Water electrolysis uses electrical work to convert water into hydrogen and oxygen. For the net reaction, two water molecules produce two hydrogen molecules and one oxygen molecule; four electrons pass through the external circuit per reaction as written.
The reversible voltage follows from the reaction's Gibbs free energy under specified conditions. Finite operating electric current additionally requires kinetic and transport driving forces and incurs resistive losses, so operating voltage exceeds the reversible requirement.
New to topics? Read the docs here!