Abstract: In low-power AC networks, it is desirable when a load is connected, for example an arc furnace or a vehicle, to know the load capacity of the AC network in order to avoid undesirable drops in the network voltage U.sub.N. To this end, load voltage signals which are proportional to the amplitude of a load voltage U.sub.L are detected at the point of the load L at a 1st measurement time which can be predetermined and at a 2nd measurement time which can be predetermined and is later. At the same time, load current signals are detected which are proportional to the amplitude of a load current I.sub.L. Furthermore, a phase difference .DELTA..phi. is determined, which is obtained with reference to the phase angle of the network voltage U.sub.N at the 1st measurement time. These measurements are used as the bases to calculate a magnitude of the network impedance of the AC network and the network phase angle .phi..sub.N between the network voltage U.sub.N and the network current. A maximum possible power P.sub.