We have investigated the influence of simulated outdoor illumination conditions on the functionality of photovoltaic-biased electrosynthetic (PV-EC) systems used for the production of hydrogen as a renewable and storable fuel via solar water splitting. Thin-film multijunction solar cells were adopted for the PV part of the device together with an electrosynthetic cell with a Pt/IrOx catalyst pair in a 1 molar potassium hydroxide electrolyte solution. We studied the influence of the incident illumination angle on the solar-to-hydrogen efficiency and have given a first evaluation of the long-term (1 year) performance of PV-EC systems in terms of the hydrogen volume produced for a given geographical location. In this approach, variations from the standard AM1.5G type illumination expressed as changes in the average photon energy of the spectra were used to simulate different geographical locations as well as seasonal and daily changes in the illumination spectra. Finally, we compared the impact of various types of multijunction photovoltaic devices (tandem, triple, and quadruple junctions) on the annual solar hydrogen production.