How much voltage does it take to split water?

Answer 1

If you had an absolutely perfect system - free of any inefficiencies - it would take you no more energy to split water than the amount of energy you would gain from the recombination of hydrogen with oxygen to form water. That energy level is 142 J/g.

So, the amount of energy gained from the recombination of hydrogen with oxygen to re-form water (in effect, "burning" the hydrogen) is 142 Joules per gram of hydrogen. In an absolutely perfect system it would take 142 Joules of electricity to form a gram of hydrogen from water. Since inefficiencies exist in any energy-conversion system, your mileage will vary.

Actually, it's going to take about 50% more energy to split the water than you're going to get back out of the system - depending upon your system. There are systems in laboratories and other experimental systems which are more efficient than the "50% more" mark quoted, but you won't find any of them on the shelves at Wal*Mart.

For a homebrew system, however, start your calculations at about 200 Joules per gram of hydrogen extracted from the water. Remember that depending upon the inefficiencies inherent in whatever system you build that it may take more or less energy than 200 J/g to extract the hydrogen you seek.

Answer 2

It doesn't take much and more voltage can actually interfere with the splitting process.

About 1.23 Volts (at the electrodes).

2 H2O(l) → 2 H2(g) + O2(g); E0 = +1.229 V

But you need an eletrolyte in the water (many salts will do just fine so sea water is easy to split into hydrogen and oxygen)