def normalized_efficiency(P, G_poa, P_ref, G_ref=1000):
    """
    Compute normalized efficiency defined as:

    :math:`NE = \\dfrac{\\dfrac{P}{P_{ref}}}{\\dfrac{G_{poa}}{G_{ref}}}`
    
    where 
    :math:`P` is the observed power (AC or DC), 
    :math:`P_{ref}` is the DC power rating at STC conditions, 
    :math:`G_{poa}` is the plane-of-array irradiance, and 
    :math:`G_{ref}` is the reference irradiance.
    
    Parameters
    -----------
    P : pandas Series or float
        Power (AC or DC) 
        
    G_poa : pandas Series or float
         Plane of array irradiance
         
    P_ref : float
        DC power rating at STC conditions
        
    G_ref : float (optional)
        Reference irradiance, default = 1000
        
    Returns
    -------
    pandas Series or float
        Normalized efficiency in a pandas Series (if P or G_poa are Series) or 
        float (if P and G_poa are floats) 
    """
    Yf = P / P_ref
    Yr = G_poa / G_ref
    NE = Yf / Yr
    return NE
