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 0.8, the system efficiency the calculators use (losses in the MPPT controller, maximum power point tracking, the cables and from temperature). Example: 200Wp times 4 hours times 0.8 = 640 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, absorbent glass mat, and 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 (battery to battery, a charger between the starter and house battery) charger or shore power. Always calculate your autonomy days without solar.

An energy budget is about watt hours per day: how much passes through in total. Sizing cables, fuses and the inverter is about watts at one moment: how much can run at once. Those two get mixed up regularly, producing two kinds of mistake.

A 2,000 watt kettle running for three minutes costs 100 watt hours, less than an hour of laptop. For your energy budget it hardly registers. For your inverter, battery cable and main fuse it is the decisive figure, because they have to carry those 2,000 watts without sagging.

The other way round, a 45 watt fridge is no trouble at all for your inverter, while it is the biggest item in your daily budget because it runs around the clock.

So make two lists of the same appliances. One in watt hours per day for your battery and solar panels, and one with the highest power that can run simultaneously for your inverter and cables.

A budget that balances exactly is a budget without margin. A rainy day arrives, or two, and then your battery decides how long you last.

Autonomy is the number of days you can run on the battery alone, without sun and without driving. Work it out as usable battery capacity divided by daily consumption. Usable is not the same as nominal. From a lead battery you use half at most if you want it to survive; from lithium iron phosphate 80 percent, the depth you size a bank on.

A 200 Ah battery at 12 volts is 2,400 watt hours nominal. On lithium at 80 percent you keep 1,920 watt hours. Use 800 watt hours a day and that is 2.4 days.

Two days is tight but workable for most travellers, three days is comfortable, and beyond four days you are mostly paying in weight and money for capacity you rarely touch. Someone who drives often needs less than someone parked in one spot for weeks.

Every energy budget on paper is an estimate. You do not know exactly how often the fridge cycles, how long the water pump runs or what the inverter draws at idle. After the first week on the road it always turns out differently.

A battery monitor with a shunt solves that. A shunt is a small resistor in the negative cable of your battery through which all current flows, so the monitor knows what goes in and out instead of guessing from voltage alone. On lithium that is the difference between knowing and guessing, because between 20 and 90 percent a lithium battery's voltage says almost nothing about its state.

What to do with it: note the reading at the end of the day for a week. That number is your real daily consumption, and suddenly the whole budget adds up. If it is higher than you thought, you now know where to adjust, and that is cheaper than buying another battery afterwards.

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.