code: def poa_horizontal_ratio(surface_tilt, surface_azimuth,
                           solar_zenith, solar_azimuth):
      
  
      cos_poa_zen = aoi_projection(surface_tilt, surface_azimuth,
                                   solar_zenith, solar_azimuth)
  
      cos_solar_zenith = tools.cosd(solar_zenith)
  
      # ratio of tilted and horizontal beam irradiance
      ratio = cos_poa_zen / cos_solar_zenith
  
      try:
          ratio.name = 'poa_ratio'
      except AttributeError:
          pass
  
      return ratio
docs: Calculates the ratio of the beam components of the plane of array
      irradiance and the horizontal irradiance.
  
      Input all angles in degrees.
  
      Parameters
      ----------
      surface_tilt : numeric
          Panel tilt from horizontal.
      surface_azimuth : numeric
          Panel azimuth from north.
      solar_zenith : numeric
          Solar zenith angle.
      solar_azimuth : numeric
          Solar azimuth angle.
  
      Returns
      -------
      ratio : numeric
          Ratio of the plane of array irradiance to the horizontal plane
          irradiance