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irradiance is the total irradiance incident on the tilted module surface. It is calculated by combining transposed irradiance components (beam, sky diffuse, ground-reflected) with shading factors, losses, incidence angle modifiers, and spectral corrections. For modules, rear-side irradiance is calculated separately and combined with front-side irradiance.

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Models in This Section

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Soiling

Accounts for dust, dirt, and other debris accumulation on the module surface that reduces light transmission. PlantPredict supports monthly soiling values and integration with PVRADAR soiling data.

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Incidence Angle Modifier (IAM)

Optical losses that increase as light strikes the module surface at oblique angles. Separate factors are applied to beam, sky diffuse, and ground-reflected components. PlantPredict supports ASHRAE, physical, and Sandia IAM models.

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Spectral Correction

Adjusts for between the actual solar spectrum and the reference spectrum used for module characterization. Different module technologies (c-Si, CdTe, CIGS, etc.) have different spectral responses, making some more sensitive to spectral shifts than others. The correction accounts for atmospheric conditions (, aerosols, ) that shift the solar spectrum throughout the day and year.

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Rear Irradiance

Calculates rear-side irradiance for bifacial modules based on ground-reflected light, sky diffuse, and direct beam and circumsolar (when the sun is behind the module) reaching the rear surface. The effective POA irradiance combines front and rear contributions weighted by the module’s .

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Calculation Sequence

POA irradiance is calculated through a series of transformations:
  1. Transposition: Convert horizontal irradiance (, , ) to tilted plane components using Hay-Davies or Perez model
  2. Horizon Shading: Apply far-field shading to beam component
  3. Geometric Shading: Apply near-field shading factors to beam, sky diffuse, and ground-reflected components
  4. Soiling: Apply soiling factor to all components
  5. Incidence Angle Modifier: Apply angle-dependent optical losses
  6. Spectral Correction: Apply spectral mismatch adjustment
  7. Rear Irradiance (if applicable): Calculate rear-side irradiance and combine with front-side

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Front POA Irradiance Components

The front-side POA irradiance GPOA,frontG_{POA,front} is the sum of transposed beam, sky diffuse, and ground-reflected components before any module-level optical corrections (shading, soiling, IAM, spectral): GPOA,front=Gbeam+Gsky+GgroundG_{POA,front} = G_{beam} + G_{sky} + G_{ground} The front-side effective POA irradiance GPOA,front,effG_{POA,front,eff} is the total after all optical corrections: GPOA,front,eff=Gbeam,eff+Gsky,eff+Gground,effG_{POA,front,eff} = G_{beam,eff} + G_{sky,eff} + G_{ground,eff} Beam component: Gbeam,eff=Gbeam×Uhorizon×Ushd,B×Usoil×UIAM,B×UspectrG_{beam,eff} = G_{beam} \times U_{horizon} \times U_{shd,B} \times U_{soil} \times U_{IAM,B} \times U_{spectr} Sky diffuse component: Gsky,eff=Gsky×Ushd,D×Usoil×UIAM,D×UspectrG_{sky,eff} = G_{sky} \times U_{shd,D} \times U_{soil} \times U_{IAM,D} \times U_{spectr} Ground-reflected component: Gground,eff=Gground×Ushd,G×Usoil×UIAM,G×UspectrG_{ground,eff} = G_{ground} \times U_{shd,G} \times U_{soil} \times U_{IAM,G} \times U_{spectr} where:
  • Gbeam,Gsky,GgroundG_{beam}, G_{sky}, G_{ground} are transposed irradiance components
  • UhorizonU_{horizon} is the horizon shading factor (beam only)
  • Ushd,B,Ushd,D,Ushd,GU_{shd,B}, U_{shd,D}, U_{shd,G} are geometric shading factors
  • UsoilU_{soil} is the soiling factor
  • UIAM,B,UIAM,D,UIAM,GU_{IAM,B}, U_{IAM,D}, U_{IAM,G} are incidence angle modifier factors
  • UspectrU_{spectr} is the spectral correction factor

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Effective POA Irradiance

For bifacial modules, the effective POA irradiance combines front and rear contributions: GPOA,tot,eff=GPOA,front,eff+GPOA,rear,effG_{POA,tot,eff} = G_{POA,front,eff} + G_{POA,rear,eff} where GPOA,rear,effG_{POA,rear,eff} includes bifaciality weighting, structure shading, and mismatch losses (see Rear Irradiance). For monofacial modules: GPOA,tot,eff=GPOA,front,effG_{POA,tot,eff} = G_{POA,front,eff}