AC transmission: Efficiency and losses

How higher voltage nearly halves the power lost on the way

What this tab covers How much energy is lost while it moves and while it waits in storage or transit, and how much energy the transport itself consumes, in per cent.

Higher voltage cuts losses sharply. In the United States, transmission and distribution together lose about 5% of the electricity they carry; in a MISO comparison, 765 kV lines lost a little over half as much as 345 kV lines carrying the same power.

Chart: Efficiency ranges

Whole chain Lost in transit

Losses by measure

Measure	Range	Type	What is measured
Transmission and distribution losses, United States	5%⁴	Whole chain	Share of electricity transmitted and distributed that is lost, average 2018 to 2022 (whole network, all voltages)
765 kV AC, 2 circuits carrying 5,000 MW	5.7%⁵	Lost in transit	Power lost per 1,000 km at that heavy loading (calculated by scaling 46 MW lost over 100 miles linearly)
345 kV AC, 12 circuits carrying 5,000 MW	10.1%⁵	Lost in transit	Power lost per 1,000 km at that loading (calculated by scaling 81 MW lost over 100 miles linearly)
Loss saving, Texas 765 kV plan versus 345 kV plan	5%⁶	Lost in transit	Reduction in annual transmission energy losses on the ERCOT system, about 560 GWh a year

Holding periods and inventory

Measure	Value
Time in transit	Effectively instantaneous: the electromagnetic energy travels at close to the speed of light, so 1,000 km takes about 3 milliseconds (calculated)
Inventory held in the line	None in practice: supply and demand must balance every moment, so storage has to be added separately (batteries, pumped hydro)

Losses come in two kinds. Resistive losses rise with the square of the current, so they are worst at peak load and on lower-voltage lines. Fixed losses, mainly from magnetising transformer cores and from corona (small electrical discharges into the air around very high-voltage conductors, worse in wet weather), occur even when little power flows.

In the MISO example, moving 5,000 MW over 100 miles, two 765 kV circuits lose 46 MW at peak while twelve 345 kV circuits lose 81 MW; annual energy losses are about 404,000 MWh against 710,000 MWh. AC transmission burns no fuel and has no pumps: the energy cost of moving power is the losses themselves.

AC transmission: Efficiency and losses

Losses by measure

Holding periods and inventory

Sources