Power transmission is the movement of energy from its place of generation to a location where it is applied to perform useful work. Power transmission is the movement of energy from its place of generation to a location where it is applied to perform useful work. Power is defined formally as units of energy per unit time. In SI units: Since the development of technology, transmission and storage systems have been of immense interest to technologists and technology users. With the widespread establishment of electrical grids, power transmission is usually associated most with electric power transmission. Alternating current is normally preferred as its voltage may be easily stepped up by a transformer in order to minimize resistive loss in the conductors used to transmit power over great distances; another set of transformers is required to step it back down to safer or more usable voltage levels at destination. Power transmission is usually performed with overhead lines as this is the most economical way to do so. Underground transmission by high-voltage cables is chosen in crowded urban areas and in high-voltage direct-current (HVDC) submarine connections. Power might also be transmitted by changing electromagnetic fields or by radio waves; microwave energy may be carried efficiently over short distances by a waveguide or in free space via wireless power transfer.