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 each device's wattage, estimate how many hours per day you use it. Multiply wattage by hours to get watt-hours (Wh) per day for each device. Here is a realistic example for a couple living in their van: LED lighting (20W total): 5 hours = 100 Wh Compressor fridge (45W average, cycles on/off): runs effectively 12 hours = 540 Wh Phone charging (10W x 2 phones): 3 hours = 60 Wh Laptop (65W): 4 hours = 260 Wh (add 15% inverter loss = 299 Wh) Water pump (60W): 0.25 hours = 15 Wh Roof fan (15W): 8 hours = 120 Wh Diesel heater (10W standby + running): 8 hours = 80 Wh Inverter standby (10W): 12 hours = 120 Wh Daily total: approximately 1,334 Wh, or about 111 Ah on a 12V system. Convert Wh to Ah by dividing by your system voltage. For a 12V system: 1,334 Wh divided by 12V equals 111 Ah. For a 24V system: 1,334 Wh divided by 24V equals 56 Ah. Add a safety margin of 20-25% to account for cold weather (batteries perform worse), ageing batteries and devices that draw more than rated. So 111 Ah becomes roughly 140 Ah of required usable capacity per day.

Your daily consumption in Wh directly determines your battery and solar panel requirements. For battery sizing, decide how many days of autonomy you want without any charging. One day of autonomy is standard for fair-weather touring, two days gives you a buffer for cloudy days. Multiply your daily consumption by the number of autonomy days. With LiFePO4 (lithium iron phosphate) batteries, you can use 80-90% of the rated capacity. So for 1,334 Wh per day with one day of autonomy: 1,334 divided by 0.85 equals about 1,570 Wh, or 131 Ah at 12V. A 200Ah LiFePO4 battery would give you comfortable margin. With AGM (Absorbent Glass Mat) lead-acid batteries, you should only discharge to 50% to preserve battery life. The same 1,334 Wh calculation becomes: 1,334 divided by 0.50 equals 2,668 Wh, or 222 Ah. You would need a much larger and heavier battery bank. For solar panel sizing, divide your daily consumption by the expected peak sun hours for your travel region. In Southern Europe in summer, expect 5 peak sun hours. So 1,334 Wh divided by 5 equals 267 Wp of solar panels needed as a minimum. Account for 20-25% system losses (charge controller, cables, heat), so aim for 330-350 Wp. Remember that solar is seasonal. If you travel in winter or Northern Europe, you may need supplementary charging from your alternator (via a B2B charger) or shore power to keep your system running.