Spectral effect and low irradiance
Spectral correction
The light spectrum changes with the air mass and the water vapour in the atmosphere, and cells respond differently to each colour. The spectral correction adjusts the power for that difference with respect to the reference spectrum.
Model: First Solar (2013), for monocrystalline silicon, as implemented in pvlib. It depends on:
- the absolute air mass (from the Sun position and the pressure, derived from the altitude),
- the precipitable water, calculated per month and site from the monthly mean temperature of the weather year and a relative humidity of 65 % (Gueymard formula, 1994). It is valid for any latitude and hemisphere.
The hourly factor is limited to the 0.95–1.05 range. Over the year it adds a little production in winter and removes some in summer; the annual effect is small (around +0.3 %).
It can be turned off in Advanced settings («Apply spectral correction»).
Low irradiance
In low light, modules are less efficient (they lose relatively more through internal resistances). There are two ways to calculate it:
| Model | Use |
|---|---|
| Single-diode model (default) | The power comes from the module circuit at each irradiance and temperature; it includes low irradiance naturally. See Temperature and single diode |
| Huld (PVGIS) | Empirical model of relative efficiency versus irradiance, combined with the linear temperature coefficient |
The correction can also be turned off. In the loss waterfall it appears as «Low irradiance».
Order of application
Radiaris applies irradiance first and then temperature (as PVsyst does), followed by the spectral factor. The waterfall breakdown is exact with that order.