Most electricity travels from power plants along overhead wires called transmission lines. Laying underground or underwater cables generally costs more than stringing overhead wires. Cables are therefore used much less often than overhead wires. As electric current moves along transmission lines, the lines resist the current flow. The resistance causes the current to lose energy by heating the lines.
Power plants limit energy losses by transmitting electricity at high voltages.
As voltage is increased, the amount of current needed to transmit a particular amount of electric power decreases. Because less current flows through the line, there is less energy lost due to resistance. Direct current flows in only one direction. Alternating current reverses direction many times each second. It is easier to boost the voltage of alternating current than that of direct current.