Energy

Energy could not be created or destroyed. As Sarah Jane Smith explained to Maria Jackson, storms for example were created by, used up and left behind energy. (TV: Warriors of Kudlak)

A London research centre that the Silurians infiltrated was developing a method to convert nuclear energy directly to electric. The goal was for this to be a cheaper and safer method of doing so, and Charles Lawrence boasted of a "limitless supplies of cheap, safe energy". The truth, though, was that they were supplying the Silurians with their surplus energy, so the project didn't move forward, and the Silurians down below used it up to awaken everyone from hibernation and build up again. (TV: Doctor Who and the Silurians) Another major source of electricity on Earth was solar energy; this was known as solar power. (TV: The Seeds of Death)

The Pyroviles converted the energy of the lava within Vesuvius to create a fusion matrix, to be used to convert humans to their own kind. The Tenth Doctor foiled their plans of world domination, though, when he caused the volcano to erupt and destroyed them and the power supply. (TV: The Fires of Pompeii)

Space-time rifts such as the one in Cardiff radiated large amounts of temporal energy, which the Ninth and Tenth Doctors used to fuel their TARDISes. (TV: Boom Town, Utopia) TARDISes themselves released chronon energy.

The Mandragora Helix was a spiral of pure helix energy which radiated like none other. The Fourth Doctor described this form of energy as potentially "nasty". (TV: The Masque of Mandragora)

The Weeping Angels lived off potential energy. (TV: Blink) They once tried to feed on the time energy from a crack on the Byzantium, but it proved stronger in the end and devoured them, even erasing them from ever having existed. (TV: Flesh and Stone)

The Isolus used ionic energy to keep themselves amused on long journeys. When possessing Chloe Webber, one used it to extract objects and people into her drawings, and even to create some creatures, like the Scribble Creature. (TV: Fear Her)