You make a good point about the relative fluctuation of value, but if this isn't tied to a tangible the whole system collapses. And while a newton or watt-hour might fluctuate based on all the constraints you've listed its still far more relevant than something like gold. Unless someone comes out with a cheap and easy way to make infinite energy, that is. That said, I'm a fan of dragonfly.
Why does everything have to be time dependent with you? Why can't you live in the 'moment'.
Why not use what it all comes down to in the end? Watts. Secondary benefits would be that there would be a huge push to make transferring and storing more efficient and people would actually be able to correlate what they're buying with the cost.