Returns the total complex power demand.

def s_demand(self, bus):
        """ Returns the total complex power demand.
        """
        Svl = array([complex(g.p, g.q) for g in self.generators if
                    (g.bus == bus) and g.is_load], dtype=complex64)

        Sd = complex(bus.p_demand, bus.q_demand)

        return -sum(Svl) + Sd