The limiting efficiency of an intermediate band solar cell with a non-ideal band structure is analyzed using the principle of detailed balance. Firstly, the impact of finite band width on the optimum band gap design for AM1.5 spectrum is examined. It is found that the band width may be a determining factor in the optimum band gap arrangement, but that the degradation in efficiency due to the band width up to ∼200 meV is moderate. It is also found that the band width can determine which band gap combination gives the highest global limiting efficiency. Further to the intermediate band width modification, the inclusion of band tails, analogous to those present in amorphous materials is discussed in terms of realizing an intermediate band using quantum dot arrays. In this model the worst case scenario is assumed, that of low absorption, but maximum emissivity at the band tail edge of the intermediate band. Results show that, with multiple band gap combinations giving several local peaks, the one giving the global maximum efficiency changes with band width. When band tails are included in a materials system proposed for implementing an intermediate band solar cell via quantum dot miniband formation significant drop off in efficiency is seen. The results taken together suggest the intermediate band width and any band tails should be considered in designs for intermediate band solar cells.