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In this paper a model of the efficiency of twisted- wire planar inductors for domestic induction heating appliances is derived. This efficiency is determined by the relationship between the power dissipated by eddy currents in the vessel and the losses in the windings. The model includes the dependences of several parameters of the induction system such as the excitation current frequency, the properties of the load, the magnetic substrate characteristics, the number of turns of the winding and the wire parameters. The twisted-multistranded wires show an unequal current distribution, producing an increase of the resistance with respect to the simple parallel of the constitutive strands. To analyze this excess of losses, an equivalent single-strand round-wire for the twisted multistranded one is used. The analysis shows that the losses in windings and the delivered power into the load are coupled by the magnetic field of the whole system. The coupling makes impossible an experimental validation of the model through two independent decoupled measurements (with the winding both loaded and not loaded). Thus, in order to verify experimentally the theoretical predictions, a mixed method, combining calculations and measurements is proposed.