How many joules of energy is required to split one mole of H2O into one mole of H2 and 1/2 mole of 02?

So I want to now the amount of energy, in joules to split distilled water into hydrogen and oxygen per mole at standard temperature and pressure. Also if you could explain it's efficiency factor, that would be very helpful. I want this so I can figure out the efficiency of a hydrogen fuel cell.