Size a campervan solar system
Weighs solar AND alternator against each other — and the sun hours come from the hourly grid of your location, not from a rule of thumb. On a horizontal van roof June and December differ by a factor of twelve: in Berlin 200 Wp deliver 810 Wh a day in June and 68 in December. A 30 A DC-DC charger delivers a constant 486 Wh at 1.5 hours of driving — seven times the solar array in December.
The formulas behind the calculator
Every number above can be recomputed: the full calculation path, all assumptions and the data source with retrieval date — plus cross-validation against independent references. Disclosed, not claimed.
An estimate based on the stated assumptions. The final design must be checked by a qualified professional against the rules that apply where you are.
Data as of: 2026-06-15
| Step | Formula | Value | Provenance |
|---|---|---|---|
| Solar yield per day | P_Array * PSH * eta_Kette | 252.4 Wh/d | assumed |
| Alternator yield per day | I_boost * U_sys * h_drive * eta_boost | 486 Wh/d | exact |
| Booster equals module power | E_Booster / (PSH * eta_Kette) | 385.1 Wp | exact |
| Required nominal capacity | E_d * d_autonomy / (U * DoD * f_age * eta_path) | 312.63 Ah | exact |
| Usable energy cross-check | C_erf * U * DoD * f_alter * eta_Pfad | 2,400 Wh | exact |
- Formula
E_solar = P_array · PSH(site, month, β = 0°) · η_chain · E_boost = I · U · h_drive · η · C_req = E_d · d_autonomy / (U · DoD · f_age · η_path)- Valid for
- Campervan or van with horizontal roof mounting (β = 0°), one design month, peak sun hours from the hourly TMY grid of the site (1,155 grid points, PVGIS-SARAH3 2005–2023).
- Not covered
- Shading from roof fixtures and hatches, the alternator charge curve over driving time, shore power, the temperature dependence of battery capacity in winter camping (that is calculator 39), and tilted or tracked modules — the calculator assumes a horizontal roof, because that is what a panel van has. What the travel profile derives is shown in the result and can be changed.
- Data sources
- Manufacturer datasheets (Victron, Fronius, BYD) and IEC 61427-1, aggregated · retrieved 2026-06-15
Frequently asked questions
Does the alternator deliver more energy than my campervan's solar array?
For daily drivers, often yes: a 30 A charge booster over about an hour and a half of driving delivers roughly four times what 200 Wp of solar yields in December at 0.8 peak sun hours. The calculator weighs both sources and reports how many watts-peak of solar the booster effectively replaces — the number a purchase decision hinges on.
Do I need a DC-DC charge booster, or is a split-charge relay enough?
Without a booster the alternator charges only very little through a split-charge relay — with lithium batteries practically nothing. If you drive daily without a booster fitted, the calculator raises exactly this warning.
What does the campervan calculator not cover?
Shading from roof fittings, the alternator's charge curve over driving time, shore power, and the temperature dependence of battery capacity in winter camping. Everything derived from your travel profile — such as autonomy days — is shown with its reasoning in the method block and can be overridden there, and winter camping with lithium triggers an additional warning about charging below freezing.
How much solar does a 30 A DC-DC charger replace?
In the example the 30 A booster delivers about 486 Wh with 1.5 h of daily driving — at wintry 0.8 peak sun hours and 70% chain efficiency you would need about 868 Wp of solar on the roof for the same energy, more than four times the installed 200 Wp. That is why the calculator reports the booster equivalence as its own figure: it makes the purchase decision between “more panels” and “retrofit a booster” comparable.
Where do the autonomy days come from — and can I change them?
From the travel profile: shore-power pitch 1 day, daily driving 2, off-grid weeks 4, winter camping 3. This is the number other calculators make the user guess. Here it is derived visibly and justified — and stays overridable under “advanced assumptions” if you know your usage pattern better.
Why does the calculator warn about winter camping with LiFePO4?
Because LiFePO4 and NMC cells must not be charged below 0 °C — lithium plating permanently destroys the cell. For winter camping that means: a heated battery (many models have heating foils), installation inside the living space, or a chemistry that tolerates charging in frost. The warning appears automatically whenever the “winter camping” profile is combined with a lithium chemistry.
How much solar does a campervan actually need?
Almost everything depends on the design month, and that is exactly what nobody else asks. Example Berlin, 200 W flat on the roof, 1,200 Wh daily demand, 30 A DC-DC charger at 1.5 h driving: in June the system covers 108 % of demand, in October 62 %, in December 46 % — and without the charger, 6 % in December. Summer travellers get by with 200 W; anyone wanting winter autonomy cannot reach it with solar alone on a van roof.
How many sun hours does a van roof get in winter?
Far fewer than rules of thumb suggest — because the roof lies flat. For Berlin the calculator works out, from 19 PVGIS years: January 0.71, March 2.40, June 5.79, September 3.15, December 0.49 peak sun hours per day. Best to worst month is a factor of twelve. A tilted surface would collect a multiple of that in December — but you cannot build it on a moving panel van. That geometry penalty is missing from every other campervan calculator.
Is a DC-DC charger worth more than another panel?
In winter, almost always. The calculator puts a number on it as a solar equivalent: to replace the 486 Wh a 30 A charger delivers at 1.5 hours of driving, you would need 1,424 W on the roof in Berlin in December — that fits on no panel van. In June it is only 120 W, barely half a module. The answer flips with the season, which is why it belongs in a calculator rather than in a rule of thumb.
Are 200 W and 100 Ah enough for a van?
For summer travel with moderate consumption yes, for winter autonomy no. In the example (1,200 Wh/day, driving daily, LiFePO4) the battery bank alone needs 313 Ah to carry two autonomy days including ageing and converter losses — not 100. On the winter camping profile with three autonomy days it is 469 Ah. The widespread “100 Ah is plenty” advice quotes nominal capacity instead of usable capacity.
Why does the calculator assume a horizontal roof?
Because a panel van does not have a pitched one. That is not a simplification but the body shape — and in winter it is the single biggest loss: with a low sun, radiation strikes a horizontal surface at a very shallow angle. Tilting mounts exist, but they are not available while driving and are deliberately not modelled here. Anyone using them while parked will beat these winter figures noticeably.