We present a study of the effect of illumination intensity on the performance of a photovoltaic-biased electrochemical (PV-EC) device for solar hydrogen production based on a triple junction thin film silicon solar cell. The influence of the illumination intensity was studied for the solar cell as well as for an integrated PV-EC device. We show that while the open circuit voltage decreases with a reduction in intensity, the triple junction solar cell still provides a sufficient voltage to drive spontaneous water splitting. Moreover, a slight improvement in the fill factor at lower intensities can relax the requirements to the utilized co-catalysts. As a consequence, the difference in the performance of PV-EC devices featuring very active (Pt/RuO2) and less active (Ni/Co3O4) catalyst materials decreases when the illumination intensity is reduced.