Liquid Energon is energy!
Generation One cartoon
After having the Heart of Cybertron installed in him, Megatron led the Decepticons in a toast to the impending destruction of the Autobots. During their consumption of liquid energon, the Decepticons became over-energized, and started to behave in a manner similar to a state of Human drunkenness: Rumble began to hiccup (even though he doesn't need to breathe!), Thundercracker went offline, Megatron began to ramble about the "good old days" back on Cybertron, and Starscream's and Laserbeak's navigational systems were impaired. Only Megatron was able to revive himself to an acceptable combat level when Powerglide showed up. This is only going to reinforce Megatron's superiority complex.
While Bumblebee was doing recon on Earth, he came across a memo by one Dina Morgan, who said that liquid energon, once converted from its crystalized state to liquid form, would be the power source of the 21st century. Intrigued that the Humans knew of the Transformers' fuel source, Bumblebee went to investigate.