How Big Does Your Dual Battery System Need To Be?

One of the most common mistakes people make when planning a dual battery system is starting with the battery. They pick a size, buy it, and then figure out what they can run off it. It should work the other way around. What you want to power is what determines the size of the system you need.

This is literally the first question we ask at BAW when someone comes in wanting a 12V setup. Once we know what you're running, we can actually spec the system properly.

Start With What You Want To Run Off Your Dual Battery System

The first question is simple: what do you actually want to power from your auxiliary battery? Are you keeping it basic — a fridge, some camp lights, charge your phone? Or do you want to run an inverter so you can power tools or appliances? Are you running one fridge or two? Is it a 40-litre fridge or a 130-litre chest fridge that runs flat out?

A fridge on a drawer slide and a full inverter setup in the canopy.

Every appliance draws a certain amount of current over time. The more you want to run, the bigger your battery needs to be. There's no shortcut around that.

A Real-World Example: 100Ah For A Weekend Away

To put it in real numbers: the BAW RAM 1500 runs a GoBlock — essentially a 100Ah lithium battery in a purpose-built box with a charger built in. That setup comfortably runs a fridge and camp lights for a full weekend without any charge input. No solar, no driving. Just parked up, fridge on, lights going. For most weekend tourers, that's genuinely enough.

It's a compact, tidy solution that doesn't require a massive install, and for the person who just wants to keep the food cold and the lights on at camp, it does the job.

When You Need More: Going Big

On the other end of the scale, the BAW caravan runs up to 900Ah of lithium. That system handles air conditioning, induction cooktops, hair dryers — full household-style power consumption while off-grid. That's a very different setup to a 100Ah weekend rig, and it costs and weighs accordingly.

A 3000W inverter pulls some serious power.

Most touring setups sit somewhere between those two extremes. A serious 4WD tourer running a fridge, a freezer, a CPAP machine, and charging multiple devices might be looking at 200 to 300Ah. Someone setting up a serious off-grid caravan will be looking at significantly more.

Don't Forget To Factor In Solar

Battery size and solar go hand in hand. The bigger your system, the more important solar becomes for keeping it topped up. If you're drawing significant power every day and relying purely on driving to recharge via the alternator, you're going to run into trouble on rest days or extended camp stays.

Solar input is only one side of the equation, and how much you can store and draw is just as important as how fast you can charge it.

Solar doesn't replace a well-sized battery — it extends how long that battery lasts and keeps it healthier over its lifespan. The two work together. When you're sizing your system, think about both at the same time.

Get The Sizing Right Before You Buy

The worst outcome is spending good money on a system that's either too small to do what you need or way bigger than necessary. Getting the sizing right upfront saves you from pulling it all out and starting again twelve months down the track.

If you're in Brisbane and want someone to help you work it out properly, book a free quote with BAW Automotive. We'll go through exactly what you want to run, how you use your vehicle, and spec the right system for your needs the first time.