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

def poa_horizontal_ratio(surface_tilt, surface_azimuth,
                         solar_zenith, solar_azimuth):
    """
    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
    """

    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