Magnetrons are the most common microwave power sources. However, variation of output frequencies of magnetrons makes heating patterns of foods in multi-mode microwave cavities random and unpredictable. It is desirable to find effective alternatives, such as solid-state microwave generators. Here, we compared the frequency spectra of magnetrons and solid-state generators to evaluate their influence on the heating performance of microwave ovens. Results showed that the spectrum (i.e., peak frequency and bandwidth) of microwaves from the magnetron varied depending on the food and food position and varied considerably between individual ovens of the same model. In contrast, the solid-state microwave generator provided microwaves not only at exactly the set frequency but also within a narrow band, regardless of food loads. That is, solid-state generators had better spectral quality than that from magnetrons. As a result, solid-state generators would provide predictable and stable heating patterns of foods that cannot be achieved with magnetrons. Therefore, solid-state generators create new opportunities in designing next-generation microwave systems with high heating performance. Heating patterns of foods in magnetron-powered multi-mode cavities are random and unpredictable, limiting the use of microwave heating to address food safety issues. Solid-state microwave generators have the potential to overcome this drawback by providing better spectral quality than magnetrons, as shown in this study. In addition, the methodology developed in this work to measure the frequency spectrum of microwave generators is helpful in improving the accuracy of computer simulations. This study provides fundamental information and useful guidance to the food industry on designing and mathematically modelling solid-state powered microwave heating systems.