Nominal Power and Standard Test Conditions (STC)
The performance of a photovoltaic module is generally measured under Standard Test Conditions (STC), defined by the IEC 60904-1 standard:
- Irradiance of 1000 W/m² (optimal sunlight)
- Module temperature at 25°C
- Standardized light spectrum (IEC 60904-3)
Bifacial modules, which capture light on both sides, can improve production through ground reflection (albedo). PVGIS does not yet model these modules, but one approach is to use BNPI (Bifacial Nameplate Irradiance), defined as: P_BNPI = P_STC * (1 + φ * 0.135), where φ is the bifaciality factor.
Limitations of bifacial modules: Unsuitable for building-integrated installations where the rear of the module is obstructed. Variable performance depending on the orientation (e.g., North-South axis with East-West facing).
Estimation of the Actual Power of PV Modules
The actual operating conditions of PV panels differ from standard (STC) conditions, which affects the output power. PVGIS.COM applies several corrections to incorporate these variables.
1. Reflection and Angle of Incidence of Light
When light hits a PV module, a portion is reflected without being converted into electricity. The more acute the angle of incidence, the greater the loss.
- Impact on production: On average, this effect causes a loss of 2 to 4%, reduced for solar tracking systems.
2. Effect of the Solar Spectrum on PV Efficiency
Solar panels are sensitive to certain wavelengths of the light spectrum, which vary by PV technology:
- Crystalline Silicon (c-Si): Sensitive to infrared and visible light
- CdTe, CIGS, a-Si: Different sensitivity, with a reduced response in the infrared
Factors influencing the spectrum: Morning and evening light is redder.
Cloudy days increase the proportion of blue light. The spectral effect directly influences PV power. PVGIS.COM uses satellite data to adjust these variations and integrates these corrections into its calculations.
Dependence of PV Power on Irradiance and Temperature
Temperature and Efficiency
The efficiency of PV panels decreases with module temperature, depending on the technology:
At high irradiance (>1000 W/m²), module temperature increases: Loss of efficiency
At low irradiance (<400 W/m²), efficiency varies according to the type of PV cell
Modeling in PVGIS.COM
PVGIS.COM adjusts PV power based on irradiance (G) and module temperature (Tm) using a mathematical model (Huld et al., 2011):
P = (G/1000) * A * eff(G, Tm)
The coefficients specific to each PV technology (c-Si, CdTe, CIGS) are derived from experimental measurements and applied to PVGIS.COM simulations.
Modeling the Temperature of PV Modules
- Factors influencing the module temperature (Tm)
- Ambient air temperature (Ta)
- Solar irradiance (G)
- Ventilation (W) – Strong wind cools the module
-
Temperature model in PVGIS (Faiman, 2008):
Tm = Ta + G / (U0 + U1W)
The coefficients U0 and U1 vary according to the type of installation:
| PV Technology | Installation | U0 (W/°C-m²) | U1 (W-s/°C-m³) |
|---|---|---|---|
| c-Si | Freestanding | 26.9 | 26.9 |
| c-Si | BIPV/BAPV | 20.0 | 20.0 |
| CIGS | Freestanding | 22.64 | 22.64 |
| CIGS | BIPV/BAPV | 20.0 | 20.0 |
| CdTe | Freestanding | 23.37 | 23.37 |
| CdTe | BIPV/BAPV | 20.0 | 20.0 |
System Losses and Aging of PV Modules
All the previous calculations provide the power at the module level, but other losses must be considered:
- Conversion losses (inverter)
- Wiring losses
- Differences in power between modules
- Aging of the PV panels
According to the study by Jordan & Kurtz (2013), PV panels lose on average 0.5% of power per year. After 20 years, their power is reduced to 90% of their initial value.
- PVGIS.COM recommends entering an initial system loss of 3% for the first year to account for system degradations, then 0.5% per year.
Other Factors Not Considered in PVGIS
Some effects influence PV production but are not included in PVGIS:
- Snow on the panels: Drastically reduces production. Depends on the frequency and duration of snowfall.
- Accumulation of dust and dirt: Decreases PV power, depending on cleaning and precipitation.
- Partial shading: Has a strong impact if a module is shaded. This effect must be managed during PV installation.
Conclusion
Thanks to advances in photovoltaic modeling and satellite data, PVGIS.COM allows for precise estimation of the output power of PV modules by taking into account environmental and technological effects.
Why Use PVGIS.COM?
Advanced modeling of irradiance and module temperature
Corrections based on climatic and spectral data
Reliable estimation of system losses and panel aging
Optimization of solar production for each region