Efficiency of electrolysis, like anything else, is probably about 30-50%. Thus, you'd need more energy than the amount specified below to electrolyze the given amount of water.
The reaction 2H2O --> 2H2 +O2 has an enthalpy (heat) value of
572 kilojoules per 2 moles of water, or 36 grams of water. Four grams of hydrogen are produced (and 32 grams oxygen).
This heat is consumed. The combustion of hydrogen, via the same reaction above but reverse (arrow going the other way) gives off 572 kilojoules of heat and produces water.
When the hydrogen is burned, only some of its energy can be reclaimed due to the low efficiency of combustion.
Fuel cells allow for more efficient usage of hydrogen fuel, for example.
Source(s):
http://en.wikipedia.org/wiki/Hydrogen


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