How to Calculate Your 12V Power Budget: The Essential Formula for Australian Touring

There is a moment every off-grid tourer knows well. It's 9 pm, the campfire is glowing, and the Engel kicks into a high-cycle run. You glance at the battery monitor and watch the voltage tick down faster than you expected. The fridge is fine — for now — but the laptop is charging, the camp lights are on, and tomorrow you're parked up for a full day at the beach. Will you make it to the next mains-power town, or will you be running the engine at lunchtime to nurse the batteries back up?

The answer comes down to one thing: knowing your 12V power budget. This is the single most useful calculation you can do for your touring setup. It tells you exactly how many amp-hours you consume per day, how much battery you need, and whether your solar or DC-DC charging can keep up. It turns guesswork into a number on a page.

TL;DR: The Quick Answer

To work out your daily power budget, list every 12V device, multiply its wattage by the hours you use it per day, add it all up, and divide by your system voltage (12V). This gives you the total amp-hours (Ah) you draw each day. Then multiply by 1.5 to build in a safety margin, and you have the minimum usable battery capacity your setup needs.

For a typical Aussie touring rig — fridge, lights, phone, laptop, Starlink Mini — you're looking at roughly 40–70Ah per day in winter, and more in summer when the fridge works harder. If that number is news to you, keep reading.

Step 1: Know Your Units (Watts, Volts and Amps)

Before you can budget, you need to speak the language. There are three numbers that matter:

• Volts (V): The pressure pushing the electricity. Your vehicle runs at roughly 12V (actually 12.6–14.4V depending on engine state).

• Amps (A): The rate of flow. The number of electrons moving through the wire.

• Watts (W): The total power. This is what devices are rated in, and it's the number printed on every appliance label.

The relationship is simple: Watts ÷ Volts = Amps. A 60W fridge draws 5A at 12V. A 100W laptop charger draws 8.3A. That single conversion is the whole trick — once you know each device's amp draw, you can add them all up.

Step 2: Build Your Daily Load List

Grab a notepad (or the notes app on your phone) and write down every device that runs off your 12V system. For each one, note two things: how many watts it draws, and how many hours per day you actually run it. Be honest with the hours — nobody runs the camp lights for 30 minutes, it's more like four hours.

Here's a realistic winter touring example:

That's your daily budget: roughly 68Ah. Notice the fridge is the elephant in the room — it accounts for more than half the total. Getting the fridge right is step one of any power setup.

Step 3: Size Your Battery Bank

Now that you know your daily draw, the next question is: how many days of autonomy do you want? If you charge every night from the vehicle or every sunny day from solar, one day of capacity is fine. If you want two or three days without any charging — say, a wet week in the high country or a long free-camp in the Pilbara — multiply accordingly.

The formula: Daily Ah × Days of autonomy × 1.5 safety margin = minimum battery capacity.

Using our example, for two days of autonomy: 68.5Ah × 2 × 1.5 = 205Ah. That's why the industry standard for a serious touring rig is a 200Ah lithium (LiFePO4) battery — it's not a coincidence. For a simple weekend setup with solar on the roof, a 100Ah battery covers one day comfortably.

There's also the chemistry question. AGM batteries can only safely discharge to about 50% of their rated capacity, which is why the 1.5 margin exists. Lithium batteries discharge to 90–100%, so a 100Ah LiFePO4 effectively gives you as much usable capacity as a 200Ah AGM. If you're sizing a new system, factor that in before you compare prices.

Step 4: Match Your Charging to the Budget

A power budget is only half the picture — you also need to know how much energy you can put back in. Three charging sources matter:

• DC-DC charging from the alternator: Typically 20–40A. An hour of driving puts roughly 30–40Ah back in. This is your most reliable source, but it only works while driving.

• Solar: A 200W panel in Australian winter sun averages around 6–10Ah per hour of good light, so roughly 30–50Ah per day in clear conditions. Cloudy winter days can cut that by half.

• Portable power stations: Units like the PeakDo LinkPower 2 (99Wh, roughly 8Ah at 12V) are great for topping up phones, laptops and a small device, but they are not a substitute for a house battery on a long trip.

The key number to compare is your daily budget versus your daily charging. If you draw 68Ah per day and your solar only puts back 40Ah, you're running a deficit of 28Ah daily — and you'll need to plan driving days or mains hook-ups to balance it. That's the real point of the exercise: knowing the deficit before it becomes a flat battery.

Step 5: Use the Monitor to Verify (Not Just Guess)

Once your budget is on paper, verify it against reality using a battery monitor with a smart shunt. These units measure the actual current flowing in and out of your battery, so you can see exactly what your devices draw rather than trusting labels and estimates. Check the monitor at the same time each evening for a week and compare the real numbers to your calculation. Most setups find their fridge draws a little more or less than the label suggests — that's your data, and it's gold for fine-tuning.

Real-World Tips for Touring

• Pre-cool the fridge at home on 240V before you leave. A fridge that starts cold draws far less than one that has to chill a warm cabinet from scratch.

• Winter is a power trap. Days are shorter, so solar yields drop, and the fridge still runs the same. If you're planning winter touring, add an extra day of autonomy to your budget.

• Keep the laptop on 12V USB-C PD, not through an inverter. Converting 12V to 240V and back to USB-C wastes up to 20% of your energy as heat. A direct 12V USB-C PD charger is dramatically more efficient.

• Charge devices during driving hours. If you're moving camp each day, run the laptop and phone charging off the vehicle's DC-DC system rather than the house battery.

• Know your fridge's real draw. Cheaper fridges with poor insulation cycle far more often. If your budget is tight, the fridge upgrade is where the biggest single improvement lives.

Building Your Power System Right

Every amp you save is an amp you don't have to generate, store or carry. That's why efficient charging and accurate monitoring matter as much as the battery itself. To keep your touring rig running clean, check out our 12V Accessories collection for battery monitors, DC-DC chargers and USB-C PD charging solutions that turn this maths into a working system. For a compact backup that keeps phones and laptops alive at camp, the PeakDo LinkPower 2 is a genuinely useful addition to any drawer system.

Close

The difference between a stress-free off-grid trip and a dead-battery emergency is usually just a few lines of arithmetic done before you leave. Calculate your budget once, verify it against a monitor, and you'll never have to guess again. What does your setup draw per day? Jump into the comments and share your numbers — the collective knowledge of the touring community is how we all get smarter.