Finding new strategies to enable the transition away from fossil fuel-based energy to the efficient use of sustainable resources is a global challenge. The implication is to capitalize on solar energy. Nevertheless, for an eventual transition away from fossil fuels, strategies for storage beyond atteries are required for the grid management and most efficient use of solar energy on demand. An alternative storage strategy involves the development of nonstoichiometric metal oxide materials with a high capacity to incorporate and release oxygen through redox reactions steered by the changes in temperature through thermal solar energy cycles. The resulting stoichiometry changes can be used for syngas (CO + H2) conversion when coupled with the introduction of appropriate reactant gases such as H2O and CO2 in the oxidation cycles;higher liquid fuels can also be converted and stored through Fischer-Tropsch or choosing catalytically active reactor materials directly. State-of-the-art in this novel and very promising energy technology is to use metal oxides such as CeOx or FeOx. Based on defect thermodynamic consideration we predict, synthesize and test the solar-to-fuel splitting performance of first perovskites in this field. Based on cationic doping we can directly implicate the oxygen non-stoichiometry and activity of the material to efficiently split H2O and CO2; viz. systematic material model cases can be generated to gain a defect chemical understanding on the split efficiencies and fuel yields to give future engineering guidelines for the reactor materials.