We developed a comprehensive detailed balance model of intermediate band solar cell (IBSC). The key feature of our model is based on the conservation of photons in solar spectrum. Together with parametric analysis of carrier partition, we calculated the power conversion efficiency and found an enhancement of 1.5 times in wide band gap material IBSC (such as GaN). On the other hand, this model can also explain the inferior performance of GaAs-based IBSC through the degradation of open-circuit voltages, which can be attributed to the strong non-radiative recombination and the increased photo-generated carriers. The resulting maximum efficiency is complied with the classical Shockley-Queisser limit, and should be considered for the future IBSC design.