Energy is a conserved quantity; it cannot be created or destroyed. Thus, the total amount of energy in the Universe is always the same. Yet, energy can pass from one system to another and change from one type of energy into another.

There are many types of energy: kinetic energy in a moving object, thermal energy in a fire, chemical energy in batteries, gravitational potential energy in the water at the top of a waterfall, elastic potential energy in a drawn bow just before firing an arrow…. The total amount of energy in a system is the sum of all the types of energy in it.

Energy conservation does not mean that any of those particular types of energy is conserved. Energy can change from one type to another. For example, when a ball speeds up as it falls, pulled down by gravity, its gravitational potential energy changes into kinetic energy. When a wind turbine uses the moving wind to generate electricity, it transforms the kinetic energy of the wind into electrical energy.

Energy conservation also does not mean that the total energy of any particular system is conserved. Energy can move from one system into another. For example, if a billiard ball that is moving pushes another ball and makes it move, the first ball is giving some of its kinetic energy to the second ball. Only the total energy of isolated systems is conserved.

Energy is a scalar quantity. Its unit in the IS is the Joule (kg·m2/s2).