Solar photocatalysis appears as a viable approach for the production of value-added chemicals from CO. However, up to now, there is no information on the influence of the light intensity on the product distribution of CO hydrogenation and the modeling of the actual local temperature at the catalytic sites for typical nanoparticulate photocatalysts. Herein, it is shown that for a photothermal catalyst containing a high density of homogeneously distributed Ru nanoparticles, the collective heating prevails, resulting in a homogeneous temperature distribution in the material that should be relatively close to that of the support and that can be measured macroscopically. Moreover, light intensity has a clear influence on product distribution due to the differences in the local temperature, and therefore, attention should be paid to stable operating conditions, temperature, and CO conversion that can result in remarkable differences in product selectivity for the same catalyst as a function of light intensity.