Camper van energy budget: how much power do you really need?

Make a list of everything that uses power in your camper van. Note the wattage and how many hours per day you use each device. Typical consumers and their daily usage: - Compressor fridge: 30-45W, 24 hours (but the compressor runs about 30-50% of the time) = 200-500 Wh/day - LED lighting: 5-20W, 4-6 hours = 30-100 Wh/day - Phone charging: 10-15W, 2-3 hours = 25-45 Wh/day - Laptop: 40-65W, 3-5 hours = 120-325 Wh/day - Water pump: 30-60W, 0.5 hours = 15-30 Wh/day - Diesel heater fan: 10-40W, 8-12 hours = 80-480 Wh/day The formula: Wh per day = wattage times effective running time (duty cycle). A fridge is on 24 hours, but the compressor only runs 8-12 hours. The duty cycle is then 33-50%.

Your energy balance varies significantly by season. In summer you use more cooling but less heating and lighting. In winter the opposite applies, plus shorter days mean less solar energy. Summer (Southern Europe): fridge consumes more (40-50% compressor time), no heater, less lighting. Solar yield high (4-6 peak sun hours). Winter (Northern Europe): fridge consumes less, heater runs 8-12 hours, more lighting. Solar yield low (1-2 peak sun hours). The Energy Timeline tool simulates this per month, so you can see which months you have a deficit or surplus.

Now that you know your daily consumption, compare it with your yield from the three charging sources. Solar: panel wattage times peak sun hours times 0.75 (efficiency loss from MPPT, cables, temperature). Example: 200Wp times 4 hours times 0.75 = 600 Wh/day. Alternator (B2B): amperage times average charge voltage times driving hours. The alternator does not output exactly 12V but around 14.2-14.4V; use 13V as a realistic average. Example: 30A times 13V times 2 hours = 780 Wh/day. Shore power: charger amperage times voltage times charging hours. Example: 30A times 12V times 4 hours = 1440 Wh. If your total daily yield exceeds your daily consumption, you are fine. If not, you need to generate more, consume less, or add a larger battery bank as a buffer.

Your battery is not an energy source but a buffer. On days when you consume more than you generate, you live from the buffer. On good days you refill it. Rule of thumb for lithium (LiFePO4): you can use 80% of the capacity. The nominal voltage is 12.8V (not 12V). A 200Ah LiFePO4 battery gives 200 times 12.8V times 0.8 = 2048 Wh usable. Rule of thumb for lead-acid (AGM/gel): use a maximum of 50%. The nominal voltage is 12V. A 200Ah lead-acid battery gives 200 times 12V times 0.5 = 1200 Wh usable. How much buffer you need depends on the number of autonomy days you want: the number of days you can go without charging. Two days of autonomy at 600 Wh daily consumption = 1200 Wh buffer needed. Watch for the overcast-days factor: in cloudy conditions, solar panels often produce only 5-10% of their peak output. A system that runs perfectly on solar in summer can go flat within 24 hours in autumn without a B2B charger or shore power. Always calculate your autonomy days without solar.

Replace halogen with LED: saves 80% on lighting energy. Choose an efficient fridge: the difference between brands can be 30-40% at the same cooling volume. Charge devices via USB-C PD instead of via the inverter (230V): the inverter itself consumes 10-30W at no load. Switch off the inverter when not needed. Many van builders leave the inverter on 24/7, which can cost 250-700 Wh/day in standby power. Run your diesel heater at the lowest comfortable setting: the difference between setting 1 and setting 3 can save 300 Wh/day in fan consumption.