We have fabricated Dye Sensitised Solar Cells (DSSCs) using only water as the solvent and guanidinium iodide–iodine as the redox couple that operate at 4% energy efficiency under 1-sun illumination. This result is 5 times higher than the best previously reported values. We show that it is critical to facilitate the wetting of water electrolytes into the mesoporous TiO₂ dye films, especially when using hydrophobic dyes. We show chenodeoxycholic acid to be a good surfactant for this purpose. By separate variation of iodide and iodine concentrations in series of cells we show that the optimum concentrations for water based DSSCs are quite different from those used in organic electrolytes. We argue that this is due to the much lower stability constant of tri-iodide in water, relative to organic solvent. Finally we also vary the TiO₂ thickness and pore structure to achieve the above stated efficiency.