The hydrogen production from photocatalytic water decomposition is one of the best approaches that convert solar energy to hydrogen energy. At present, the hydrogen production from photocatalytic water decomposition has a low efficiency. In order to determine a valuable hydrogen production process from photocatalytic water decomposition in the aspect of energy conversion, the concept of critical conversion efficiency of light energy is proposed in this paper. Critical conversion efficiency of light energy is defined as the conversion efficiency of light energy when the hydrogen energy output is equal to an equivalent energy input of the system. The criterion introduced is that only if the conversion efficiency of light energy is greater than the critical one, an energy profit can be achieved in a system. A hypothetical system with 20,000 tons annual hydrogen production by using Na2S/Na2SO3 as the sacrificial reagent is analyzed and calculated in this paper to determine the critical conversion efficiency of light energy by means of energy balance, cumulative exergy balance, and emergy balance, respectively. The results show that the critical conversion efficiency of light energy determined by emergy balance is the most reasonable, which gives the figure around 15%.