Not all energy beings are created equal. The TOS creatures from "Day of the Dove" and particularly "Wolf in the Fold" were incorporeal but there was no indication that they were transcendent like the Organians. It would be hard to call them immortal and definitely not omnipotent as the "Dove" being was weakened through starvation and the "Rejak" creature was presumably destroyed via the transporter (Spock predicts it will eventually die diffuse and powerless in space).
Nothing is ever said on-screen about the energy source of the Thasians (the race who adopted Charlie X), the Organians, Trelane, the Q, the Douwd, etc.. One could imagine that the Q feeds off the Universe itself - being able to go anywhere in space and time, and manipulate matter and energy as desired would imply the ability to access whatever energy source is needed whenever required. Despite the apparent omnipotence of his incorporeal parents, Trelane seemed to have limits and require some sort of technology to do all that he did. Presumably, it had some underlying energy source not evident to the Enterprise crew. One might imagine that the Organians, the Douwd, and Thasians were similar.