How much power is lost when transported to distant users?

There are several key factors that limit electricity transmission, such as the distance between the generator and the end user, the power of the original transmission, the material used to carry the electricity, and the location of transformers and capacitors. Any of these factors can limit the strength of the electrical power to the end user. Consequently, all these issues must be carefully monitored at all times to ensure safe transmission of electricity

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Electricity is transmitted using a set of high voltage cables to carry the electrical current from the power generation station to a series of transformers. The transmission cables used are very thick in size and are designed to withstand the high amount of heat generated by the electricity as it moves through the cables. The heat threshold of the cables, such as the thermal load as well as the current-carrying capacity, is a factor that limits electricity transmission rates. As the volume of electricity being transported along the cables increases, so does the temperature.

As energy is transmitted along a circuit, a certain percentage of the power is lost. This is due to the energy required to move the electricity from the power generation source to the user. The rate of loss is defined by Joule's Law which states that the amount of energy lost is in proportion to the squared value of the current. For a given amount of power, a higher voltage reduces the current and thus the resistive losses in the conductor. For example, increasing the voltage by a factor of 10 reduces the current by a corresponding factor of 10 and therefore the power losses by a factor of 100, provided the same sized conductors are used in both cases.

Today, transmission-level voltages are usually considered to be 110 kV (110,000 volts) and above. The power produced at generating plants is typically between 2,300 volts and 30,000 volts, depending on the size of the plant. The power is then stepped-up at a transmission substation to anywhere from 110kV to 765kV for the overhead long distance transmission lines. Once the power reaches distribution areas, transformers at the step-down transmission substations reduce the voltage to a lower level for distribution to commercial and residential users. This distribution is accomplished with a combination of sub-transmission (33 kV to 115 kV) and distribution (3.3 to 25 kV) lines, some of which can be underground. Finally, at the point of use, the energy is transformed to low voltages (100 to 600 V) for output.

Right now in the US, 7-8% of total power generated on average is lost during transmission.