The hard part is proving that you will always be able to get x hydrogen for your x energy credits. How do you issue energy credits? Currently the only solution I can think of isn't really a solution - having a central issuer take your hydrogen and give you credits and a promise to give you hydrogen back if you should return those credits. The promise is easier to transport and weighs less than the hydrogen it is purportedly backed by, but centralizing things is a step backward - we'd have to instead first find a way for a computer to easily verify that a difficult computation took place that... resulted in electrolysis of water. I'd love to hear of a way to produce such a proof, but I can't be sure that it is possible.