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Generally speaking, array losses can be defined as all events which penalize the available array output energy with respect to the PV-module nominal power as quoted by the manufacturer for STC conditions. This is the philosophy stated by the  JRC/Ispra European Center recommendations, through the Normalized performance index and the Performance Ratio. Several of these loss sources are not directly measurable.

Starting with incident irradiation in the collector plane (after taking irradiation shading effects into account), one can imagine that an ideal PV-array should yield one kW/kWp  under an irradiance (Ginc) of 1 kW. That is, assuming a linear response according to Ginc, the ideal array will produce one kWh energy under one kWh irradiance for each installed kWp (as defined at STC).

This ideal yield is diminished by the following losses:

-     Shading losses  (irradiance deficit and electrical effect). These are included in the official PR definition established by JRC/Ispra, as well as the IAM loss. It is not clear if the Horizon loss (far shadings) is included or not in the official PR.

-        Incidence angle modifier (IAM), is an optical effect (reflection loss) corresponding to the weakening of the irradiation really reaching the PV cells surface, with respect to irradiation under normal incidence.

-        Irradiance Loss: the nominal efficiency is specified for the STC (1000 W/m²), but is decreasing with irradiance according to the PV standard model.

-        Thermal behavior of the PV array. The standard test conditions are specified for a cell temperature of 25°C, but the modules are usually working at much higher temperatures. The thermal loss is calculated following the one-diode model. For crystalline silicon cells, the loss is about  -0.4 %/°C at MPP. For fixed voltage operating conditions, the temperature mainly affects the I/V curve voltage, and effective losses are strongly dependent on the array overvoltage with respect to the operating voltage.

 The parameters available to the user (thermal loss factor) involve the cell temperature determination with respect to given external conditions.

-        Real module performances of the module with respect to the manufacturer specifications. PVsyst uses effective specification parameters to calculate the primary PV-array characteristics. The user may define a relative loss factor, which is related to the average effective module power at STC, and acts as a constant penalty during all simulation conditions.

-        Mismatch losses of the PV modules, which can be evaluated by a special tool, but is only taken into account as a constant loss during simulation.

-        Dirt on the PV-modules, may be defined in % of STC, yearly or in monthly values.

-        Partial shading electrical effects, limiting each string current to the more shaded cell, are of course depending on the sun position. They are not explicitly calculated in PVsyst, but can only be roughly evaluated using the "Near shadings according to modules".

-        MPP loss, i.e. the difference between the effective operation conditions and the maximum available power point. For MPP use (grid inverters) this loss is neglected in PVsyst. For fixed operating voltage, it can be quantified from the output simulation results  (see EArrMPP, EArrUFix, MPPLoss).

-        Ohmic wiring losses, as thermal effects, essentially result in a voltage drop of the I/V-array characteristics. The real effect is different whether the array operates at MPP or fixed voltage. At MPP operation, PVsyst applies the wiring loss before computing the MPP. At fixed voltage, the effective losses are strongly dependent on the array overvoltage with respect to the operating voltage.

-        Regulation loss is the energy potentially available from the PV array, but which cannot be used by the system.

 In MPP applications, this could be the array potential PV production outside the inverter input voltage limits, or during power overloads. This is usually accounted in "Inverter losses", that is in system losses.

 In stand-alone systems, it corresponds to the excess energy which cannot be used when the battery is full.

 In DC-grid installation, this is the potential current in excess with respect to the instantaneous load current.

In Normalized performance index,  all these array losses are accounted for in the "Collection Losses" Lc, that is the difference between Yr (the ideal array yield at STC) and Ya (the effective yield as measured at the output of the array).

Nevertheless, unlike Ispra recommendations, in PVsyst the unused energy is specifically designed as Lu = Unused loss  (see Normalized performance index for details).