The detailed balance model for semiconductor solar cells is carefully examined and extended to include the impact of nonradiative recombination and other aspects of real solar cell devices, such as photon recycling, spontaneous emission coupling, and less than unity absorptance and emittance. The limiting efficiencies for single and multi-junction solar cells are analyzed taking nonradiative recombination and real material properties into account. Four fundamental loss mechanisms are clarified and discussed, including carrier and solar radiation losses, and as well carrier energy given up to thermalization and spatial relaxation. The fraction of solar power extracted and the fraction lost are investigated as a function of critical device parameters. Quantitative analysis shows that i) SRH recombination and spatial relaxation losses increase as the material quality decreases and ii) the optimal bandgap increases as the material quality decreases.