A method for operating a microwave heating device , the microwave heating device comprising a heating chamber and at least two radiating portions that are adapted to radiate microwaves to the heating chamber , wherein the at least two radiating portions can be operated in a plurality of operational configurations that differ in frequency and in one or more phase shifts between the radiated microwaves, an energy efficiency being associated with each operational configuration, the method comprising: - positioning at least one product in the heating chamber ; - obtaining data of energy efficiency, as a function of operational configurations, relating to the at least one product in the heating chamber , the obtained data relating to a plurality of frequencies; - processing the obtained data so that, for each frequency of said plurality, an efficiency difference and an efficiency standard deviation are calculated, wherein, for the calculation of the efficiency difference, an operational configuration with a maximum energy efficiency and an operational configuration with a minimum energy efficiency at that frequency are singled out and an efficiency difference is calculated as the difference between the maximum energy efficiency and the minimum energy efficiency, and wherein, for the calculation of the efficiency standard deviation, a mean value of energy efficiency at that frequency is calculated and the standard deviation of energy efficiency at that frequency from the mean value is calculated; - selecting an operating frequency that optimizes a mathematical function that includes efficiency difference and efficiency standard deviation at the same frequency; and - choosing a plurality of operational configurations for heating the at least one product , the chosen operational configurations having the selected operating frequency and respective one or more phase shifts that are chosen around the corresponding one or more phase shifts of the operational configuration having the maximum energy efficiency at the selected operating frequency. - positioning at least one product in the heating chamber ; - obtaining data of energy efficiency, as a function of operational configurations, relating to the at least one product in the heating chamber , the obtained data relating to a plurality of frequencies; - processing the obtained data so that, for each frequency of said plurality, an efficiency difference and an efficiency standard deviation are calculated, wherein, for the calculation of the efficiency difference, an operational configuration with a maximum energy efficiency and an operational configuration with a minimum energy efficiency at that frequency are singled out and an efficiency difference is calculated as the difference between the maximum energy efficiency and the minimum energy efficiency, and wherein, for the calculation of the efficiency standard deviation, a mean value of energy efficiency at that frequency is calculated and the standard deviation of energy efficiency at that frequency from the mean value is calculated; - selecting an operating frequency that optimizes a mathematical function that includes efficiency difference and efficiency standard deviation at the same frequency; and - choosing a plurality of operational configurations for heating the at least one product , the chosen operational configurations having the selected operating frequency and respective one or more phase shifts that are chosen around the corresponding one or more phase shifts of the operational configuration having the maximum energy efficiency at the selected operating frequency. <EOS>