12V fridges and water: amp-hours, litres, and what they weigh
These two get bought at the same time and for the same trip, and they fail in opposite ways: the fridge runs your battery flat, and the water runs your payload out. Both are arithmetic problems with published numbers, so both are solvable before you spend anything.
A fridge's sticker draw is not its real draw
A 12V compressor fridge is quoted with a current draw — say 3.5 amps. That's what it pulls while the compressor is running, which is not all the time. What matters is the duty cycle: the fraction of each hour it spends running.
Duty cycle depends on ambient temperature, how cold you've set it, how full it is, whether it's in shade, and how often the lid gets opened. On a mild day in the shade a well-packed fridge might run 25% of the time. In a hot vehicle in central Australia in summer it can approach 60%.
So the honest calculation is amps × duty cycle × 24 = amp-hours per day.
At 3.5 A and 30% duty, that's about 25 Ah a day. At 55% it's over 46 Ah — nearly double, for the same fridge.
Two consequences:
- Size the battery for the bad day, not the brochure day. A 100 Ah lead-acid battery gives you roughly 50 Ah of usable capacity before you start damaging it; lithium gives you most of its rating. Read that against the number above and you'll see how quickly a hot week gets tight.
- Insulation and placement beat capacity. A fridge in direct sun through a rear window, or packed warm, will cost you more amp-hours than any battery upgrade buys back. A cover and shade are the cheapest efficiency you can fit.
The dual battery side of this — DC-DC charging, wiring size, why an alternator alone won't fill a battery properly — is its own subject: dual battery and 12V systems.
Thermoelectric ("Peltier") coolers are a different product. They draw continuously, cool only relative to ambient, and are not suitable for keeping food safe on a hot day. If a listing quotes no compressor and a very low price, that's what it is.
Water weighs a kilogram a litre, and it does not get lighter
This is the single most useful fact in touring storage. One litre of water is one kilogram. A 40 L tank is 40 kg. Two jerries in the back are 40 kg. An 80 L under-body tank is 80 kg, plus the tank and the mounts.
Put that against your payload rather than assuming it disappears. On a dual cab already carrying a steel bar, a canopy, drawers and a fridge, water is very often the thing that pushes it over. Payload calculator.
Where the water sits matters as much as how much of it there is:
- Under-body tanks put the weight low and between the axles, which is the best place for it. They're also exposed to rock and to corrugation-driven fatigue at the mounts.
- In-canopy or in-tub tanks sit higher and further back, which loads the rear axle. Check your rear axle rating, not just GVM — it's possible to be under GVM and over the rear axle limit at the same time.
- Jerry cans on a rear bar or roof are the worst case for handling, and on the roof they count against roof load. A 20 L jerry on the roof is 20 kg plus the holder, on a rating that's usually somewhere between 75 and 100 kg dynamic. Roof load calculator.
Potable, and staying that way
If you're drinking it, the tank, the fittings and the hose all need to be food-grade. Listings usually say so where it's true. Some points worth knowing:
- Food-grade tank plus garden hose to fill it is not a food-grade system.
- Tanks fitted for washing and recovery water don't need to meet the same standard, and plenty are sold for exactly that.
- A tap and a pump are separate purchases from the tank on most under-body kits. Check what's in the box.
Fitment: tanks are chassis-specific, fridges are not
A fridge fits wherever it physically fits, and the fitment question is really about the slide, not the fridge — slides and drop-down mounts are usually sold for a drawer system or a canopy rather than for a vehicle.
Water tanks are the opposite, and they're strongly series-specific:
- An under-body tank is shaped around a particular chassis, exhaust route and spare wheel position. A tank for one wheelbase will not suit another.
- Cab configuration changes the available space — a dual cab and a single cab of the same model have very different room under the tray.
- Some tanks require relocating the spare, which is its own purchase and its own compliance question.
What to check before you buy
- The fridge's draw and your realistic duty cycle, turned into amp-hours per day, against your usable battery capacity.
- Whether it's a compressor fridge, stated plainly.
- The filled weight of any tank, in your payload sum, before you buy it.
- Which axle the water loads, and that axle's rating.
- Food-grade, if you're drinking it — tank, fittings and filler.
- For a tank: your exact chassis and cab configuration, and what happens to the spare wheel.
Where FITSPEC fits
Fridge listings are indexed as products, and their fitment is usually about the slide rather than the vehicle — the index says so rather than inventing a vehicle match. Tank listings carry whatever series the retailer published, attributed to them, because an under-body tank is one of the few accessories where getting the wheelbase wrong means it simply will not go on.
This guide is general information, not fitting or engineering advice. Fitment, compliance and safety depend on your specific vehicle — confirm with the retailer and a qualified fitter before you buy.