At its simplest electricity is an energy. It is useful to civilisation because it can be efficiently generated, transferred long distances, subdivided and used to power a myriad of machines. It is created by expending another energy (e.g. steam, solar radiation, water, wind) which is converted into electricity. It can then be transported, subdivided or stored. Ultimately, it can only be useful if it is again converted into another energy (e.g. kinetic or thermal). The law of conservation of energy states that the total amount of energy in an isolated system remains constant over time. It is said to be conserved over time. Energy can neither be created nor destroyed, it can only be transformed from one state to another.