Broadly defined, is the capability of doing work. More specifically, it is the capacity for doing work as measured by the capability of doing work (potential energy) or the conversion of this capability to motion (kinetic energy). Forms of energy include: thermal, mechanical, electrical and chemical. Energy may be transformed from one form into another form useful for work. Most of the world¹s convertible energy comes from fossil fuels that are burned to produce heat that is then used as a transfer medium to mechanical or other means in order to accomplish tasks. Electrical energy is usually measured in kilowatt-hours, while heat energy may be measured in British thermal units or other traditional non-metric speciallized units in addition to metric units. In the electric power industry, energy is more narrowly defined as electricity supplied over time, expressed in kilowatts.