> ## 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.

# Horizontal Irradiance Components Pre-Processing

export const PlaneOfArrayIrradiance = () => <Tooltip tip="Solar irradiance on the tilted module surface (beam + diffuse + reflected); drives PV power generation.">
    plane-of-array irradiance
  </Tooltip>;

export const ClosureEquation = () => <Tooltip tip="GHI = DNI × cos(θz) + DHI; relates the three horizontal irradiance components.">
    closure equation
  </Tooltip>;

export const ZenithAngle = () => <Tooltip tip="Angle between the local vertical and the sun (0° = overhead, 90° = horizon).">
    zenith angle
  </Tooltip>;

export const Decomposition = () => <Tooltip tip="Separating GHI into direct (DNI) and diffuse (DHI) components using empirical models.">
    decomposition
  </Tooltip>;

export const Transposition = () => <Tooltip tip="Converting horizontal irradiance components to plane-of-array (tilted surface) irradiance.">
    transposition
  </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 GHI = () => <Tooltip tip="Global Horizontal Irradiance: total solar radiation on a horizontal surface (direct + diffuse).">
    GHI
  </Tooltip>;

## Summary

Irradiance at a given site is typically reported as three complementary components: Global Horizontal Irradiance (<GHI />), Direct Normal Irradiance (<DNI />), and Diffuse Horizontal Irradiance (<DHI />). PlantPredict validates these components and pre-processes them before passing them to <Transposition /> models.

When the **Frontside POAI** toggle is enabled, this pre-processing is bypassed—measured <PlaneOfArrayIrradiance /> is reverse-decomposed into horizontal components via the [POAI Diffuse-Direct Decomposition](/models/horizontal-irradiance/poai_diffuse_direct_decomposition) model instead.

## Inputs

| Name                              | Symbol     | Units   | Description                                                   |
| --------------------------------- | ---------- | ------- | ------------------------------------------------------------- |
| **Global Horizontal Irradiance**  | $GHI$      | W/m²    | Total irradiance on horizontal surface from weather data      |
| **Direct Normal Irradiance**      | $DNI$      | W/m²    | Beam irradiance perpendicular to sun's rays from weather data |
| **Diffuse Horizontal Irradiance** | $DHI$      | W/m²    | Diffuse irradiance on horizontal surface from weather data    |
| **Solar Zenith Angle**            | $\theta_z$ | degrees | Angle between sun and local vertical                          |

***

## Outputs

| Name              | Symbol | Units | Description                                             |
| ----------------- | ------ | ----- | ------------------------------------------------------- |
| **Processed GHI** | $GHI$  | W/m²  | Global horizontal irradiance (validated or calculated)  |
| **Processed DNI** | $DNI$  | W/m²  | Direct normal irradiance (validated or calculated)      |
| **Processed DHI** | $DHI$  | W/m²  | Diffuse horizontal irradiance (validated or calculated) |

***

## Detailed Description

Irradiance components are:

* **Global Horizontal Irradiance (GHI)**: Total solar radiation on a horizontal surface from the entire sky hemisphere, including direct beam, diffuse sky radiation, and reflected radiation.
* **Direct Normal Irradiance (DNI)**: Solar radiation on a surface perpendicular to the sun's rays, representing only the direct beam component.
* **Diffuse Horizontal Irradiance (DHI)**: Solar radiation from the sky dome after scattering by atmospheric constituents, excluding the direct beam.

The three components are related by the <ClosureEquation />:

$$
GHI = DNI \cos(\theta_z) + DHI
$$

where $\theta_z$ is the solar <ZenithAngle />.

When Frontside POAI is **off** (standard path), PlantPredict processes irradiance data in the following order:

1. **Validate GHI**: Negative GHI values are set to zero
2. **Resolve DHI**:
   * If <Decomposition /> model selected (Erbs, Reindl, or DIRINT) → calculate DHI from GHI (overwrites any existing DHI)
   * If no decomposition model → use DHI from weather data (prediction will fail if missing)
3. **Resolve DNI**:
   * If DNI is missing or the "Weather File DNI" option is set to OFF, calculate DNI from closure: DNI = (GHI − DHI) / cos(θz). If GHI ≤ DHI or θz > 87.9°, DNI = 0.
   * Else use DNI from weather data
4. **Pass to transposition**: Validated components feed into <Transposition /> models

When Frontside POAI is **on**, all of the above steps are skipped. The [POAI Diffuse-Direct Decomposition](/models/horizontal-irradiance/poai_diffuse_direct_decomposition) (GTI-DIRINT) algorithm derives GHI, DNI, and DHI from the measured POAI, and these derived values are passed to transposition.

***

## References

* Duffie, J. A., & Beckman, W. A. (2013). *Solar Engineering of Thermal Processes* (4th ed.). John Wiley & Sons. DOI: [10.1002/9781118671603](https://doi.org/10.1002/9781118671603)
* Iqbal, M. (1983). *An Introduction to Solar Radiation.* Academic Press.
* Maxwell, E., Wilcox, S., & Rymes, M. (1993). *User's Manual for SERI QC Software: Assessing the Quality of Solar Radiation Data.* NREL/TP-463-5608. DOI: [10.2172/10125711](https://doi.org/10.2172/10125711)