Degradation Losses (DC Applied)

​

Summary

Degradation Losses (DC Applied) model the time-dependent reduction in PV system output due to module aging and performance decline. When Linear DC or Non-Linear DC degradation models are selected, PlantPredict applies degradation to DC power before inverter conversion. This page documents the DC-applied degradation algorithms. For AC-applied degradation (Linear AC and Stepped AC), see Degradation Losses (AC Applied).

​

Inputs

---

​

Outputs

---

​

Detailed Description

​

Application Point

When Linear DC or Non-Linear DC degradation is selected, degradation is applied to DC power at the inverter level, after DC field power calculation and before inverter efficiency conversion. This differs from AC-applied degradation models which are applied after the inverter output.

​

None (No Degradation)

When degradation model is set to None: $$U_{deg} = 0$$ $$P_{DC,out} = P_{DC,in}$$

​

Linear DC Degradation

Linear degradation applies a constant annual rate over the system lifetime. Delayed Onset Calculation: If First Year Degradation is disabled: $$t_{delayed} = t_0 + 1 \text{ year}$$ If First Year Degradation is enabled: $$t_{delayed} = t_0$$ Time Elapsed: $$\Delta t = \frac{(t - t_{delayed})}{8760 \times 60} \text{ (in years, from minutes)}$$ If $t < t_{delayed}$, then $\Delta t = 0$. Degradation Coefficient: $$U_{deg} = r_{deg} \times \Delta t$$ Degraded Power: $$P_{DC,out} = (1 - U_{deg}) \times P_{DC,in}$$ $$L_{deg} = P_{DC,in} - P_{DC,out}$$

​

Non-Linear DC Degradation

Non-linear degradation allows specification of different degradation rates for each year of operation. This model always applies degradation from the energization date (no delayed onset option). Years Elapsed (excluding leap days): First, calculate the number of complete days elapsed: $$n_{days} = \lfloor t - t_0 \rfloor + 1$$ where $\lfloor t - t_0 \rfloor$ is the integer number of days between timestamps. Count leap days (February 29th occurrences) in the elapsed period: $$n_{leap} = \text{count of February 29th in } [t_0, t]$$ Calculate elapsed years, excluding leap days: $$y = \frac{\Delta t_{min} - (n_{leap} \times 1440)}{8760 \times 60}$$ where $\Delta t_{min}$ is the total elapsed time in minutes from $t_0$ to $t$. Cumulative Degradation: The degradation coefficient sums all complete years plus a pro-rated portion of the current year: $$U_{deg} = \sum_{i=0}^{\lfloor y \rfloor - 1} r_i + \left( (y \mod 1) + \frac{n_{leap}}{365} \right) \times r_{\lfloor y \rfloor}$$ Degraded Power: $$P_{DC,out} = (1 - U_{deg}) \times P_{DC,in}$$ $$L_{deg} = P_{DC,in} - P_{DC,out}$$

​

Linear AC and Stepped AC (Brief Description)

Linear AC and Stepped AC degradation models apply degradation to AC power after inverter conversion rather than to DC power. These models are documented in detail in Degradation Losses (AC Applied).

• Linear AC: Applies continuous linear degradation to inverter AC output
• Stepped AC: Applies degradation in annual steps rather than continuously

When Linear AC or Stepped AC is selected, no DC-level degradation is applied; the DC power passes through unchanged to the inverter.

​

Light and Elevated Temperature Induced Degradation (LeTID)

LeTID is an additional degradation mechanism that can be enabled independently of the primary degradation model. When enabled, LeTID losses are applied at the same level as the primary degradation (DC for Linear DC/Non-Linear DC, AC for Linear AC/Stepped AC). LeTID Calculation: The LeTID algorithm follows the same cumulative approach as Non-Linear DC degradation: $$n_{leap} = \text{count of February 29th in } [t_0, t]$$ $$y = \frac{\Delta t_{min} - (n_{leap} \times 1440)}{8760 \times 60}$$ where $\Delta t_{min}$ is the total elapsed time in minutes from $t_0$ to $t$. $$U_{LeTID} = \sum_{i=0}^{\lfloor y \rfloor - 1} l_i + \left( (y \mod 1) + \frac{n_{leap}}{365} \right) \times l_{\lfloor y \rfloor}$$ Power After LeTID: $$P_{after,LeTID} = (1 - U_{LeTID}) \times P_{DC,in}$$ $$L_{LeTID} = P_{DC,in} - P_{after,LeTID}$$ When both primary degradation and LeTID are applied at the DC level: $$P_{DC,out} = P_{DC,in} - L_{deg} - L_{LeTID}$$

​

First Year Degradation Setting

The First Year Degradation setting controls whether degradation begins immediately at energization or is delayed by one year:

• Enabled (On): Degradation accumulation begins at the energization date ($t_0$)
• Disabled (Off): Degradation accumulation begins one year after energization ($t_0 + 1$ year)

This setting applies to Linear DC, Linear AC, and Stepped AC models. Non-Linear DC degradation always begins at energization.

​

Use Leap Years Setting

The Use Leap Years setting controls how elapsed time is calculated for degradation:

• Enabled (On): Leap days (February 29th) are included in elapsed time calculations
• Disabled (Off): Leap days are excluded, using a standard 365-day year (8760 hours)

When disabled, the algorithm counts leap day occurrences in the period and subtracts them from the total elapsed time, then adds a fractional adjustment to the current year’s degradation calculation.

---

​

References

• Jordan, D. C., & Kurtz, S. R. (2013). Photovoltaic degradation rates—an analytical review. Progress in Photovoltaics: Research and Applications, 21(1), 12-29.
• Jordan, D. C., Silverman, T. J., Wohlgemuth, J. H., Kurtz, S. R., & VanSant, K. T. (2017). Photovoltaic failure and degradation modes. Progress in Photovoltaics: Research and Applications, 25(4), 318-326.
• Kersten, F., Engelhart, P., Ber, H. C., 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.