Because it's trivial to figure out in your head.
All one-bar charge moves use the same amount of energy.
Two-bar charge moves use half that energy.
Three-bar charge moves use one-third that energy.
So, to get a comparison of "Damage per energy used", just multiply the damage number by the number of bars. Bang. Comparison done.
Psychic - 100 dmg x 1 bar = 100, kinda sucks
X Scissor - 45 dmg x 3 bars = 135, not so great
Solar beam - 180 dmg x 1 bar = 180, frickin' awesome
Outrage - 110 dmg x 2 bars = 220, BOOYEAH!