how fuch hydrogen do you need to produce a kilo watt of electricty in a fuel cell?

There are many different kinds of fuel cells: http://www.repp.org/hydrogen/hydrogen_fuelcell_types.html each one has its own operating temperatures, efficiency levels, and power density.

As a rough figure you could say that a fuel cell is 50% efficient and then examine hydrogen on a chart of gasoline gallon equivilents: http://en.wikipedia.org/wiki/Gasoline_gallon_equivalent to get 357.37 cu ft as equal to a gallon of gasoline. Also on the chart electricity is 33.4 kwh. So dividing we get that about 10.7 cu ft of hydrogen has the energy equivilent of about 1 KWh of electricity. Something that is 50% efficient would require double this amount to produce the 1kwh of electricity or 21.4 cu ft.

By weight this would be 997 grams/33.4 or 29.85 grams x 2 or 59.7 grams for a 50% efficient fuel cell.