def efficiency(th):
    """
    Calculates thermal-to-electricity conversion efficiency.
    Parameters:
    -----------
    th: hot temperature [C]
    Returns:
    --------
    eta: Thermal-to-electricity conversion efficiency.
    """
    tc = 27  # C
    tc += 273  # K
    th += 273  # K
    eta = 1-tc/th
    eta *= 0.68
    return eta