[Sherman Logan]

Takes 31.7 megajoules (about 30,046 btu) of energy to split a liter of water into hydrogen and oxygen using electrolysis.

That's only a part of the energy needed. You have to compress the hydrogen considerably to use it. Compressing gases takes a LOT of energy.

[Olog-hai]

That it does. The number I cited is just the energy that needs to go into the water molecule to break apart the hydrogen bonds to oxygen, and IIRC there is a thermodynamic penalty as well when the hydrogen atoms join to form H₂ molecules.

[thackney]

Compressing gases takes a LOT of energy.

What I remember from compressor stations, the lighter the gas specific gravity, the more waste heat generated in the compression, ie more energy to compress.

Hydrogen, then is more inefficient to compress than natural gas.