Energy is the capacity of a physical system to do work. The common symbol for energy is the uppercase letter E. The standard unit is the joule, symbolized by J. One joule (1 J) is the energy resulting from the equivalent of one newton (1 N) of force acting over one meter (1 m) of displacement. There are two main forms of energy, called potential energy and kinetic energy.
Potential energy, sometimes symbolized U, is energy stored in a system. A stationary object in a gravitational field, or a stationary charged particle in an electric field, has potential energy.
Kinetic energy is observable as motion of an object, particle, or set of particles. Examples include the falling of an object in a gravitational field, the motion of a charged particle in an electric field, and the rapid motion of atoms or molecules when an object is at a temperature above zero Kelvin.
Matter is equivalent to energy in the sense that the two are related by the Einstein equation:
E = mc2
In electrical circuits, energy is a measure of power expended over time. In this sense, one joule (1 J) is equivalent to one watt (1 W) dissipated or radiated for one second (1 s). A common unit of energy in electric utilities is the kilowatt-hour (kWh), which is the equivalent of one kilowatt (kW) dissipated or expended for one hour (1 h). Because 1 kW = 1000 W and 1 h = 3600 s, 1 kWh = 3.6 x 106 J.
Heat energy is occasionally specified in British thermal units (Btu) by nonscientists, where 1 Btu is approximately equal to 1055 J. The heating or cooling capability of a climate-control system may be quoted in Btu, but this is technically a misuse of the term. In this sense, the system manufacturer or vendor is actually referring to Btu per hour (Btu/h), a measure of heating or cooling power.