A wet porous slab was divided into a dried zone and a wet zone during dielectric drying, and an "accurate method" was proposed to predict thermal power generated in each zone for both microwave drying and high-frequency drying. Furthermore, a "uniform material (UM) approximate method" and a "uniform electric field intensity (UEI) approximate method" were proposed through simplifications of the accurate method. The two approximate methods were compared with the accurate method through calculation examples. As a result, in a low range of moisture content, the thermal power generated in each zone can be predicted by the UEI approximate method, and the sum of the powers can be predicted by both approximate methods. On the other hand, in the high range of moisture content, the sum of the powers can be predicted only by the UM approximate method, while the thermal power generated in each zone and the sum of the powers cannot be predicted by the UEI approximate method.