Basically the efficiency is the ratio of the produced electrical power, divided by the input power (irradiance).
Therefore the efficiency of a PV module at STC will be :
Effic = 0.001 * PNom [Wp] / Module Area [m2].
- The nominator is a power at STC
- The denominator is indeed the STC Irradiance across the module area, i.e. 1000 W/m2 * Module Area [m2], which explains the units.
Now we have different possibilities of defining the module area:
- The Gross area is from the module's sizes: Length * Width. This includes the frame and spacing between cells. This is the most usual, always available in the database.
- The Sensitive area may be defined in the PVsyst database, when defining the cell's area (facultative). In this case the module's sensitive area = NbCells * Cell area.
This will lead to another efficiency related to the sensitive area, characteristic of the cell's technology.
- In some cases we can define Tile modules (with mutual covering). In this case PVsyst defines the sizes of the "apparent" area for the evaluation of the efficiency.
Use of the Efficiency
The efficiency is never used in the calculations: in any electrical calculations (simulations, etc), PVsyst uses the "One diode model".
The efficiency is only an indicator for display.
In the Loss diagram, the mentioned efficiency is related to the Gross area of the PV system (sum of all modules), which is also mentioned on the plot.
It allows to get a quick check of the "Array Nominal Energy at STC":
EArrNom = GlobEff * System Area * Efficiency[%]/100
NB: The performance ratio (PR) doesn't depend on the efficiency in any way, as it is normalized to the Pnom value.
If you have a better efficiency, you will have a better yield but a higher PNom at the denominator.
Effects of the area on shadings
The only part where the distinction of module area versus sensitive area could have an effect, is the calculation of the shadings. In PVsyst the fraction of light lost due to direct shadings is calculated by determining the shaded fraction on the PV surfaces in the 3D drawing (blue rectangles and polygons). These include typically the frames and spacings between PV modules. In some rare cases, this can lead to very small deviations, e.g. when the shadow touches only the frames. We deem that these deviations have no significant effect on the simulation results.