@classmethod
def annual_useable_generation(cls, genmix, gentypes):
    """
        Returns the useable generation in TWh for genmix.
        """
    result = genmix[genmix.index.isin(gentypes)]['Generation (TWh)'].sum()
    if 'Curtailment' in genmix.index:
        result += genmix.loc['Curtailment', 'Generation (TWh)']
    return result
