def peak_load(self):
        """
        Cumulative peak load capacity of generators of this grid

        Returns
        -------
        float
            Ad-hoc calculated or cached peak load capacity
        """
        if self._peak_load is None:
            self._peak_load = sum(
                [_.peak_load.sum()
                 for _ in self.graph.nodes_by_attribute('load')])

        return self._peak_load