1,243 posts

@BayLisa Don't panic — a 200Ah 12v Fogstar is a perfectly decent foundation, especially for a narrowboat where space and weight distribution matter. The 24v argument mainly holds water when you're running higher loads (inverter above 2kW, serious bow thruster, that sort of thing) or you've got long cable runs where voltage drop becomes a headache. For typical narrowboat usage — fridge, lighting, water pump, charging devices — 12v is absolutely workable. You can always add a second 200Ah battery in parallel later if capacity becomes the issue rather than voltage. What loads are you actually planning to run? That's really the question that should drive the decision, not voltage as an abstract concept.

@BayLisa that 200Ah Fogstar isn't going anywhere in a hurry, so you might as well call it a 12v system and crack on — analysis paralysis sinks more boats than damp bilges do 🚢

ValleyTony | 312 posts

One thing worth adding that nobody's mentioned — your cable runs matter a lot here. On a narrowboat you've likely got decent-length runs to a bow thruster or inverter, and at 12v those cable sizes get chunky fast to keep losses acceptable. That's where 24v genuinely earns its keep.

That said, @ShesBeRight is right that a 200Ah Fogstar isn't going anywhere. If your loads are mostly 12v native stuff — lighting, pumps, USB — you're fine. Where 24v really shines is if you're running a sizeable inverter (1kW+). You'd be converting down anyway and the thinner cabling makes life easier.

My shepherd's hut is 12v throughout and works perfectly. But if I were starting fresh on something with longer runs, I'd probably go 24v just for the cable savings alone.

Wardy26 | 847 posts

Good point from @ValleyTony on the cable runs — that's the real sting with 12v on a boat. To add something different: think about your inverter load specifically. If you're planning to run anything substantial — a microwave, induction hob, even a decent hair dryer — 12v means massive current draw through those cables. A 1000w load at 12v is pulling ~85 amps, whereas at 24v you'd halve that. For a narrowboat with relatively short runs it's manageable, but factor in proper busbar sizing and decent fusing. That said, @RayTaylor's right that your Fogstar isn't a bad starting point — plenty of liveaboards run perfectly happy 12v systems. Just be honest with yourself about what you're actually powering day-to-day before deciding whether to expand or convert.