The global irradiation onto the inclined plane $E_\text{GTI}$ (Global Tilted Irradiance) results to:

$$E_\text{GTI} = E_\text{BTI} + E_\text{DTI} + E_\text{RTI}$$

The direct radiation to the horizontal $E_\text{BHI}$ (Beam Horizontal Irradiance) can be calculated from the geometric conditions (cosine) over the elevation angle of the $\gamma_\text{M}$ to the inclined plane to the direct radiation to the inclined plane $E_\text{BTI}$ (Beam Tilted Irradiance).

For more information on reflected radiation onto the inclined plane $E_\text{RTI}$ (Reflected Tilted Irradiance), see Calculation > Irradiation > Ground reflection.

For the calculation of diffuse irradiation onto the inclined plane $E_\text{DTI}$ (Diffuse Tilted Irradiance) the following models are available:

#### Hay & Davies (Default model)

In previous versions of PV*SOL® only this model was available

Hay, J. E., & Davies, J. A. (1980). Calculation of the Solar Radiation Incident on an Inclined Surface. In Proceedings of First Canadian Solar Radiation Data Workshop. Toronto.

#### Liu & Jordan

Liu, B. Y. H., & Jordan, R. C. (1960). The interrelationship and characteristic distribution of direct, diffuse and total solar radiation. Solar Energy, 4(3), 1–19. https://doi.org/10.1016/0038-092X(60)90062-1

#### Klucher

Klucher, T. M. (1979). Evaluation of models to predict insolation on tilted surfaces. Solar Energy, 23(2), 111–114. https://doi.org/10.1016/0038-092X(79)90110-5

#### Perez

Perez, R., Seals, R., Ineichen, P., Stewart, R., & Menicucci, D. (1987). A new simplified version of the perez diffuse irradiance model for tilted surfaces. Solar Energy, 39(3), 221–231. https://doi.org/10.1016/S0038-092X(87)80031-2

#### Reindl

Reindl, D. T., Beckman, W. A., & Duffie, J. A. (1990). Evaluation of hourly tilted surface radiation models. Solar Energy, 45(1), 9–17. https://doi.org/10.1016/0038-092X(90)90061-G