I was having a lot of trouble predicting the outcomes of combat until I realized that if I ignored the defense on both sides things started making sense:
Here is the ship I am attacking (The ship is effected by the bug where after you kill the first ship in a stack others stay invisible):
Here is my fleet: the ships are identical, so each has 22 energy weapons, 18 armor, and a full 20/20 hp.
Here is the result of combat:
My fleet now has 28/40 hp, meaning it took the full 12 points of damage from the Terran guardian's kinetic weapons despite having 18 armor per ship. I know that combat will be completely changing for beta 3, but please don't wait to fix this issue, it has a major impact on game-play and balance.