Thermodynamic analysis of energy conversion from light-to-chemical, light-to-electric and electric-to-chemical is presented by the case study of water photoelectrolysis on TiO₂ surface. It is demonstrated that at the current state-of-the-art energy conversion efficiency of water photoelectrolysis can be increased ∼17 times by separating the processes of solar-to-electric and electric-to-chemical energy conversion and optimizing them independently. This allows to mitigate a high overvoltage of oxygen evolution reaction with respect to thermodynamic $E_{O_2/H_{2}O}^0\hspace{0.17em}=1.23\hspace{0.17em}\text{V}$ potential as well as spectrally narrow absorbtivity of solar light by TiO₂ which determine the low efficiency (∼ 1.0%) of direct light-to-chemical energy conversion. Numerical estimates are provided illustrating practical principles for optimization of the solar energy conversion and storage processes.