Pure graphene is limited by its gapless energy band structure in catalytic reactions that are important for energy conversion applications. However, impurities can create a band gap and improve the electrical and photocatalytic properties of graphene, leading to a wide range of energy-related applications. In this study, the Kubo-Greenwood formula was used to investigate the thermopower properties of monolayer graphene doped with B, N, and Si atoms under various concentrations of impurities and magnetic fields. The impurity concentration was controlled by varying the number of unit cells. The electronic structure of the doped graphene are strongly influenced by the type and concentration of impurities, with a created energy gap that was sensitive to the impurity concentration. Furthermore, applying a magnetic field resulted in subband splitting and band gap reduction for all selected structures, regardless of the impurity type. The thermal properties of the doped structures are dependent on the impurity type, concentration, and magnetic field strength. It was observed that the thermal properties decreased with increasing impurity concentration and were affected by the magnetic field. The thermal properties of the nitrogen-doped structure is higher than those of the boron-doped and silicon-carbide-doped structures. Overall, these findings are significant for developing more efficient energy conversion devices.