It should also be noted that solar concentration is very likely necessary for low-cost H2 production using PEC: modelling PEC without solar concentration shows that even for the limit case, LCOH2 does not drop below 15 $/kg(H2) (see SI Section 5.4). Solar concentrators will likely be significantly cheaper than PEC cells (per m 2 ) for the foreseeable future. Hence, replacing PEC cell area with solar concentrator area enables effective cost reduction. In the limit case, however, solar concentrator cost actually becomes the dominant factor in PEC CAPEX (see SI Figure . 3-2). With respect to research targets these results imply the following: the lifetime of PEC cells has to be significantly improved (reaching on the order of years) to address the high costs resulting from regular replacements. Furthermore, PEC cells should be optimized to operate under high solar concentration factors (> 50) to reduce the required cell area. This implies that highly active PEC materials (absorbers and electrocatalysts) are required which have a sufficient space-time yield to convert the high incoming energy flux (peak activity likely has to exceed 400 mol H2 h -1 m -2 (PEC cell)). It is uncertain if such progress is realistically achievable, especially to allow PEC to compete with PV+E (see below). For PC, a much larger region of the parameter space gives rise to models in the target cost range (see Figure ). For cost-competitive hydrogen production, STH efficiency has to be at least 6%. Catalyst concentration can be as high as 0.5 g(Catalyst)/L, but concentrations below 0.2 g(Catalyst)/L open up much more flexibility with regards to efficiency and cost. For most cases, catalyst cost has to be below 1000 $/kg(Catalyst), except when the catalyst concentration is very low (< 0.05 g(Catalyst)/L). In all cases, the catalyst lifetime is relatively short (between 0.5 and 1 year), which necessitates regular catalyst replacements. In contrast to PEC, however, replacing the catalyst is straightforward because no other components have to be replaced with it: it can be removed from the water/catalyst mixture (e.g. by nanofiltration ) and new catalyst is added. Looking at research targets, this data suggests that it is crucial to lower the catalyst concentration. Systems with low catalyst concentrations can tolerate lower STH efficiencies and higher catalyst costs, while reduction of the catalyst amount also reduces resource consumption for catalyst production. To achieve lower catalyst concentrations the main consideration is the absorption behavior of the photocatalyst: it needs to have a sufficiently high absorption cross section so that most sunlight is still absorbed even when lower concentrations are employed. The demands for catalytic activity are less stringent to enable low catalyst concentrations. Even for the limit case (highest efficiency, lowest concentration), peak catalytic activity does not have to exceed 7 mol H2 g -1 h -1 (activities on the order of 1 mol H2 g -1 h -1 have already been achieved ), which corresponds to a turnover frequency of < 1 s -1 for a homogeneous catalyst (assuming a molar mass of 500 g/mol). Hence, light absorption performance is likely more important than highly active catalytic sites for H2 and O2 evolution. To identify effective paths to a cost-competitive system, it insightful to look at the parameter values of the model with the shortest development distance that reaches the target cost range: with a distance of 0.48, this model has a STH efficiency of 17.5%, catalyst concentration of 0.01 g(Catalyst)/L, catalyst cost of 2950 $/kg(Catalyst) and catalyst lifetime of 0.52 years. Hence, high catalyst cost and low lifetime can be tolerated through the combination of high STH efficiency and low catalyst concentration. These characteristics point to the potential of homogenous photocatalysts, which can be used at low concentrations due to high molar absorptivity. Furthermore, it was recently shown that it is theoretically possible for homogeneous photocatalysts to achieve dual absorber STH efficiencies (> 20%) with a single catalyst. With progress on catalyst concentration and STH efficiency, there is a pathway for PC to cost-competitive hydrogen production. Due to the simple plastic baggie reactor construction, this approach has a low CAPEX (but rather high OPEX due to catalyst replacements, see SI Figures 4.2-1 and 4.3-1), which is complementary to PV+E (high CAPEX, low OPEX, see SI Figure .2-1) and could allow for easier construction of new plants. Safety issues due to generation of a H2/O2 mixture in the reactors have to be considered, but these risks were found to be manageable. PV+E shows a well-defined region of the parameter space that gives rise to models with an LCOH2 in the target cost range (Figure ). The symmetry and gradients indicate that each parameter (PV CAPEX, electrolyzer CAPEX, conversion efficiency) has a roughly linear effect on LCOH2 and there are no significant interactions between parameters (which was also shown by the linear development distance/LCOH2 relationship). To reach the target cost range, PV CAPEX has to be below 650 $/kW(PV) and electrolyzer CAPEX can be as high as 800 $/kW(Electrolyzer) but only if the conversion efficiency approaches the theoretical maximum of 0.025 kg(H2/kWh(Electricity). Improving the conversion efficiency in general opens up possibilities to tolerate higher PV and electrolyzer CAPEX. However, the model with shortest development distance in the target cost range (distance of 0.41) has the base case conversion efficiency and stack replacement cost (0.0185 kg(H2)/kWh(Electricity) and 40% of electrolyzer CAPEX), PV CAPEX of 300 $/kW(PV) and electrolyzer CAPEX of 330 $/kW(Electrolyzer). Hence, even without other improvements, it is possible to enable hydrogen production at the target cost by reducing PV and electrolyzer CAPEX 2 or 3-fold each. The implication for research and development targets is that cost reductions are key, and efficiency improvements are not strictly necessary but open up possibilities to enable costcompetitive H2 production even with more expensive components.