Transposition factor
The Transposition Factor is the ratio of the incident irradiation (GlobInc) on the plane, to the horizontal irradiation (GlobHor). I.e. what you gain (or loose) when tilting the collector plane.
It may be defined in hourly, daily, monthly or yearly values.
It is computed by applying a transposition model (Hay or Perez) to the horizontal hourly values. The result depends namely on the diffuse irradiance.
See the tool Orientation optimization