Energy was the capacity of a system to perform work.
Some lifeforms could directly feed upon a system's energy, while others - such as most Earth lifeforms - had to extract it through interconnected biochemical processes. "Pure energy" life-forms are defined by their ability to alter the environment without a material form. All of these processes contributed to entropy within their universe.
Time itself performed work on space, thus exerting energy. The chronon energy lost from this interaction could be detected, and it accumulated to the point of forming a "fire at the end of the universe". (TV: Flesh and Stone) This was not to be confused with temporal energy, released when a system acted on a previous or future version of itself.
The Weeping Angels lived off potential energy. (TV: Blink) The work that would have been performed over the life of their prey was instead available for the Angels, making complex intelligent species targets. 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)
Wenley Moor nuclear research facility was developing a method to convert nuclear energy directly to electric. The goal, as Charles Lawrence boasted, was a "limitless supplies of cheap, safe energy". The Silurian base nearby drew power from this reactor after the reactor appeared, waking the Silurians from their hibernation units when the mechanism had failed to wake them up for millions of years. (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 (TV: Boom Town) and Tenth Doctors (TV: Utopia) used to fuel their TARDISes. TARDISes themselves released chronon energy. (PROSE: Attack of the Snowmen)
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)