From a purely chemical perspective, the water required for electrolysis is split into its components hydrogen and oxygen.
18 g/mol H₂O → 16 g/mol O + 2 g/mol H₂
To produce one kilogram of hydrogen, exactly nine kilograms of water are theoretically required.
However, electrolysis requires ultra-pure water. Ordinary tap water or groundwater must therefore be filtered and purified beforehand (e.g. by reverse osmosis). During this treatment, approximately 20% of the water is lost as concentrate. In addition, the electrolysis process itself is not 100% efficient (e.g. due to water carryover on the oxygen side).
Overall, a water requirement of approximately 12–13 kg per kilogram of hydrogen produced must be expected. Based on the heating value, this corresponds to just under 300 milliliters of water per kilowatt-hour of hydrogen.
For comparison: the average consumption of a hydrogen vehicle is 1 kg/100 km, meaning approximately 12 liters of water are needed per 100 km. A gasoline car requires approximately 18–36 liters of water (production of gasoline: water consumption 3–6 liters; assumption: fuel consumption 6 L/100 km).