In physics, energy is a scalar physical quantity that describes the amount of work that can be performed by a force, an attribute of objects and systems that is subject to a conservation law. Different forms of energy include kinetic, potential, thermal, gravitational, sound, light, elastic, and electromagnetic energy. The forms of energy are often named after a related force. (Wikipedia) Energy units are often used in relation to environmental protection, energy reserves and geopolitics.
You are currently converting energy units from watt second to megatonne TNT