Irradiance and transposition
Weather data give the irradiance on the horizontal plane. The modules are tilted, so the irradiance they actually receive has to be calculated: the plane-of-array irradiance (POA).
Sun position
For each hour the solar zenith angle and azimuth are calculated from the latitude, longitude and altitude of the site (pvlib solar position algorithm). By default the time of the timestamp is used; an optional offset (in minutes) shifts that time (PVsyst uses +10). At night, the plane irradiance is set to zero.
POA components
The irradiance on the plane is split into three parts:
| Component | Source |
|---|---|
| Direct | DNI × cos(angle of incidence) |
| Sky diffuse | The part of the horizontal diffuse that reaches the plane; calculated with the transposition model |
| Ground-reflected | GHI × albedo × (1 − cos(tilt)) / 2 |
Transposition models
They differ in how they distribute the sky diffuse. It is chosen in Advanced settings:
| Model | Description |
|---|---|
| Perez (default) | Empirical anisotropic: splits the diffuse into circumsolar, horizon band and dome. The most accurate and the recommended one |
| Hay-Davies | Simple anisotropic: part of the diffuse comes from around the Sun and the rest from the whole sky |
| Reindl | Variant of Hay-Davies that adds horizon brightening |
| Klucher | Corrected isotropic: more diffuse with clear skies and near the horizon |
| King | Isotropic diffuse with an empirical correction based on the zenith angle; for reference only |
| Isotropic | Diffuse arrives equally from the whole sky; the simplest, it tends to underestimate tilted planes |
All are calculated with pvlib; the in-house Hay-Davies implementation is checked against pvlib in the tests (difference below 1 % in six orientations).
Shading on the POA
Before the optical losses, the shading module reduces the direct and circumsolar components (with the k factor of the electrical model) and the isotropic diffuse by the blocked sky view. See Objects and shading.
Far horizon
When the Sun is below the horizon profile, the direct component is set to zero. See Horizon.
What to expect
Transposition is one of the largest sources of difference between programs. At mid latitudes, a south-facing, well-tilted plane receives more annual irradiation than the horizontal. In the loss waterfall it appears as the «Transposition» gain.