I often get confused by the flow of energy when I'm in cat form. Here's a handy reference for how it works for the useful PvE cat skills. Energy costs assume 5 points in Ferocity and 2 points in Shredding Attacks.
Ability Costs
50 Pounce
60 Ravage
40 Mangle (Ferocity)
42 Shred (Shredding Attacks)
20 Cower
30 Rip
35 Maim
Energy gains
20 energy / 2 second tick
Clearcasting (roughly once every 30 seconds, 3.3% proc rate or so)
20 energy / 2pc Tier 4 proc (roughly once every 25 seconds, 4% proc rate)
What trips me up about this is that Shred's 42 energy is not a round number. Also sometimes the tick based energy gain doesn't give an even 20 energy, randomly you'll get an extra 1 or so energy from time to time.