How to calculate power consumption for your camper van

Your power consumption is the foundation of your entire electrical system. Every other component depends on it: battery capacity, solar panel area, charge controller size, cable thickness and inverter rating. If your consumption estimate is off, everything downstream is wrong too.

Many van builders skip this step and simply copy what others have done. But your needs are unique. Someone who works remotely from their van with a laptop and monitor has very different requirements from a weekend traveller who just wants lights and a fridge.

Overestimating your consumption means you spend money on batteries and panels you do not need, and carry extra weight for nothing. Underestimating means you run out of power on day two of your trip and have to find a campsite with shore power, which defeats the purpose of building an off-grid system.

A proper consumption calculation takes about 30 minutes and can save you hundreds of euros in correctly sized components. It also gives you confidence that your system will actually work in the conditions you plan to use it in.

The process is straightforward: list every device, note its power draw, estimate daily usage hours and multiply. Let us walk through each step.

Start by listing every electrical device in your camper van. Split them into two categories: 12V DC devices that run directly from the battery, and 230V AC devices that require an inverter.

Common 12V DC loads include LED lighting, USB chargers, a compressor fridge, a water pump, a diesel heater control unit, a roof fan (like a MaxxFan or Fiamma) and a stereo system. These devices draw current directly from the battery without conversion losses.

Common 230V AC loads include a laptop charger, a phone charger (if using a standard wall charger), a coffee machine, a blender, a hair dryer, a TV or monitor and power tools. These devices run through the inverter, which adds 10-15% conversion loss on top of the device's own consumption.

For each device, find its power consumption in watts (W). Check the label on the device or its charger. If only amps (A) are listed, multiply by the voltage: watts = amps times volts. A 12V fridge drawing 4A uses 48W. A laptop charger rated at 65W uses 65W from the AC outlet, but about 75W from the battery after inverter losses.

Do not forget standby power. An inverter on standby draws 5-20W continuously, your diesel heater controller uses 1-3W even when not heating, and a fridge cycles on and off throughout the day.

Now that you have a list of every appliance with its wattage and hours of use, you can work out the daily consumption. The formula is simple:

Daily use (Wh) = Power (W) x Hours per day

A worked example using the default values from the electrical planner, so you can check it yourself:

• Compressor fridge: 18 W on average x 12 hours of running = 216 Wh
• LED lighting: 5 W x 5 hours = 25 Wh
• Roof fan: 25 W x 6 hours = 150 Wh
• Water pump: 60 W x 0.5 hours = 30 Wh
• Diesel heater: 25 W x 8 hours = 200 Wh
• Laptop charger: 65 W x 3 hours = 195 Wh, and 229 Wh through the inverter
• Tank sensor that stays on day and night: 1 W x 24 hours = 24 Wh

Total: around 875 Wh a day.

Watch that fridge. You often read 45 W, and that is right: it draws that much while the compressor runs. But it does not run continuously, and measured across the day it averages around 18 W. Work with 45 W over twelve hours and you size your whole battery bank twice too large.

To convert this to Ah (amp hours), divide by the system voltage:875 Wh / 12V = 73 Ah a day

This is your baseline on an average day. Also estimate a heavy day (bad weather, more time inside, more cooking) and a light day (mostly outdoors, little electronics). That gives you the band your system has to perform within.

Tip: be honest in your estimates. Most people underestimate their use. Better to allow a little extra than too little.

The same appliance does not use the same amount all year, and some appliances only run for half of it. Make one list and hold it all year and you will be wrong in one of the two seasons.

What rises in summer: the fridge, which at 30 degrees outside runs two to three times as much as at 15, and the ventilation.

What rises in winter: lighting, because it is dark by four, the heater with its fan and fuel pump, and everything you do inside because you are not sitting outside.

So put two columns next to your appliance list and fill in both. The summer total decides whether your cooling and solar match; the winter total decides how much you have to top up from the alternator or shore power, because solar delivers a fraction then.

The largest item differs per season, and that is exactly why both are worth calculating. A system that is generous in July can fall short every day in January.

Beyond the appliances you switch on, every camper has a low draw that never stops. That is the item most often missing on paper and most often responsible for a flat battery in practice.

What sits in there:

• The inverter in standby, often 5 to 20 watts, which over a day is 120 to 480 watt hours
• The battery monitor, the charge controller and the battery management system, together usually a few watts
• A gas or carbon monoxide alarm, permanently on as it should be
• Standby lights on radios, amplifiers and USB outlets
• A heater at rest with its own electronics

Together that is often 10 to 30 watts, or 240 to 720 watt hours a day. For a camper standing still for two weeks that is the difference between a battery that holds and a battery that is flat when you return.

The answer is a main switch that isolates everything except the alarms when you leave the van, and an inverter that is not on by default.

A calculation is an assumption until you have measured it. Only after a week of travel do you know whether it held, and it usually differs, in both directions.

The simplest way is a battery monitor with a shunt: it counts everything in and out, and at the end of the day you read one number. Do that for a week and you have your real daily consumption, including every standby item you forgot.

A cheaper intermediate step is a plug-in meter on one appliance to see what it actually does. On the fridge that is especially instructive, because it is the item where estimates are furthest off.

What to do with the result: compare it with your calculation and find the gap. If there is 200 watt hours a day between them, something was left out, and it is almost always the inverter at idle or the fridge in warm weather. Adjust the calculation, not reality.

With your daily use in Wh you can now pick a suitable battery and solar panels.

For battery size the rule is that you do not want to empty the battery every day. With lithium (LiFePO4) you count on 80 percent of the capacity, with lead-acid and AGM (absorbent glass mat, a sealed lead battery) on 50 percent. For our example of 875 Wh a day:

• Lithium: 875 Wh / 0.8 / 12V = around 91 Ah, so a 100 Ah battery
• AGM or lead: 875 Wh / 0.5 / 12V = around 146 Ah, so 150 to 200 Ah

If you want two days off-grid without charging, double the capacity.

For solar you work back from the yield per day. A system delivers roughly 4 Wh per watt-peak per day in summer, 2.4 Wh in spring and 1.2 Wh in winter, after losses in the controller and the wiring. So a 100 Wp panel gives 400 Wh in summer and 120 Wh in December.

For 875 Wh a day that means:

• Summer: around 220 Wp
• Spring and autumn: around 365 Wp
• Winter: around 730 Wp

That winter figure is why almost nobody manages the whole year on solar alone. It is why most campers combine solar with a DC-DC charger (charging from the alternator while driving) and possibly a shore power connection.

Use our calculators to work out the sizes for your situation.