Campervan battery and solar sizing: work it out from your actual loads

“How much solar do I need?” is the most-asked question in van electrics and the least answerable as posed. Solar is the last number you calculate, not the first. The sizing chain runs one direction: what you use per day decides the battery, and the battery's daily deficit decides the charging — solar, alternator, or shore. Skip the first step and every number downstream is a guess dressed up as a spec. So: build a load table, do two divisions, and the system sizes itself. Here is the method, with the math shown.

Step 1: the load table — watts × hours = your real number

List every electrical consumer, its power draw in watts, and how many hours a day it actually runs. Multiply across each row, sum the column, and you have your daily watt-hours — the single number the whole system hangs on. A realistic table for a couple working part-time from the van:

Two things jump out. First, the fridge is the quiet giant: it never turns off, so a modest 45 W compressor cycling at roughly half duty ends up at 500–700 Wh a day — usually the single biggest line in a build without induction, and it grows in summer when the van is hot. Second, the induction hob is the budget-buster. One appliance, used for under an hour, can equal everything else combined and roughly double the battery and solar you need. Cook on gas or diesel and the summer table sums to around 1,100–1,300 Wh/day; go all-electric and you are designing a different, much more expensive system. Decide this before you buy a single component.

A note on honesty: use hours you will actually run, not hours you hope to. The diesel heater's fan looks trivial per hour, but on a cold night it runs all night — and winter is exactly when solar is weakest, which matters in Step 3.

Step 2: battery bank — daily Wh × autonomy ÷ usable depth

The battery formula is one line:

Bank size (Wh) = daily consumption × days of autonomy ÷ usable depth of discharge.

Days of autonomy is how long you want to run with no charging at all — grey skies, engine off. Two days is the sensible default; three if you park up for long stretches in shoulder season. Usable depth of discharge is where chemistry earns its price difference: LiFePO4 (lithium iron phosphate) can be cycled to roughly 90% of its rated capacity without meaningful lifespan cost, while lead-acid — flooded or AGM — should not be taken much past 50% if you want it to survive more than a season or two. A “100 Ah” lead-acid battery is, in practice, a 50 Ah battery.

Worked example, the 1,200 Wh/day couple from Step 1, gas cooking:

• 1,200 Wh × 2 days = 2,400 Wh needed as usable energy.
• LiFePO4: 2,400 ÷ 0.9 ≈ 2,670 Wh of rated capacity. At a nominal 12.8 V that is 2,670 ÷ 12.8 ≈ 208 Ah — so a 200 Ah LiFePO4 bank (one 200 Ah unit or two 100 Ah in parallel) lands the spec almost exactly.
• The same target in lead-acid: 2,400 ÷ 0.5 = 4,800 Wh → 400 Ah — twice the rated capacity and, at ~28 kg per 100 Ah AGM, well over 100 kg of battery versus ~40 kg of lithium.

That weight comparison is most of why the lead-acid era in van builds is over: lithium costs more per rated amp-hour but less per usable amp-hour, cycles several times longer, and carries a fraction of the mass. Add the induction hob to the load table and the same math points at 400 Ah or more of lithium — plus an inverter sized around 2,000–3,000 W, which itself pushes toward a 24 V or 48 V system in bigger builds. Again: the hob is a system decision, not an appliance purchase.

Step 3: solar — daily Wh ÷ realistic harvest hours

A solar panel's nameplate wattage is what it produces in laboratory conditions: perpendicular sun, 25 °C cell temperature, no losses. A flat-mounted panel on a hot van roof sees none of that. The honest planning unit is the peak sun-hour— the day's total irradiance compressed into equivalent full-power hours — and for a flat roof it ranges from roughly 4–5 h in a southern-European summer, through 2–3 h in spring and autumn at mid latitudes, down to 1–1.5 h in a northern November. Then knock off another 20–30% for cable, controller, charging losses, and the heat-soaked roof. The formula:

Array size (W) = daily Wh ÷ sun-hours ÷ ~0.75 system efficiency.

Run the 1,200 Wh/day couple through it:

• Summer (4.5 sun-hours): 1,200 ÷ 4.5 ÷ 0.75 ≈ 355 W → a 400 W array covers the day with margin. This is why 400 W is the standard couple's roof.
• October (2.5 sun-hours): the same 400 W harvests roughly 400 × 2.5 × 0.75 ≈ 750 Wh — a daily 450 Wh deficit that eats the 200 Ah bank in about five days.
• November at 50°N (1.5 sun-hours, grey weeks): ~450 Wh on a good day, near zero on a bad one — a third of demand, and demand is higher because the heater fan runs all night.

The conclusion is not “buy more panels.” A Ducato roof fits maybe 600–800 W once you keep the fan and skylight, and tripling the array still leaves you short through a dark fortnight. The winter answer is the engine: a DC-DC (B2B) charger pushing 30–50 A from the alternator puts 400–650 Wh into the bank per hour of driving. Solar carries you April to September; the alternator is the backstop the rest of the year. Every serious build has both.

Three reference systems

The same three-step math, run for three archetypes. Costs are rough 2026 mid-range component prices (batteries, panels, chargers, wiring — not labour); weights include batteries, panels, inverter, and cabling.

Notice the jump between rows two and three: roughly double the battery, but well over double the cost, because the inverter, charging hardware, and cabling all step up a class with it. The induction hob's true price is the whole third row. For where electrics sit in the total build budget, see the cost breakdown — on most builds the electrical system is the second-biggest line after the base van.

Weight and placement: the part the wiring diagrams skip

An electrical system is also ballast. The third-row build — 600 Ah of lithium, a 3,000 W inverter, 600 W of glass on the roof, plus copper — comes to 80–120 kg, and a lead-acid equivalent would be double that. That mass has a correct location: low, central, and between the axles, typically under a seat or bench ahead of the rear axle, never high in an overhead cabinet and never crammed into the rear overhang where it multiplies its effect on the rear axle. Panels are the exception you cannot move — 25–40 kg on the roof raises the centre of gravity, which is one more reason not to chase winter sun with roof glass. Where the kilograms land matters as much as how many there are: see what a conversion actually weighs and the payload guide for the axle-load math that decides whether your 3.5 t plate survives the build.

Do the math where the layout lives

The whole method is three multiplications and two divisions — the hard part is keeping the load table honest and remembering that the battery box occupies real floor, real payload, and a real line in the budget. VanPlot's power autonomy check runs exactly this load-table math inside your 3D layout: list your appliances, and it sizes the bank and array next to the weight and cost readouts, so the 200 Ah box under the bench shows up in the floor plan and on the axles at the same time. Free in the browser — build the table in VanPlot once, and “how much solar do I need?” stops being a forum question and becomes a line in your spec.