> ## Documentation Index
> Fetch the complete documentation index at: https://docs.plantpredict.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Plane-of-Array Irradiance Overview

export const BifacialityFactor = () => <Tooltip tip="Ratio of rear-side efficiency to front-side efficiency (typically 0.65–0.90); used to weight rear-side irradiance.">
    bifaciality factor
  </Tooltip>;

export const PrecipitableWater = () => <Tooltip tip="Total water vapor in an atmospheric column, expressed as equivalent depth of liquid water (typically 0.5–5 cm).">
    precipitable water
  </Tooltip>;

export const AirMass = () => <Tooltip tip="Ratio of atmospheric path length to path length at zenith; equals 1 when sun is overhead.">
    air mass
  </Tooltip>;

export const SpectralMismatch = () => <Tooltip tip="Difference between actual solar spectrum and reference spectrum (AM1.5G), affecting module performance vs. rated efficiency.">
    spectral mismatch
  </Tooltip>;

export const Bifacial = () => <Tooltip tip="PV module that absorbs light from both front and rear surfaces, generating additional energy from ground-reflected irradiance.">
    bifacial
  </Tooltip>;

export const Soiling = () => <Tooltip tip="Accumulation of dust, dirt, and debris on module surfaces, reducing light transmission and energy production.">
    soiling
  </Tooltip>;

export const IAM = () => <Tooltip tip="Incidence Angle Modifier: correction factor for optical losses at non-perpendicular light angles.">
    IAM
  </Tooltip>;

export const GHI = () => <Tooltip tip="Global Horizontal Irradiance: total solar radiation on a horizontal surface (direct + diffuse).">
    GHI
  </Tooltip>;

export const DHI = () => <Tooltip tip="Diffuse Horizontal Irradiance: solar radiation from the sky dome (excluding direct beam) on a horizontal surface.">
    DHI
  </Tooltip>;

export const DNI = () => <Tooltip tip="Direct Normal Irradiance: solar radiation received perpendicular to the sun's rays (direct beam only).">
    DNI
  </Tooltip>;

export const POA = () => <Tooltip tip="Plane-of-Array: solar irradiance on the tilted module surface (beam + diffuse + reflected).">
    POA
  </Tooltip>;

<POA /> 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, <Soiling /> losses, incidence angle modifiers, and spectral corrections. For <Bifacial /> modules, rear-side irradiance is calculated separately and combined with front-side irradiance.

## Models in This Section

### [Soiling](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.

### [Incidence Angle Modifier (IAM)](iam)

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

### [Spectral Correction](spectral_correction)

Adjusts for <SpectralMismatch /> 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 (<AirMass />, aerosols, <PrecipitableWater />) that shift the solar spectrum throughout the day and year.

### [Rear Irradiance](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 <BifacialityFactor />.

## Calculation Sequence

POA irradiance is calculated through a series of transformations:

1. **[Transposition](/models/transposition/overview)**: Convert horizontal irradiance (<GHI />, <DNI />, <DHI />) to tilted plane components using Hay-Davies or Perez model
2. **[Horizon Shading](/models/shading/horizon_shading)**: Apply far-field shading to beam component
3. **[Geometric Shading](/models/shading/overview)**: Apply near-field shading factors to [beam](/models/shading/3d_shading_site_level), [sky diffuse](/models/shading/sky_diffuse_shading), and [ground-reflected](/models/shading/ground_reflected_shading) components
4. **[Soiling](/models/poa-irradiance/soiling)**: Apply soiling factor to all components
5. **[Incidence Angle Modifier](/models/poa-irradiance/iam)**: Apply angle-dependent optical losses
6. **[Spectral Correction](/models/poa-irradiance/spectral_correction)**: Apply spectral mismatch adjustment
7. **[Rear Irradiance](/models/poa-irradiance/rear_irradiance)** (if applicable): Calculate rear-side irradiance and combine with front-side

## Front POA Irradiance Components

The **front-side POA irradiance** $G_{POA,front}$ is the sum of transposed beam, sky diffuse, and ground-reflected components before any module-level optical corrections (shading, soiling, IAM, spectral):

$$
G_{POA,front} = G_{beam} + G_{sky} + G_{ground}
$$

The **front-side effective POA irradiance** $G_{POA,front,eff}$ is the total after all optical corrections:

$$
G_{POA,front,eff} = G_{beam,eff} + G_{sky,eff} + G_{ground,eff}
$$

**Beam component:**

$$
G_{beam,eff} = G_{beam} \times U_{horizon} \times U_{shd,B,elec} \times U_{soil} \times U_{IAM,B} \times U_{spectr}
$$

**Sky diffuse component:**

$$
G_{sky,eff} = G_{sky} \times U_{shd,D} \times U_{soil} \times U_{IAM,D} \times U_{spectr}
$$

**Ground-reflected component:**

$$
G_{ground,eff} = G_{ground} \times U_{shd,G} \times U_{soil} \times U_{IAM,G} \times U_{spectr}
$$

where:

* $G_{beam}, G_{sky}, G_{ground}$ are transposed irradiance components
* $U_{horizon}$ is the horizon shading factor (beam only)
* $U_{shd,B,elec}, U_{shd,D}, U_{shd,G}$ are shading factors (beam includes [electrical shading effect](/models/shading/electrical_shading_effect))
* $U_{soil}$ is the soiling factor
* $U_{IAM,B}, U_{IAM,D}, U_{IAM,G}$ are incidence angle modifier factors
* $U_{spectr}$ is the spectral correction factor

## Effective POA Irradiance

For bifacial modules, the effective POA irradiance combines front and rear contributions:

$$
G_{POA,tot,eff} = G_{POA,front,eff} + G_{POA,rear,eff}
$$

where $G_{POA,rear,eff}$ includes bifaciality weighting, structure shading, and mismatch losses (see [Rear Irradiance](/models/poa-irradiance/rear_irradiance)).

For monofacial modules: $G_{POA,tot,eff} = G_{POA,front,eff}$