Dye-sensitized solar cells (DSSCs) composed of aqueous electrolytes represent an environmentally friendly, low-cost, and concrete alternative to standard DSSCs and to typical solar cells. Notwithstanding flammable and toxic organic-solvent-based electrolytes are so far more employed than simpler (iodide) aqueous solutions, recently recorded efficiencies of water-based DSSCs suggest a trend inversion in the near future. Here we present a study, based both on experiments and on ab initio molecular dynamics simulations, in which assessments on the efficiencies of three water electrolytes commonly employed in DSSCs (i.e., LiI, NaI, and KI) are reported. In particular, by atomistically tracing the iodides ability as charge carriers and by experimentally measuring the generated currents, we demonstrate that NaI aqueous solutions are more efficient electrolytes than LiI and KI - in descending order - in transporting electrons in DSSCs under bias. Monitoring the role played by the hydration shells of the ionic species under an electric field we interpret, by first-principles, the various iodide mobilities. This finding, when combined with general considerations on the cations-induced effects on the TiO2 electronic structure, is able to account for the distinct efficiencies of the investigated electrolytes.