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Summary

Degradation Losses (AC Applied) model the time-dependent reduction in PV system output due to module aging and performance decline. PlantPredict offers five degradation models—None, Linear DC, Non-Linear DC, Linear AC, and Stepped AC—differing in where the loss is applied (DC power upstream of the vs. AC power downstream of the inverter) and how the rate evolves over time (constant, continuous, or annual steps). This page documents the two AC-applied models and the optional AC-applied model. For DC-applied degradation, see Degradation Losses (DC Applied).

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Inputs

NameSymbolUnitsDescription
AC PowerPACP_{AC}WAC power from inverter efficiency model
Energization Datet0t_0datetimeBlock energization date (system commissioning)
Linear Degradation Raterdegr_{deg}%/yearAnnual degradation rate (linear or stepped)
LeTID Annual Rates[l0,l1,...,ln][l_0, l_1, ..., l_n]%/yearPer-year LeTID rates starting at year 0

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Outputs

NameSymbolUnitsDescription
Degraded AC PowerPAC,degP_{AC,deg}WAC power after degradation
Degradation LossLdegL_{deg}WPower loss due to degradation
LeTID LossLLeTIDL_{LeTID}WPower loss due to LeTID

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Detailed Description

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Application Point

When Linear AC or Stepped AC degradation is selected, degradation is applied to AC power at the array level, after summing inverter outputs and before auxiliary loads and transformer losses. Because degradation is applied after inverter conversion, AC-applied models do not affect behavior—unlike DC-applied models, which reduce the power the inverter sees and can change the operating point as modules age (see Degradation Losses (DC Applied)). Before the , the system is not yet commissioned, so all models set PAC,deg=0P_{AC,deg} = 0.

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No AC Degradation (None, Linear DC, Non-Linear DC)

When None, Linear DC, or Non-Linear DC degradation is selected, no AC-level degradation is applied: Udeg=0U_{deg} = 0 PAC,deg=PACP_{AC,deg} = P_{AC}

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Linear AC Degradation

Linear AC degradation applies a constant annual rate rdegr_{deg} over the system lifetime. Degradation accumulates from the energization date tonset=t0t_{onset} = t_0 when First Year Degradation is enabled, or from tonset=t0+1t_{onset} = t_0 + 1 year when disabled. The degradation coefficient is: Udeg=rdeg⋅ΔtU_{deg} = r_{deg} \cdot \Delta t where Δt=max⁡(t−tonset, 0)\Delta t = \max(t - t_{onset},\, 0) is the elapsed time expressed as a fractional number of years (using an 8760-hour year). The degraded power is: PAC,deg=(1−Udeg)×PACP_{AC,deg} = (1 - U_{deg}) \times P_{AC} Ldeg=PAC−PAC,deg=Udeg×PACL_{deg} = P_{AC} - P_{AC,deg} = U_{deg} \times P_{AC}

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Stepped AC Degradation

Stepped AC degradation applies the same constant annual rate rdegr_{deg} but in discrete annual increments rather than continuously. Degradation onset follows the same First Year Degradation logic as Linear AC: tonset=t0t_{onset} = t_0 when First Year Degradation is enabled, or tonset=t0+1t_{onset} = t_0 + 1 year when disabled. The degradation coefficient is: Udeg=rdeg×⌈Δh8760⌉U_{deg} = r_{deg} \times \left\lceil \frac{\Delta h}{8760} \right\rceil where Δh\Delta h is the number of hours since tonsett_{onset} (Δh=(t−tonset)×24\Delta h = (t - t_{onset}) \times 24 if t>tonsett > t_{onset}, Δh=0\Delta h = 0 otherwise) and ⌈⋅⌉\lceil \cdot \rceil denotes the ceiling function (round up to the nearest integer). Because of the ceiling, each degradation step takes effect at the start of the year—the first step applies immediately once t>tonsett > t_{onset}. With First Year Degradation enabled, the system degrades from the moment of energization. Versions 3–4 used a floor function (⌊⋅⌋\lfloor \cdot \rfloor) instead, which deferred each step to the end of the year. The degraded power is: PAC,deg=(1−Udeg)×PACP_{AC,deg} = (1 - U_{deg}) \times P_{AC} Ldeg=PAC−PAC,deg=Udeg×PACL_{deg} = P_{AC} - P_{AC,deg} = U_{deg} \times P_{AC}

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Light and Elevated Temperature Induced Degradation (LeTID)

is an additional degradation mechanism that can be enabled independently of the primary degradation model. Unlike conventional degradation, LeTID is partially reversible—modules typically degrade over the first few years of operation, then partially recover (Repins et al., 2020). Per-year rates lil_i can therefore be negative in later years to capture this recovery. LeTID losses are reported separately from primary degradation. When Linear AC or Stepped AC degradation is selected, LeTID is applied at the AC level. The algorithm uses the same cumulative approach as Non-Linear DC degradation (see DC degradation): ULeTID=∑i=0⌊Δt⌋−1li+(Δt−⌊Δt⌋+nleap365)×l⌊Δt⌋U_{LeTID} = \sum_{i=0}^{\lfloor \Delta t \rfloor - 1} l_i + \left( \Delta t - \lfloor \Delta t \rfloor + \frac{n_{leap}}{365} \right) \times l_{\lfloor \Delta t \rfloor} where Δt\Delta t is the elapsed time in fractional years (with leap days excluded from the year length), ⌊Δt⌋\lfloor \Delta t \rfloor is the number of complete years elapsed, and nleapn_{leap} is the count of February 29th occurrences between t0t_0 and tt. The first term sums the rates of all complete years; the second term pro-rates the current year’s rate. LLeTID=ULeTID×PACL_{LeTID} = U_{LeTID} \times P_{AC} When both primary degradation and LeTID are active, their losses are additive: PAC,deg=PAC−Ldeg−LLeTIDP_{AC,deg} = P_{AC} - L_{deg} - L_{LeTID}

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References

  • Jordan, D. C., & Kurtz, S. R. (2013). Photovoltaic degradation rates—an analytical review. Progress in Photovoltaics: Research and Applications, 21(1), 12–29. DOI: 10.1002/pip.1182
  • Repins, I., et al. (2020). Light and elevated temperature induced degradation (LeTID) in a utility-scale photovoltaic system. IEEE Journal of Photovoltaics, 10(4), 1084–1092. DOI: 10.1109/JPHOTOV.2020.2989168
  • Kersten, F., Engelhart, P., et al. (2015). Degradation of multicrystalline silicon solar cells and modules after illumination at elevated temperature. Solar Energy Materials and Solar Cells, 142, 83–86. DOI: 10.1016/j.solmat.2015.06.015