Time‐resolved microwave conductivity is used to compare aqueous‐soluble organic nanoparticle photocatalysts and bulk thin films composed of the same mixture of semiconducting polymer and non‐fullerene acceptor molecule and the relationship between composition, interfacial surface area, charge‐carrier dynamics, and photocatalytic activity is examined. The rate of hydrogen evolution reaction by nanoparticles composed of various donor:acceptor blend ratio compositions is quantitatively measured, and it is found that the most active blend ratio displays a hydrogen quantum yield of 0.83% per photon. Moreover, it is found that nanoparticle photocatalytic activity corresponds directly to charge generation, and that nanoparticles have 3× more long‐lived accumulated charges relative to bulk samples of the same material composition. These results suggest that, under the current reaction conditions, with ≈3× solar flux, catalytic activity by the nanoparticles is limited by the concentration of electrons and holes in operando and not a finite number of active surface sites or the catalytic rate at the interface. This provides a clear design goal for the next generation of efficient photocatalytic nanoparticles. Time‐resolved microwave conductivity shows that the rate of photocatalytic hydrogen evolution by aqueous‐soluble organic donor–acceptor nanoparticles is limited by charge concentration, and that nanoparticles have 3× more long‐lived accumulated charges relative to bulk samples of the same material composition.