where and are the solar azimuth and zenith angles, respectively. and are the tilt and azimuth angles of the array, respectively.
Azimuth angle convention is defined as degrees east of north (e.g. North = 0, East = 90, West = 270). Array azimuth is defined as the horizontal normal vector from the array surface. An array facing south has an array azimuth of 180 deg. Array tilt is defined as the angle from horizontal.