Size a cabin solar system
Shows both designs side by side — and finds the design month for each season itself: it is the weakest month the season contains, from the hourly grid of your location. For a cabin near Berlin with a 45° south roof that is October, not April, for the summer season. Year-round operation costs a factor of 2.5 in module power there. Plus the idle-period calculation: a garden cabin stands empty in winter, and for lead chemistries self-discharge plus deep discharge is what kills the battery.
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 |
|---|---|---|---|
| Array power for summer use | E_d / (PSH_summer * eta_chain) | 297.58 Wp | exact |
| Array power year-round | E_d / (PSH_winter * eta_chain) | 752.05 Wp | exact |
| Factor between the designs | P_yearround / P_summer | 2.5272 | exact |
| Required nominal capacity | E_d * d_autonomy / (U * DoD * f_age * eta_path) | 371.28 Ah | exact |
| State of charge after idle | SoC_start - r_self,month * months | 0.91 | assumed |
- Formula
P_array = E_d / (PSH(site, weakest month of the season) · η_chain) · SoC_end = SoC_start − r_self · months- Valid for
- One design month per season — derived as the weakest month of that season from the hourly TMY grid (1,155 DACH grid points, PVGIS-SARAH3 2005–2023), for the roof pitch and orientation you enter. Summer season is April to October. No recharging during the idle period.
- Not covered
- Shading, snow cover on the modules, temperature dependence of self-discharge, trickle charging from a small extra module. Self-discharge is computed linearly — over a few months that is the more conservative form.
- Data sources
- Manufacturer datasheets (Victron, Fronius, BYD) and IEC 61427-1, aggregated · retrieved 2026-06-15
Frequently asked questions
How much bigger does a cabin solar system need to be for year-round use?
The design month moves from the autumn edge of the summer season to December — and the calculator finds both itself. For a 45° south roof near Berlin that is October (2.88 peak sun hours) and December (1.14), a factor of 2.5 in module power: 298 versus 752 W at 600 Wh daily demand. The frequently quoted factor of five only arises from blanket assumptions of 4.5 and 0.8 peak sun hours, which do not hold for a tilted south roof. The calculator shows both designs with module power and cost side by side, instead of quietly sizing for year-round use and selling the larger array.
Will my battery survive the winter while the cabin stands empty?
The idle-period calculation answers exactly that: state of charge at departure minus monthly self-discharge times the idle months. With lead chemistries, self-discharge combined with deep discharge kills the battery — in spring it is not flat but ruined. If the state of charge falls below the critical threshold the calculator warns you; leaving with a full charge is the main countermeasure.
How far can I trust the winter design for a cabin?
With caveats: snow cover on the modules is not modelled and is the most common total outage in December — below 1 peak sun hour in winter the calculator warns about it explicitly. Shading, temperature dependence of self-discharge and trickle charging from a small extra module are also outside the model; self-discharge is calculated linearly, which over a few months is the more conservative choice.
Why is AGM the default chemistry in the cabin calculator?
Because a cabin’s load profile barely touches lead’s weaknesses and uses its strengths: AGM tolerates charging in frost, which destroys LiFePO4 cells — and an unattended cabin cannot babysit a battery heater in winter. With few cycles per year, lithium’s cycle life advantage never comes into play. If you heat year-round or cycle daily, switch the chemistry in the field above — the calculator converts the deep-discharge limit automatically.
What role does the cell chemistry play for the idle period?
A double one: flooded lead batteries lose about 8% of charge per month, AGM, gel and lithium about 3%. And the deep-discharge limit sits at 50% remaining charge for lead versus 20% for lithium. A flooded battery locked up half full is below the limit after three months — the curve in the chart shows exactly this trajectory for your inputs.
Is a small trickle-charge module enough for winter?
Usually yes — and the calculator deliberately does not assume it: the idle-period maths covers the case with no recharging at all. Even 10–20 Wp with a simple charge controller covers the self-discharge of a typical 100 Ah battery several times over and keeps it full for months. Important: a snow-free mounting spot (vertical on the south wall instead of flat on the roof) and a controller with float charging.
Which month is the design month for a garden cabin?
The weakest month of the season in question — and the calculator finds it rather than assuming it. For a south-facing roof at 45° near Berlin, year-round operation lands on December with 1.14 peak sun hours. For the April-to-October season it is not April but OCTOBER, with 2.88 — on a tilted surface the autumn edge sits lower than the spring edge. Design against April instead and the array comes out too small, leaving you dark in October.
How much does a steeper roof pitch help on a cabin?
For year-round use, surprisingly much, because December decides. For the same cabin near Berlin (600 Wh/day): 20° needs 1,036 W, 45° only 752 W, 70° even 673 W. A very steep surface is worse in summer and better in winter — and winter sets the size. For the summer season the pitch barely matters (298 versus 295 W between 45° and 70°).
What does an east or west orientation cost on a cabin?
More than most expect, because the loss bites hardest in winter. The same cabin at 45° facing west instead of south needs 1,691 W year-round instead of 752 — more than double, and roughly 1,015 instead of 451 euros in modules. For the summer season the gap is far smaller at 505 versus 298 W. Orientation is not a side issue on a cabin.
Why is the summer-versus-year-round gap smaller than often claimed?
Because the widely quoted factor of five comes from two assumptions that are too extreme: 4.5 peak sun hours for summer and 0.8 for winter. Neither holds for a tilted south roof. Computed from real site data for 45° south near Berlin the factor is 2.5 (752 versus 298 W) — still substantial, but not fivefold. This calculator carried those old assumptions itself until site data replaced them.
How big does the array need to be for a cabin using 600 Wh a day?
About 298 W for the April-to-October season and about 752 W for year-round operation — both for a 45° south roof in the Berlin region at 70 percent chain efficiency. In module cost that is roughly 179 versus 451 euros. The battery bank comes on top and follows the autonomy days, not the array power.