In order to measure energy, we need a unit for it. In the metric system, the standard unit of energy is the joule. The formal definition of a joule is: A joule is the amount of energy expended when an object is moved 1 meter against a resisting force of 1 newton. (You can learn all about the concept of force in a physics class.) As for the joule, here are some statements that may help you visualize this unit.  A joule is… …enough energy to lift a one kilogram object 10.2 centimeters. …enough energy to heat one milliliter of water from 20ºC to 20.24ºC. …enough energy to keep a 60 watt light bulb glowing for 0.0167 seconds. Obviously, a joule is a very small amount of energy, and in fact it is an inconveniently small amount when we describe chemical reactions. Chemists usually report energies for reactions in kilojoules (1 kJ = 1000 J).