Faradaic efficiency

The fraction of supplied charge that produces the intended product.

Faraday's laws fix the quantity of hydrogen that a given quantity of charge should produce. Faradaic efficiency is the ratio of what is actually collected to that theoretical quantity.

In water electrolysis the losses come mainly from gas crossover, where product that has been made recombines or ends up on the wrong side of the separator, and from shunt currents, where part of the supplied current bypasses the intended path through conductive electrolyte channels.

It is distinct from voltage efficiency, which concerns how much energy each unit of charge costs. Overall energy efficiency is the product of the two, so a system can have excellent voltage efficiency and still perform poorly if a meaningful fraction of its charge is not producing collectable hydrogen.