Question: how much energy to separate hydrogen from oxygen?
Answer: That means that 1 kilowatt hour of electricity can split about 270 grams of water and produce about 350 liters of hydrogen and 175 liters of oxygen. It depends on the pressures and concentrations. Assuming standard conditions (1 atmosphere of pressure, 1 mole per liter of H+ ions), then you'd need a voltage of 1.23V.