Not 100% of the energy, because even if no current is wasted, the voltage applied to the electrolysis cell is higher than the minimum value that corresponds to 100% energy efficiency.
To know the energy efficiency, besides the current efficiency, which is close to 100%, we need to know how big is the overvoltage needed for electrolysis.
To know the energy efficiency, besides the current efficiency, which is close to 100%, we need to know how big is the overvoltage needed for electrolysis.