Energy quantity refers to the amount of energy present in a system, which can be measured in various units such as joules, calories, or kilowatt-hours. It represents the capacity to perform work or produce heat. Understanding energy quantity is essential in fields like physics, engineering, and environmental science. Different forms of energy, such as kinetic energy, potential energy, and thermal energy, contribute to the total energy quantity in a system. The principle of conservation of energy states that energy cannot be created or destroyed, only transformed from one form to another, maintaining a constant total energy quantity in an isolated system.