Question: how much energy is required to separate water into hydrogen and oxygen?
Answer: The standard enthalpy of formation of water is -286kJ/mol. Which means 286kJ of energy is produced when oxygen gas and hydrogen gas reacted to form 1 mole of water. This also means that we need 286 kJ to break water and form H2 and O2. 1kWh = 3600 kJ.