There is no canon answer as to what specific type of energy Galactus is feeding upon when a world is selected by a Herald. Depending on the writer, the medium or the continuum being described, Galactus has used a variety of energy forms to survive on from nebulous and ill-defined "life-force energies" to the anti-matter energy of the Negative Zone or the background energy of the universe itself (perhaps vacuum energy), which may be just another way to describe the Power Cosmic.
Here's what we do know:
Galactus (as the humanoid Galan) is a survivor of the previous universe. He survives the cosmic crush of the Universe with the help of the Phoenix Force and is reborn as a fundamental/abstract being in our Universe. He is reborn from the Cosmic Egg as a brother to Eternity and Death. He is seen as a force between the two challenging all life in the Universe.
Galactus does not have to completely destroy a world he has fed upon. If he is able to set up and use his Elemental Converter, he can leave a shred of life energy so that world can barely support lower level life (lichens and their ilk).
He is able to draw the lifeforce from the world without his Elemental Converter, but if he does, the planet is often completely destroyed and unable to support life in the process. This is not his desired method since it wastes precious life-force energies.
In a conversation/confrontation with the Phoenix, Galactus indicates both of them utilize the same energy, he using the spent remains of a civilization or barren life-bearing planet, and she the energies of life as yet unborn.
Even at his earliest stages, he was a power almost without equal. Discovered by one of the first most advanced races in the universe, a Watcher is horrified to discover how powerful Galactus was.
The Watcher after studying Galactus flees when he, as was the want of his people refused to destroy the potential threat he correctly perceives Galactus to eventually become. As Galactus leaves his incubation chamber, he consumes his first planet without technology leaving it a husk unable to support life, forcing its inhabitants to become the first Wanderers (beings forced from their world by Galactus).
Despite his humanoid appearance Galactus is a being who defies description, his powers are dwarfed by his physical appearance, which is now more than anything a construct perhaps in homage to his previous humanoid appearance. The Enchantress describes him: "What we normally see of Galactus is the merest tip, he's an iceberg of cosmic dimensions. No. Worse. He HAS no dimensions. No boundaries at all."
He is equated on the same level of cosmic necessity as the other great conceptual powers of the Universe, including Death, The Living Tribunal, The Inbetweener, and Epoch, to name a few. Within his energy-filled body lives the destructive force of Eternity, Abraxas. Only the Other Cosmic/Abstract beings or the Celestials have proven to be his equal. In a confrontation against three Celestials however, Galactus was defeated (Fantastic Four #602, #603, #604) and rescued by a future Franklin Richards.
The life force that Galactus feeds upon does not appear to need to be sentience, for Norrin Rad made it his duty during the time he served Galactus to find him worlds filled with the potential for life, but having no sentient life upon them. In the Marvel Universe, planets can have life-force like energies which can lead to beings such as Gaea/Jord (Earth-616) coming into existence and leading to an explosion of lifeforms over time. Perhaps it is this energy upon which he feeds.
Galactus' recent death and resurrection implies Galactus' continued existence having been restored by the "beyond Omega-Level" powers of Franklin Richards and the threat of the Mad Celestials defeated. Galactus was repowered by Franklin Richards who has no known connection to the Phoenix Force. Galactus was restored to full strength and was capable of destroying a Celestial after his renewal.
- Considering Galactus' transition from the previous Universe to this one was through the interaction with the Phoenix Force, it may be he requires the re-infusion of Phoenix energy to sustain himself. The Phoenix Force is considered to be the unbridled energy of life in the Universe. Since his life was recreated by the Phoenix after the death of his Universe, he may be lacking a particular universal constant which is not available in our universe.
Taa's civilization was one of the last still in existence. Lethal radiation caused by the "Big Crunch" this universe was experiencing was wiping out all life across the universe. Galan, a space explorer, was dispatched to travel through the cosmos to find a means of saving Taa, but he found none. The radiation eventually killed off all but a tiny fraction of the population of Taa. Knowing their deaths were inevitable, Galan proposed to the remaining survivors that they die gloriously by piloting one of their starships directly into the heart of the "Cosmic Egg." As the starship containing Galan and his fellow survivors approached the focal point of the Big Crunch, the heat and radiation killed all the passengers except Galan, who strangely found himself filled with new energy.
At the moment Galan's universe met its end, the Phoenix Force amassed the positive emotions of all living beings in the cosmos to preserve them from eternal damnation, enabling the Sentience of the Universe — the previous universe's equivalent to Eternity — to meet with Galan. Within the "Cosmic Egg" the Sentience of the Universe revealed itself to Galan and informed him that though they both would die in the final moments of the universe, they would both survive through a joint heir born into the next universe. The Sentience of the Universe merged itself with the mortal Galan and thus Galactus, the devourer of worlds, was conceived. --Handbook of the Marvel Universe, Galactus Entry
It may also be that containing the destructive element of Abraxas within his "body" may require an infusion of life energy (Phoenix Force) to maintain his prison. In the very early years, Galactus could go for extremely long periods of time without feeding. This may have been due to his primal exposure to the Phoenix Force after the Big Bang. As the Phoenix Force dissipated and possible collected with worlds, it became harder for him to harvest from the background radiation of the Universe and he was forced to seek it out.
Noting that Galactus has complete control of all normal electromagnetic phenomenon, the energy he is seeking must be very subtle or very unique, otherwise he would simply feed from stars or other highly energetic objects in our universe. If a star cannot feed his powers, he is seeking a very rare energy indeed. Since the Phoenix Force can manifest anywhere, it is possible he is seeking planets whose potential of that energy is still in an abundance enough for him to replenish his supply.
It is noted that Galactus can utilize energy from the EM spectrum and can harvest the power of a star to replenish his cosmic energy for combat. He has also absorbed energy from a Protector of the Universe (Gravity) completely draining him of his powers but was able to temporarily sate Galactus enough to spare the Cosmic being Epoch from Galactus' hunger. It is possible Galactus can feed on alternative energy sources, but this varies from writer to writer just as his power level does.
Galactus is now thought to be a Herald of Franklin Richards (this is implied from a statement from Nathaniel Richards, Reed Richard's father) after a confrontation with the Mad Celestials in which Galactus appears to be defeated and potentially mortally wounded. He is resurrected and restored with the powers of Franklin Richards. Whether this is hyperbole or a statement of fact remains to be determined. It is also noted Franklin and Galactus will be aware of each other for billions of years in the future. Stand by for your moment of awesome:
Galactus being raised from the dead by Franklin Richards. Fantastic Four #604