With those low cost energy decks, here are a few examples of them.
Gyarados (Stormfront). For 0 energy, Gyarados will do 30 damage for each Magikarp in the discard pile. Since there are several Poké-Powers as well as Supporters that can easily get the required 3 Magikarp discarded, as well as trainers to immediately salvage Gyarados when it goes down, Gyarados can win without using any energy to begin with.
Beedrill (Great Encounters/Rising Rivals). For 1 energy, Beedrill does 30 damage times the number of Beedrill in play. Thanks to the power of the Rising Rivals Beedrill, it can keep up a flow of Bees easily and as such dish out great damage for only 1 energy, making the deck need only like 6 energy.
Shuppet (Platinum) or Uxie (Legends Awakened). For 1 energy, these cards deal 30 or 20 damage and return to your hand/bottom of your deck respectively, taking all cards attached to them with them. Since this includes most damage boosting cards (Pluspower/Expert Belt), they can hit for figures up to 90 for only 1 energy, and then send out something that's hard to hit properly (Mr. Mime Mysterious Treasures/Surpreme Victors). Since they only need 1 energy, theres VERY little required for them to keep going, and the trainers used let them draw almost their entire deck turn 1.
Kingdra (Legends Awakaned) is in a bit of a different vein, it can deal 40 damage for 1 energy, or 60 and 20 to a benched Pokemon if you discard 2 cards from your hand. Since his other attack works on energy in the discard, there are Kingdra builds with more energy, but most commonly they only use the energy needed for the attack and discard other cards, not bothering for the energy-fueled attakc.
Those are just some examples. If you'd like to know more about the TCG and how the big decks work, you could throw me a line on AIM or MSN if you feel like it, I don't mind. You'll find my contact info in my profile.