The efficient conversion of solar energy to fuels through electrochemical processes requires optimizing the photovoltage and current for an ideal coupling with the electrolysis reaction. A modular architecture for tandem photovoltaics is explored and modeled as a strategy to drive an arbitrary electrolysis reaction from sunlight to produce the maximum fuel product in a day. Non-monolithic tandem solar cells based on Si and organometal halide perovskites are simulated in two-terminal and four-terminal arrangements and coupled with experimental data on water-splitting and CO₂ reduction to predict the performance of an integrated solar fuels system. An appropriately designed four-terminal system is modeled to match or exceed the output of a two-terminal system. The four-terminal configuration leads to a 15.8% increase in daily H₂ production with a 1.5 eV/1.12 eV system, and a 5.3% increase with a more ideal 1.74 eV/1.12 eV combination. The four-terminal system is also simulated to match the production of formic acid and increase the production of ethylene by 20.4% in a Cu-catalyzed CO₂ reduction process compared to a two-terminal tandem arrangement. The effects of series resistance in non-monolithic tandem devices are modeled as well, showing a much greater tolerance to cell width in the four-terminal systems.