Sizing your 12V fridge to your battery: the sums most tourers skip

Our second van came with a 100Ah lithium and a 65L compressor fridge, and the bloke at the dealership told us it'd "run forever." It didn't. Three cloudy days in a row in the Grampians and we were down to 20% by dinner on day three, with the kids' school laptops charging off the same bank. Nobody had done the sums. I hadn't either, if I'm honest, because I trusted a sales pitch instead of a calculator.

The maths behind fridge sizing and battery capacity isn't hard. It's just skipped, constantly, because everyone wants a number someone else worked out for them. There isn't one universal number. There's your fridge, your climate, your battery chemistry, and how many other things are drawing off the same bank. Get those four right and you stop guessing.

Start with what your fridge actually draws

Every compressor fridge has a rated draw, usually somewhere between 0.5A and 3.5A depending on size and ambient temperature, but that number on the spec sheet is the draw when the compressor is running, not a continuous figure. Compressor fridges cycle. A 60-80L 12V fridge in a 25°C camp kitchen might run at maybe 30-40% duty cycle, meaning it's actually drawing current less than half the time. In 40°C Cape York heat with the fridge in direct sun, that duty cycle climbs towards 60-70%, sometimes higher if the door's getting opened by kids grabbing icy poles every ten minutes.

The number you actually want is daily amp-hour consumption, not the instantaneous draw. Most manufacturers publish an average daily figure for a stated ambient temperature — check the spec sheet, not the box. A typical 60L compressor fridge might sit somewhere around 20-35Ah per day in mild conditions, and climb from there in genuine heat. If your fridge doesn't list this, work it backwards from the rated amp draw multiplied by an assumed duty cycle, and be conservative. Assume worse conditions than you think you'll get, because Cairns in November will find you out.

Then add everything else sharing the bank

This is where most people's sums fall over. The fridge isn't drawing alone. Lights, the water pump, phone and laptop charging, a router if you're working remotely from the van, and — if anyone in the family needs one — a CPAP overnight, all come off the same bank unless you've split circuits.

I tally ours like this: fridge roughly 30Ah, LED lighting and USB charging maybe 8-10Ah, water pump a couple of amp-hours, and the kids' devices another 10Ah on a big screen-time day. That's close to 50Ah a day before anything unusual happens. Add a CPAP and you're looking at another 15-20Ah overnight, on top. It adds up fast, and it's genuinely easy to lowball this if you only ever think about the fridge in isolation.

Lithium vs AGM changes the sizing equation, not just the price

We've covered the full cost breakdown elsewhere on the site — see lithium vs AGM: the real cost per usable amp-hour — but for fridge sizing specifically, the number that matters is usable capacity, not rated capacity. A 100Ah AGM is realistically good for about 50Ah before you're pushing into voltage territory that shortens its life and can trip fridge low-voltage cutoffs. A 100Ah lithium will comfortably give you 80-90Ah usable.

That difference alone can be the gap between "fine for three cloudy days" and "flat by day two." If you're running AGM and doing the sums off the full rated capacity, you're kidding yourself. Halve it, roughly, and work from there.

Match the battery to your worst realistic day, not your average one

Average conditions don't strand you. Three overcast days back to back at a free camp with no genset does. My rule of thumb, after plenty of trial and error: work out your daily draw including the fridge and everything else, then size the battery to cover two to three days of that draw with zero solar input, before you even think about what your panels will recover.

So if our tally is 50Ah a day, I want at minimum 100-150Ah of usable capacity sitting in reserve for a genuinely grey run — which for lithium means roughly 120-170Ah of rated battery, and for AGM closer to double that in rated terms. That's before solar does any work at all. It sounds conservative until you're the family in the Grampians in April with a flat bank and a fridge full of the kids' yoghurt turning warm.

Solar recovers the bank, it doesn't replace the sizing

A 200W panel on the roof will, in reasonable sun, put back somewhere around 60-80Ah across a full charging day, less in winter, less again under tree cover or cloud. That's a genuine help and it's why most family vans run comfortably on lithium plus 200-300W of fixed panel. But solar is a top-up to a properly sized bank, not a substitute for one. If your battery can't carry you through a no-sun stretch, more panel doesn't fix that; it just means good days recover faster while bad days still strand you.

This is also where your charging path matters as much as your battery size. If you're relying on driving to charge the bank, an old alternator-only setup won't cut it with a modern smart alternator — worth reading DC-DC chargers explained if you haven't sorted that side of the system yet, because a fridge sized correctly to a battery that never actually charges properly is still going to leave you flat.

Watch the compressor after corrugations, not just the battery

One thing the sums don't capture: a fridge that's copped a hammering on washboard roads draws more than its spec sheet suggests, because a compressor working harder against wear or a loose mount cycles longer and less efficiently. We've written separately about why corrugations kill 12V fridges early, but it's relevant here too — a fridge that's been rattled on the Birdsville or the Gibb will often start drawing noticeably more than it did new, which throws your sizing out even if nothing else about your setup changed. If your battery seems to be draining faster than it used to and you've recently done a rough stretch of road, check the fridge mounts before you blame the battery.

What I'd actually tell a mate buying today

If you're speccing a new setup from scratch: work out your fridge's rated daily Ah at a realistic ambient temperature, add your lighting, charging and pump loads honestly, size lithium to cover two to three days of that total with no solar, then add 200W-plus of panel to keep the bank topped up on the road. Don't buy the fridge and battery separately and hope they get along. I'd argue most of the "our battery doesn't last" complaints I hear from other families at caravan parks aren't a battery problem at all — they're a sizing problem that was never actually calculated, just assumed.

We run a 65L fridge off 200Ah of lithium now, with 300W on the roof, and it's the first setup where I've stopped mentally rationing power by day three of a free camp. That peace of mind is worth more to me than a jumping pillow, and that's saying something with two kids in tow.

— Kellie Nguyen, Family Touring & Parks Editor