12v fridge power consumption calculator

Most people buying their first 12V fridge focus entirely on the litres. Fair enough — that’s the number on the price tag. But the number that actually decides whether your weekend away goes smoothly is the amp-hour draw, and almost nobody checks it until the battery’s already flat at 2am with a esky full of warming sausages.

This calculator gives you a realistic daily power draw for your fridge, in Ah and Wh, and tells you how long a given battery will keep it running. It’s the same maths a 4WD auto-electrician would scribble on the back of an invoice, just faster.

If you’ve never sized a battery system before, the numbers below will feel abstract for about thirty seconds — then they’ll click, and you’ll wonder why every fridge box doesn’t just print this on the side.

The Formula Your Solar Installer Assumes You Already Know

A 12V fridge doesn’t run constantly. The compressor cycles on and off to hold temperature, and how often it cycles is called the duty cycle. A fridge with a 45W compressor running at a 40% duty cycle isn’t drawing 45W all day — it’s drawing 45W for 40% of the time and roughly nothing for the rest.

The formula:

• Average watts = compressor wattage × duty cycle
• Watt-hours per day = average watts × 24
• Amp-hours per day = watt-hours ÷ system voltage

Worked example: a 50W compressor running at a 35% duty cycle on a 12V system. Average watts is 50 × 0.35 = 17.5W. Over 24 hours that’s 17.5 × 24 = 420Wh. Divide by 12V and you get 35Ah per day. That’s the number that matters when you’re sizing a battery or comparing solar panel output against fridge draw.

Where the Duty Cycle Number Actually Comes From

Manufacturers rarely print duty cycle on the box, and that’s the bit most calculators get wrong by assuming a fixed number. In reality it shifts with ambient temperature, how full the fridge is, how often the lid opens, and whether it’s in direct sun. A chest-style fridge in a shaded camp on a 20°C day might sit around 25–30%. The same fridge in the back of a ute tray on a 38°C day, opened every twenty minutes for drinks, can push past 55%. If you don’t know your actual duty cycle, 35–40% is a sensible starting assumption for most 40–60L compressor fridges in Australian conditions.

A Weekend at Wilpena Pound, Worked Through Properly

Say you’re heading to the Flinders Ranges for four nights, no shore power, running a 50L compressor fridge with a 50W compressor. Daytime temps are sitting around 30°C, so you assume a 40% duty cycle. That gives you 50 × 0.4 × 24 ÷ 12 = 40Ah per day. Over four nights, that’s 160Ah total — before you’ve even switched on a light or charged a phone.

If you’re running a 100Ah lithium battery with 90% usable depth of discharge, that’s 90Ah usable — only just over two days’ worth on fridge alone, with zero solar input. This is exactly why most touring setups pair a dual battery system with at least one solar panel; without it, the fridge alone will flatten a single 100Ah battery well before the trip’s over.

Three Inputs That Quietly Wreck the Whole Calculation

Get any one of these wrong and the rest of the maths is just confidently incorrect:

• Voltage drop on long cable runs. If your fridge is wired with thin or lengthy cable back to the battery, voltage drop forces the compressor to work harder and draw more current than its rated wattage suggests.
• Ambient temperature on the day. Duty cycle isn’t fixed — it climbs fast once the cabin or tray temperature passes 30°C.
• Battery age and chemistry. An old AGM battery rarely delivers its full rated capacity. Lithium holds capacity far better across hundreds of cycles, which is part of why it’s become the default for touring setups.

For general guidance on 12V electrical setups and towing regulations relevant to camper trailers and caravans, the Australian Bureau of Statistics publishes data on vehicle and caravan ownership trends, while Wikipedia’s overview of compressor and thermoelectric cooling is a reasonable starting point if you want to understand why compressor fridges outperform thermoelectric ones on power efficiency.

