What LL has put up is probably the closest anyone has ever tried.
The problem is that in HERO, the basic unit of energy is the Endurance Pip.
The problem with converting directly to a specified unit of energy is this:
Mini Man, who shrinks down the size and mass of a vintage Star Wars action figure by Kenner. When he does so, he gains several powers, including thirty inches of Flight.
Gargantu-Juan the Human Tank activates his power and grows to a height of twelve feet as his body turns to solid steel. He also has thirty inches of Flight.
Professor Technical has built a diesel-powered rocket capable of attaining low-earth orbit. This mighty manned missile majestically moves it's entire thousand-ton mass with thirty inches of Flight.
No matter which character you are using, barring other modifiers to the power, they all use the same amount of END to power their thirty inches of Flight.
Now that is _not_ to say that you can't work something up such that one END is equal to X joules, and enforce that by requiring larger characters to spend more END to do the same thing as a less massive character, etc. If that is what you want to do, then by all means, go for it. I heartily endorse do,kering with the rules, after all.
I am simply saying that there is nothing in the exiating system that lends itself easily to what you are doing. Honestly, even the END rules are wonky, when you realize that the energy used to power a human being for one day wont keep a lightbulb burning for more than a moment or two.