# What equation does EnergyPlus use to calculate Surface Outside Face Beam Solar Incident Angle Cosine Value?

The equation used in the EnergyPlus source code for Surface Outside Face Beam Solar Incident Angle Cosine Value calculation does not match other sources for this same value.

Energy Plus: cos(θ_i ) = sin(α)*sin(β) + cos(α)*cos(β)*cos(γ-γ_s)

Other Sources: cos(θ_i ) = sin(α)*cos(β) + cos(α)*sin(β)*cos(γ-γ_s)

θ_i=angle of incidence

α=solar altitude

β=surface tilt

γ=wall azimuth

γ_s=solar azimuth

The EnergyPlus Reference manual does not explicitly state this equation. Is there another reference that states how EnergyPlus calculates Surface Outside Face Beam Solar Incident Angle Cosine Value?

@mademarc Where do you get the EnergyPlus equation quoted above? In the code, I only see

`CTHETA`

calculated as a product of the solar direction cosines and the surface outward facing normal vector.