Short answers, pulled from the story.
Energy is a quantitative property transferred to a body or system, recognizable through work, heat, and light. It exists as a conserved quantity that changes form but never disappears from an isolated system.
Thomas Young used the word energy in its modern sense for the first time in 1807 after centuries of philosophical debate. This usage followed earlier concepts like Aristotle's energeia in the 4th century BC and Gottfried Leibniz's vis viva in the late 17th century.
A kilowatt-hour delivers 3.6 million joules when running at one kilowatt power for an hour. The International System of Units defines the joule as the standard measure for all forms of energy transfer.
Plants capture sunlight's radiant energy during photosynthesis converting carbon dioxide and water into carbohydrates and oxygen. This process allows complex organisms to occupy ecological niches unavailable to simpler life despite lower physical efficiency rates.
The equation E equals mc squared quantifies this mass-energy equivalence between relativistic mass and energy. Albert Einstein published special relativity in 1905 showing rest mass corresponds to equivalent amounts of rest energy.