Let's look at ideal conditions first. 1molH2O is electrolyzed to generate H2 and O2 (the pressure of the gas generated is 100 kPa) in a reversible manner (i.e. very close to the equilibrium state and without energy dissipation) at 298 K ( 25 degrees Celsius) and an external pressure of 100 kPa, which requires 237.1 electric currents. kJ of energy, in addition to absorbing 48.7 kJ of heat from the environment.
Suppose hydrogen is an ideal gas. According to the ideal gas equation of state pV = nRT, the amount of matter in 1 cubic meter of hydrogen at 298 K and 1 standard atmosphere is 40.9 moles, and requires at least 237,100 × 40.9 = 9.697 million joules of electrical energy.
Actual electricity required is 5 kilowatt hours. 1 kilowatt hour is 1000 J/s × 3600 s = 3.6 million joules. 5 kilowatt hours equals 18 million joules, 969.7 ÷ 1,800 = 54%. In many cases it would be good if the actual result could be 20 to 30% of the theoretical value. It is not easy to have a utilization rate above 50%.