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Array losses, general considerations

Array losses are all factors that reduce the array output energy below the PV module nominal power specified by the manufacturer for STC conditions. This concept aligns with JRC/Ispra European Center recommendations, as reflected in the Normalized performance index and Performance Ratio. Several of these loss sources are not directly measurable.

Starting with incident irradiation in the collector plane (after accounting for shading effects), an ideal PV array would produce 1 kW/kWp at an irradiance (Ginc) of 1 kW. In other words, assuming a linear response to Ginc, the ideal array would produce 1 kWh of energy per 1 kWh of irradiance for each installed kWp (as defined at STC).

This ideal yield is reduced 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): an optical effect (reflection loss) corresponding to the reduction in irradiation reaching the PV cell surface compared to irradiation at 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. PV modules nominal performances are specified at 25°C, but modules typically operate at much higher temperatures which leads to thermal losses (about -0.2-0.4%/°C).
  • Actual module performance compared to manufacturer specifications. PVsyst uses effective specification parameters to calculate the primary PV array characteristics. The user can define a relative loss factor related to the average effective module power at STC, which applies as a constant penalty across 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".
  • Optimizer Loss: When optimizers are selected, additional losses may occur before ohmic wiring losses. These losses typically result from optimizer efficiency. Additionally, optimizers may need to select an operating point other than the MPP if input power, current, or voltage exceeds their limits. In such cases, additional clipping power loss is applied.
  • 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, they essentially result in a voltage drop in the I/V array characteristics. The actual effect differs depending on whether the array operates at MPP or fixed voltage. At MPP operation, PVsyst applies the wiring loss before computing the MPP. At fixed voltage, effective losses strongly depend on array overvoltage relative 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 the Normalized performance index, all array losses are accounted for in "Collection Losses" (Lc), which is the difference between Yr (the ideal array yield at STC) and Ya (the actual yield measured at array output).

However, unlike Ispra recommendations, PVsyst specifically designates unused energy as Lu = Unused loss (see Normalized performance index for details).