The problem with energy is that it's plentiful, and we get better and better at creating it. A currency should be scarce afaik. Interesting thoughts anyway.
you're thinking of 'energy of today'.
Unless of course you have an insight into what types of energy will be used in the near/far future and with that insight you already know what planets and asteroid bodies will contain such energy, and that you know of the locations these planetoids and asteroids (and probably stars) and that they'll contain such energy sources in plentiful supply.
I'm going to guess that you don't actually know this stuff.
My thoughts is also with energy at the 'top of the food chain', but that a huge mixture of different currencies will exist underneath this.