Temperature and single-diode model
Cell temperature: Faiman model
The cell temperature determines how much the power drops in hot hours:
T_cell = T_ambient + G / ( u_c + u_v · v_wind )
with G = effective irradiance on the plane (W/m²), v = wind speed (m/s), and:
| Coefficient | Default value | Meaning |
|---|---|---|
| u_c | 25,0 W/(m²·K) | Losses through radiation and natural convection |
| u_v | 6,84 W·s/(m³·K) | Forced convection by the wind |
These are the Faiman (2008) / IEC 61853 values for modules on a ventilated roof. The module absorptivity is implicit: it is not multiplied again. Both can be changed in Advanced settings: lower values of u_c and u_v (poorly ventilated, roof-integrated modules) give higher temperature and more loss.
Module power: single-diode model
The module is represented by the single-diode equivalent circuit with the PVsyst parameterisation (implemented with pvlib): a current source (photocurrent), a diode, a series resistance R_s and a shunt resistance R_sh that depends on irradiance. For each hour the maximum power point (MPP) is calculated with the effective irradiance and the cell temperature.
| Parameter | Meaning |
|---|---|
| Number of cells in series | – |
| R_s | Series resistance |
| R_sh,ref and R_sh,0 | Shunt resistance at 1,000 W/m² and at zero irradiance |
| γ | Diode ideality factor |
| α_sc | Isc temperature coefficient (A/°C) |
| μ_γ | Variation of the ideality factor with temperature |
| R_sh exponent | 5.5 by default |
The parameters come from the manufacturer's .PAN file or are estimated from the datasheet: the saturation current is fitted so that Voc(STC) matches the datasheet, and μ_γ so that the model reproduces the declared Pmax temperature coefficient.
What this gains
Compared with the classic empirical model (linear loss with temperature + Huld correction), the single-diode model:
- reproduces both the irradiance effect and the temperature effect,
- also gives the maximum power point voltage of each hour, which is used in the inverter and in the cable losses,
- allows the module's I-V and P-V curves to be drawn.
Separation in the waterfall
The power is split into three factors for the loss waterfall:
P = P_STC(G) · low_irradiance_factor(G, 25 °C) · thermal_ratio(G, T_cell)
Validation
With a 445 W module (Jinko JKM445N, PVsyst parameters) and three cases in Cordovilla (Navarre), the annual DC energy is within +0.10 %, +0.16 % and +0.26 % of PVsyst. See Validation and limits.
Alternative mode
In Advanced settings you can choose the empirical Huld + temperature coefficient model instead of the single-diode one. It is less accurate but does not need circuit parameters.