Help CenterLocation & Climate
Seasonal irradiation variation and worst-month sizing
Solar irradiation (peak sun hours, PSH) varies significantly month to month with the season. Sizing a system on the annual average produces an underpowered system during the least favorable months.
| Site | Annual average PSH | Worst-month PSH | Gap |
|---|---|---|---|
| Bamako (Mali) | 5.8 h | 4.9 h (August, rainy season) | −16% |
| Douala (Cameroon) | 4.6 h | 3.4 h (July, rainy season) | −26% |
| Paris (France) | 3.2 h | 1.1 h (December) | −66% |
| Dakar (Senegal) | 5.9 h | 5.1 h (August) | −14% |
The gap between annual average and worst month is especially large in Europe due to seasonal day-length variation, and in West/Central Africa due to the rainy season (high cloud cover July–September).
SolarDim Pro sizes the PV array based on the PSH of the year's least favorable month (not the average), ensuring the system stays functional year-round rather than only on a 12-month average.
For a hybrid system with grid backup, this conservative choice can be relaxed: the grid compensates for weak months, allowing sizing closer to the annual average to reduce cost.
Common mistakes
- Sizing an off-grid system on the annual irradiation average instead of the worst month
- Ignoring the rainy season in a Central or West African project
- Applying the same safety margin in Europe (large seasonal gap) and near the equator (small gap)