Feedback wanted: free tool that sizes battery/solar/inverter/wiring for a 12V camper build

Been playing around with this over the weekend whilst finalising the wiring spec for my van build and I have to say it's genuinely useful — particularly the wiring sizing section, which is where most people get themselves into trouble (seen some frightening 2.5mm² runs on 30A circuits in the wild...).

A few observations from someone who's done this the hard way:

• The battery sizing logic seems solid for leisure use. It correctly flagged that my 200Ah LiFePO4 (Fogstar Drift, for reference) was marginal for a two-day off-grid stint with my 2000W inverter running a coffee machine and small induction hob
• The solar suggestions felt realistic rather than optimistic — a lot of calculators seem to assume you're parked in Malaga year-round rather than a layby near Carlisle in February
• Inverter recommendations aligned with what Victron publish in their own sizing guides, which gave me confidence

What I'd love to see added is a cable voltage drop calculator that accounts for ambient temperature derating — that's the bit that catches people out in engine bays and roof conduit runs.

Would also be brilliant if it factored in shore power charging scenarios for those of us using this for emergency home backup as well as the van.

Has anyone else run their existing completed build through it to see how close it gets? Curious whether it matches what you've actually ended up with in practice — that's the real test. Drop your results below, would make for a cracking comparison thread.

Sounds interesting but I'd want to know what assumptions it's making under the hood before I trusted it for anything critical. Does it account for voltage drop across longer cable runs, or is it just plugging in a simple Ohm's law calc?

Also curious whether it handles 12V vs 48V systems differently — I'm looking at potentially bumping my boat setup up to 48V with Victron kit and the wiring gauge recommendations change dramatically.

@MarinePhil what inverter sizes is it covering? Some of these tools cap out at 2-3kW and aren't much use if you're running anything serious.

Link?

Ran my Fenland setup through it last night — 200Ah Fogstar Drift LiFePO4, 400W Renogy panels, Victron 75/15 MPPT. The output was pretty close to what I'd calculated manually, which gave me some confidence.

@ExBrickie fair point on the assumptions. Would be good to know if it's using worst-case UK solar irradiance figures or something more optimistic — makes a massive difference whether you're sizing for January in the Fens or August in Cornwall. My real-world winter yield is about 40% of what the panel specs suggest.

One thing I noticed: the inverter sizing didn't seem to account for surge current on startup loads. Worth double-checking that manually if you're running anything motorised.

@ExBrickie raises the point I'd want answered first too — specifically around the cable sizing methodology. Is it using a voltage drop percentage threshold (the standard 3% at 12V is already quite tight for longer runs) or is it working from fusing/breaker ratings? Those two approaches can give you meaningfully different conductor sizes, especially on inverter feeds where you've got high transient current draw.

@FenlandVanLifer — curious whether it flagged your 75/15 as undersized for 400W? At 12V nominal that's theoretically ~33A, and the 15A MPPT will clip in decent sun. That's the kind of thing a decent sizing tool should catch automatically rather than leaving it to the user to spot.

For the shepherd's hut and van builds I've done, the wiring section is genuinely where most DIY errors originate. Will give this a proper run-through this week.

@LesWood78 ran my whole garden office setup through it on a whim and it told me my existing cable run was basically a fire hazard, which explains so much about that burning smell last August 🔥