The maximum transfer of electric energy to the metal in an arc furnace depends on the length of arc and the impedance of the electrical supply system from the generators to the arc itself. The use of directly-reduced sponge iron by continuous feeding results in long periods of flat-bath operation, when it is particularly important to keep a short high-current arc to get the heat into the metal rather than to the refractories, which would suffer excessive wear. The paper illustrates, by reference to a 125 tonne furnace, the method of assessing the optimum operating currents and power factors and the effects of differing powersupply systems. The importance of a low-impedance power system is illustrated, and the possibility of being unable to use the maximum furnace power without excessive refractory wear is noted. The particular problems of connecting arc-furnace loads to electrical supply systems are reviewed, and consideration is given to the problem of voltage flicker. The use of compensators is discussed with reference to existing installations, in which strong supplies from the supply-authority system are not economically available. The furnace operating characteristics, which indicate the optimum points of working, have to be checked on commissioning, and the paper outlines the test procedures. The optimum points for each type of charge and steel can be assessed only during their actual production. The importance of proper recording of relevant data is stressed, and reference is made to the use of computers and automatic power-input controllers.