Getting More Confident Numbers Out of It

If you want to skip estimating duty cycle altogether, a cheap inline battery monitor (a shunt-based one, not just a voltmeter) will show you actual Ah drawn over 24 hours. Run it for a full day at home in the fridge or freezer before your trip, and you’ll have a real number instead of an assumption — then plug that daily Ah figure straight into your battery sizing instead of guessing duty cycle.

What Changes If You’re Running a Freezer Setting Instead

Dropping the thermostat to freezer temperatures (below 0°C) increases duty cycle, sometimes by ten to fifteen percentage points, because the compressor has to work harder to hold a lower set point against the same ambient heat. If you’re running a dual-zone fridge/freezer, run this calculator separately for each compartment if it has independent compressors, or add 10–15% to your duty cycle estimate if it’s a single compressor working harder to chill one zone colder.

When These Numbers Actually Change What You Buy

It’s easy to run a calculator like this once and forget about it. But the numbers genuinely should change a buying decision in a few situations. If your daily Ah draw is creeping above what a single 100Ah battery and one solar panel can sustain through a few overcast days, that’s the point to consider a second battery or a larger panel — not after you’ve already been caught out flat in the middle of nowhere. Worth checking the latest guidelines from energy.gov.au on solar panel sizing if you’re building this into a permanent off-grid setup rather than just weekend touring.

Questions Real Users Ask After Seeing the Output

How much power does a 12V fridge actually use a day?

Most portable 12V compressor fridges between 40L and 60L draw somewhere between 25Ah and 55Ah per day, depending on ambient temperature, insulation quality, and how often the lid is opened. A well-insulated fridge in mild weather sits at the lower end; a poorly insulated one in summer heat sits well above it.

How long will a 100Ah battery run a 12V fridge?

With a typical 35–40Ah daily draw and a lithium battery offering around 90Ah usable capacity, you’re looking at roughly two to two and a half days before the battery is fully discharged. An AGM battery with the same 100Ah rating but only 50% safe depth of discharge will give you closer to a single day.

How many watts of solar does it take to keep up with a 12V fridge?

As a rough rule, a 150W solar panel in good Australian sun can produce somewhere around 50–70Ah on a clear day, which comfortably covers most single-fridge setups with some buffer for cloudy days. Panel angle, shading, and the quality of your solar regulator all affect real-world output.

How many amps does a typical 12V compressor fridge draw when running?

While actually cycling, most 12V compressor fridges draw between 3 and 6 amps. Because the compressor isn’t running constantly, this peak figure is much higher than the average daily draw — which is exactly why duty cycle matters so much in the calculation.

Does a chest-style fridge use less power than an upright one?

Generally yes. Chest-style fridges lose less cold air when opened because cold air sinks and stays in the bottom of the unit, while upright fridges spill cold air out every time the door opens. For the same litre capacity, a chest fridge will typically run a lower duty cycle.

Is it worth running a 12V fridge straight off the car battery?

Only for very short periods with the engine running. Most vehicles aren’t set up to handle deep discharge from a single starter battery, and running a fridge off it while parked risks leaving you without enough charge to start the car. A proper dual battery system or dedicated auxiliary battery is the standard setup for a reason.

Why does my fridge’s actual runtime seem shorter than the calculator suggests?

Battery capacity ratings are usually based on ideal lab conditions. Real-world factors — cold weather reducing battery performance, an older battery that’s lost capacity, voltage drop in wiring, or a higher-than-assumed duty cycle — all eat into the runtime the maths predicts. Treat the calculator’s output as a best-case estimate and build in a buffer.

What’s the difference between Ah and Wh, and which one should I use?

Amp-hours (Ah) measure current over time at a specific voltage, while watt-hours (Wh) measure energy regardless of voltage. Wh is more useful when comparing devices or batteries running at different voltages — like comparing a 12V fridge against a 24V one — because it normalises the comparison.

Get the duty cycle roughly right and the rest of this maths holds up reliably, whether you’re sizing a single weekend battery or a full off-grid setup.