<p>Photoelectrochemical systems harness onsite solar energy to drive chemical processes, enabling improvements in sustainability and decarbonization. Photoelectrochemical systems have been extensively studied for reactions such as hydrogen production; however, competitive costs are difficult to attain due to the limited solar efficiency of low-cost photoelectrochemically stable materials. Building on this premise, we propose that applications that do not require high solar-efficiency materials to deliver meaningful throughput are needed for photoelectrochemical systems. Using rigorous thermodynamic modeling grounded in experimental data, we demonstrate the existence of such applications in chemical separations, which comprise processes critical to tackling global challenges in water treatment and resource recovery. Operating domains and scales at which photoelectrochemical separations utilizing low solar efficiency materials can be practical and cost-competitive against modular photovoltaic-electrochemical systems are identified. This study demonstrates that photoelectrochemical separations have a design space broader than classical applications, and establishes thermodynamic limits and targets, paving the way for real-world impact with photoelectrochemical technology.</p>