Optimal efficiency calculations for 3C-SiC- and 6H-SiC-based intermediate band (IB) solar cells are presented. Using detailed balance methods, it is shown that the conversion efficiency of IB SiC-based solar cells is very sensitive to IB position, type of spectrum, and concentration of the incident light. Under a 1000 suns concentration and AM1.5 spectrum irradiation, a maximum efficiency of 55.9% is theoretically achieved for 3C-SiC with the IB located at 0.79 eV up to the valence band of the host material. Under the same conditions, a theoretical maximum efficiency of 49.7% is achieved with an IB located at 0.84 eV below the conduction band of the 6H-SiC. Based on the obtained theoretical results, the formation of an isolated IB in the appropriate position is demonstrated for Ni-doped 3C-SiC and Mn-doped 6H-SiC using the density function theory method which leads to an enhancement in the absorption coefficient in the ranges of the solar spectrum.