inverter-battery-cable-size-no-bullshit-guide-for-aussie-off-grid
Dave Miller, OffGrid Masterplan author

By Dave Miller · off-grid sparky · 22 years wiring solar, batteries and sheds in central QLD

“I melted a 25mm cable on a 1200-watt inverter in my 40°C shed after assuming the 35mm run was enough for the 4-metre distance.”

OffGrid Masterplan

— Dave. Measure twice, buy once.

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Inverter Battery Cable Size: Get It Right or Get Screwed

Look, if you're running an off-grid system in Australia, battery cables aren't a "nice-to-have" — they're the lifeline. Get them wrong and your inverter fries, your batteries die, or you get a fire. No exceptions. Here's the raw deal.

Look, if you're running an off-grid system in Australia, battery cables aren't a "nice-to-have" — they're the lifeline. Get them wrong and your inverter fries, your batteries die, or you get a fire. No exceptions. Here's the raw deal.

Battery to Inverter Cable SizingBattery48 V bankInverter5 kWPositiveNegativeUnder 1 m: 70 mm² · 1–2 m: 95 mm²Keep runs short, fuse within 200 mm of battery
Cable sizing guide between battery bank and inverter based on run length.

Short Runs: Don't Waste Money on Beefy Cables

For runs under 1.5 metres (like battery to inverter in a shed), you don't need 50mm² cable. It's overkill and costs more. Here's the sweet spot:

Using 50mm² for a 2m run? You're just paying for weight. BatteryCables.com.au sells 25mm² at $8.50/m — that's $17 for a 2m run. 50mm² is $18/m. Save the cash.

Voltage Drop: The Silent Killer

Forget "it's only 10cm" — voltage drop kills efficiency. For every 1% drop, you lose 1% of power. At 3%? Your system runs like it's on a diet.

Rule of thumb: Keep voltage drop under 2% for battery-to-inverter runs. Use this formula:

Voltage Drop = (2 × Current × Cable Length × 0.017) / Cable Area

Real example: 100A at 12V, 2m run, 25mm² cable:
Voltage Drop = (2 × 100 × 2 × 0.017) / 25 = 0.27% (safe).
Same setup with 16mm²? 0.43% (still okay, but push it).
10mm²? 0.65% (too high — skip it).

Don't calculate. Just use 25mm² for 12V/1000W, 35mm² for 24V/2000W. Done.

Terminals: Don't Be a Dumb Aussie

Using bare wire? Or cheap crimp lugs? You're asking for a fire. Terminals must match the cable and battery terminals. No exceptions.

Bad terminals = high resistance = heat = fire. We've seen it. It's not a "maybe" — it happens.

Where to Buy in Australia (No Fluff)

BatteryCables.com.au (Sydney): Best for cable and lugs. 25mm² cable: $8.50/m. 25mm² lugs: $3.50/pair. Fast shipping. $17 for 2m cable + lugs.

Powerhouse (Nationwide): Cable starts at $12/m (thinner than needed). Only buy if you're desperate. Avoid for off-grid.

Supercheap Auto: Cable is $15/m, but it's 16mm² (too thin for 100A). Don't waste your money.

⚠️ SAFETY WARNING: Working with electrical systems, structural modifications, or gas installations carries inherent risks. If you are not confident in your abilities, always engage a licensed professional.

Worth a watch: Sizing Fuses and Breakers 101 - Fuses, Breakers, & Overcurrent Protection | Ep: 5/6 · EXPLORIST life Mobile Marine & Off-Grid Electrical

It shows how to size fuses and breakers that match the cable ampacity you just calculated, so you don’t end up with a fire starter instead of a working system. — Dave Miller

When to Call a Professional

While many off-grid projects are achievable as DIY, certain situations require licensed professionals:

Always check local regulations and obtain necessary permits before commencing work.