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Summary

DC System Losses account for non-ideal effects that reduce power from the calculated maximum power point to actual DC power available at inverter DC terminals. PlantPredict applies loss factors for module mismatch, light-induced degradation, module quality variation, DC health, and DC wiring resistance. These losses are applied as a combined coefficient that reduces effective irradiance before power calculation. DC wiring losses are modeled as additional series resistance derived from a user-specified percentage loss at STC. Time-dependent degradation is documented separately in the Degradation Losses (DC Applied) and Degradation Losses (AC Applied) pages.

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Inputs

NameSymbolUnitsDescription
Module Mismatch CoefficientfMMf_{MM}%Module-to-module mismatch loss percentage
Light-Induced DegradationfLIDf_{LID}%LID loss percentage
Module Quality FactorfMQf_{MQ}%Power deviation from nameplate due to module binning and manufacturing tolerances
DC Health FactorfDCHf_{DCH}%User-defined DC system loss to account for factors such as module soiling non-uniformity or connection degradation
DC Wiring Loss at STCLwire,STCL_{wire,STC}%Percentage power loss in DC wiring at standard test conditions
Module STC Max PowerPmp,STCP_{mp,STC}WModule maximum power at STC
Modules Wired in SeriesNsN_s—Number of modules in series per string
Number of Parallel StringsNpN_p—Number of parallel strings in DC field
Initial Current at STCImp,STCI_{mp,STC}AMaximum power point current at STC (per string)

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Outputs

NameSymbolUnitsDescription
Combined CoefficientCcombC_{comb}—Combined loss coefficient applied to effective irradiance
Mismatch LossLMML_{MM}WPower loss due to module mismatch
LID LossLLIDL_{LID}WPower loss due to LID
Module Quality LossLMQL_{MQ}WPower loss due to module quality deviation
DC Health LossLDCHL_{DCH}WPower loss due to DC health factors
Effective Resistance at STCRs,effR_{s,eff}ΩEquivalent series resistance for DC wiring
Ohmic Power LossLohmicL_{ohmic}WPower loss in DC wiring

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Detailed Description

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Combined Coefficient

PlantPredict applies module mismatch, LID, module quality, and DC health losses as a combined coefficient that multiplies the effective irradiance components (direct, diffuse, and ground-reflected). All input percentages are converted to fractions before calculation. fMM=MismatchPercent100f_{MM} = \frac{\text{MismatchPercent}}{100} fLID=LIDPercent100f_{LID} = \frac{\text{LIDPercent}}{100} fMQ=ModuleQualityPercent100f_{MQ} = \frac{\text{ModuleQualityPercent}}{100} fDCH=DCHealthPercent100f_{DCH} = \frac{\text{DCHealthPercent}}{100} The combined coefficient is calculated as: Ccomb=(1−fMM)×(1−fMQ)×(1−fLID)×(1−fDCH)C_{comb} = (1 - f_{MM}) \times (1 - f_{MQ}) \times (1 - f_{LID}) \times (1 - f_{DCH}) This coefficient is applied to each irradiance component before power calculation: Geff,B=GB×CcombG_{eff,B} = G_{B} \times C_{comb} Geff,D=GD×CcombG_{eff,D} = G_{D} \times C_{comb} Geff,G=GG×CcombG_{eff,G} = G_{G} \times C_{comb} where GBG_B, GDG_D, and GGG_G are the beam, diffuse, and ground-reflected effective irradiance components respectively.

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DC Wiring Resistance Derivation

The user specifies DC wiring loss as a percentage of power at STC rather than directly inputting resistance in Ohms. PlantPredict derives the effective series resistance from this percentage input. First, the percentage is converted to a fraction: Lwire,STC=WiringLossPercent100L_{wire,STC} = \frac{\text{WiringLossPercent}}{100} The effective resistance at STC is then calculated: Rs,eff=Lwire,STC×Pmp,STC×Np×Ns(Imp,STC×Np)2R_{s,eff} = \frac{L_{wire,STC} \times P_{mp,STC} \times N_p \times N_s}{(I_{mp,STC} \times N_p)^2} This derivation assumes that the power loss percentage applies at the maximum power point current under STC conditions. The resulting resistance is added to the module’s series resistance during the single diode model solution.

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Individual Loss Calculations

Individual losses are calculated from the initial power (before losses) for reporting purposes: LMM=Pinitial×fMML_{MM} = P_{initial} \times f_{MM} LMQ=Pinitial×fMQL_{MQ} = P_{initial} \times f_{MQ} LLID=Pinitial×fLIDL_{LID} = P_{initial} \times f_{LID} LDCH=Pinitial×fDCHL_{DCH} = P_{initial} \times f_{DCH}

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Loss Application Sequence

DC losses are applied in the following sequence:
  1. Combined Coefficient: Applied to effective irradiance components before power calculation
  2. DC Wiring Resistance: Added to module series resistance during single diode model solution
  3. Degradation: Applied after DC power calculation (see Degradation Losses (DC Applied) or Degradation Losses (AC Applied))

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References

  • King, D. L., Boyson, W. E., & Kratochvil, J. A. (2004). Photovoltaic array performance model. SAND2004-3535, Sandia National Laboratories.