Um, well, first of all, stored energy is measured in kilowatt-hours, not kilowatts.
Second of all, I assume you mean a gallon of liquid hydrogen - which I beleive to be considerably more energy dense than gasoline. Third of all, a typical car gets between 220 and 300 wh a mile, so a kwh will take you between four and five miles. For a real world example, my battery pack holds 21kwh (about), and will take me (about) 100 miles. Anyway, by definition, aside from thermal losses in the electrolysis, compressing the gaseous hydrogen into liquid hydrogen, and converting the hydrogen back into electricity in the fuel cell (each process is about 80% efficient, if my memory servces), you have to get back all the energy you put in. Unfortunately, by the time you shuttle energy through three 80% efficient systems, you're only getting half as much out as you put in. So you can expect a gallon of liquid hydrogen to take you about a hundred miles - still, not bad, although if the containment system for that liquid hydrogen ever fails, it will be a _lot_ more exciting than a gasoline leak. ;-) S.
