Calculates the SAPM angle of incidence loss coefficient, F2.

    Parameters
    ----------
    aoi : numeric
        Angle of incidence in degrees. Negative input angles will return
        zeros.

    module : dict-like
        A dict, Series, or DataFrame defining the SAPM performance
        parameters. See the :py:func:`sapm` notes section for more
        details.

    upper : None or float, default None
        Upper limit on the results.

    Returns
    -------
    F2 : numeric
        The SAPM angle of incidence loss coefficient.

    Notes
    -----
    The SAPM traditionally does not define an upper limit on the AOI
    loss function and values slightly exceeding 1 may exist for moderate
    angles of incidence (15-40 degrees). However, users may consider
    imposing an upper limit of 1.

    References
    ----------
    [1] King, D. et al, 2004, "Sandia Photovoltaic Array Performance
    Model", SAND Report 3535, Sandia National Laboratories, Albuquerque,
    NM.

    [2] B.H. King et al, "Procedure to Determine Coefficients for the
    Sandia Array Performance Model (SAPM)," SAND2016-5284, Sandia
    National Laboratories (2016).

    [3] B.H. King et al, "Recent Advancements in Outdoor Measurement
    Techniques for Angle of Incidence Effects," 42nd IEEE PVSC (2015).
    DOI: 10.1109/PVSC.2015.7355849

def sapm_aoi_loss(aoi, module, upper=None):
    """
    Calculates the SAPM angle of incidence loss coefficient, F2.

    Parameters
    ----------
    aoi : numeric
        Angle of incidence in degrees. Negative input angles will return
        zeros.

    module : dict-like
        A dict, Series, or DataFrame defining the SAPM performance
        parameters. See the :py:func:`sapm` notes section for more
        details.

    upper : None or float, default None
        Upper limit on the results.

    Returns
    -------
    F2 : numeric
        The SAPM angle of incidence loss coefficient.

    Notes
    -----
    The SAPM traditionally does not define an upper limit on the AOI
    loss function and values slightly exceeding 1 may exist for moderate
    angles of incidence (15-40 degrees). However, users may consider
    imposing an upper limit of 1.

    References
    ----------
    [1] King, D. et al, 2004, "Sandia Photovoltaic Array Performance
    Model", SAND Report 3535, Sandia National Laboratories, Albuquerque,
    NM.

    [2] B.H. King et al, "Procedure to Determine Coefficients for the
    Sandia Array Performance Model (SAPM)," SAND2016-5284, Sandia
    National Laboratories (2016).

    [3] B.H. King et al, "Recent Advancements in Outdoor Measurement
    Techniques for Angle of Incidence Effects," 42nd IEEE PVSC (2015).
    DOI: 10.1109/PVSC.2015.7355849
    """

    aoi_coeff = [module['B5'], module['B4'], module['B3'], module['B2'],
                 module['B1'], module['B0']]

    aoi_loss = np.polyval(aoi_coeff, aoi)
    aoi_loss = np.clip(aoi_loss, 0, upper)
    # nan tolerant masking
    aoi_lt_0 = np.full_like(aoi, False, dtype='bool')
    np.less(aoi, 0, where=~np.isnan(aoi), out=aoi_lt_0)
    aoi_loss = np.where(aoi_lt_0, 0, aoi_loss)

    if isinstance(aoi, pd.Series):
        aoi_loss = pd.Series(aoi_loss, aoi.index)

    return aoi_loss