def calculate_storage_u_value(s_iso, lamb_iso, alpha_inside, alpha_outside):
    """
    Calculates the thermal transmittance (U-value) of a thermal storage.

    .. calculate_storage_u_value-equations:

    :math:`U = \\frac{1}{\\frac{1}{\\alpha_i} + \\frac{s_{iso}}{\\lambda_ {iso}} + \\frac{1}{\\alpha_a}}`

    Parameters
    ----------
    s_iso : numeric
        Thickness of isolation layer [mm]

    lamb_iso : numeric
        Thermal conductivity of isolation layer [W/(m*K)]

    alpha_inside : numeric
        Heat transfer coefficient at the inner surface of the storage [W/(m2*K)]

    alpha_outside : numeric
        Heat transfer coefficient at the outer surface of the storage [W/(m2*K)]

    Returns
    -------
    u_value : numeric
        Thermal transmittance (U-value) [W/(m2*K)]
    """
    denominator = (1 / alpha_inside + s_iso * 0.001 / lamb_iso + 1 /
        alpha_outside)
    u_value = 1 / denominator
    return u_value
