def mean_irrad(arr):
    """Calc the annual irradiance at a site given an irradiance timeseries.

    Parameters
    ----------
    arr : np.ndarray | pd.Series
        Annual irradiance array in W/m2. Row dimension is time.

    Returns
    -------
    mean : float | np.ndarray
        Mean irradiance values in kWh/m2/day. Float if the input array is
        1D, 1darray if the input array is 2D (multi-site).
    """
    mean = arr.mean(axis=0) / 1000 * 24
    return mean
