Electrical power is lost in transmission primarily as heat, generated whenever current flows through the resistance of wires, transformers, and other grid components. In the United States, these losses average about 5% of all electricity transmitted and distributed, according to the U.S. Energy Information Administration. That may sound modest, but across an entire national grid, it represents an enormous amount of wasted energy.
Resistive Heating: The Biggest Source of Loss
The dominant mechanism behind transmission loss is surprisingly simple. Every conductor, no matter how good, resists the flow of electricity to some degree. That resistance converts electrical energy into heat, a phenomenon discovered by physicist James Prescott Joule in 1840. The relationship is captured in a straightforward formula: power lost equals the current squared times the resistance of the wire (P = I²R).
The “squared” part is critical. If you double the current flowing through a line, you don’t just double the losses. You quadruple them. This single mathematical relationship drives most of the engineering decisions in power transmission, from the voltage levels chosen for long-distance lines to the thickness of the cables strung between towers.
Why High Voltage Reduces Losses
Since power equals voltage multiplied by current, you can transmit the same amount of power by using higher voltage and lower current. Reducing the current is the most effective way to cut I²R losses, because the loss drops with the square of the current. This is why electricity leaves power plants at extremely high voltages, often hundreds of thousands of volts, before being stepped down through a series of transformers closer to homes and businesses.
A transmission line carrying power at 400,000 volts needs far less current than one carrying the same power at 10,000 volts. The result is dramatically lower heat losses along the way. Without this voltage-stepping strategy, long-distance power delivery would be impractical.
Corona Discharge on High-Voltage Lines
At very high voltages, a different kind of loss appears. When the electric field around a conductor becomes strong enough, it ionizes the surrounding air, stripping electrons from gas molecules in a chain reaction. This process, called corona discharge, produces a faint glow, a buzzing or hissing sound, ozone, and radio interference. It also wastes energy.
Environmental conditions make corona losses worse. High humidity, low air pressure, elevated temperatures, and pollution on the surface of conductors all lower the threshold at which ionization begins. Rain and fog are particularly problematic, sometimes increasing corona losses several times over dry-weather levels. Engineers reduce these losses by increasing the spacing between conductors and installing corona rings, metal fittings that smooth out the electric field at vulnerable points on the line.
Losses Inside Transformers
Transformers step voltage up and down at various points in the grid, and each conversion wastes a small amount of energy through two mechanisms.
The first is eddy currents. A transformer’s iron core is a metal, and when alternating magnetic fields pass through it, they induce small circular currents within the metal itself. These currents produce heat, just like current flowing through any conductor. Eddy current losses increase with the square of both the magnetic field strength and the frequency of the alternating current. To limit them, transformer cores are built from thin, insulated layers of steel (called laminations) rather than a single solid block, which forces eddy currents into smaller, less energetic loops.
The second mechanism is hysteresis loss. Every time the magnetic field in a transformer core reverses direction (which happens 120 times per second on a 60 Hz grid), the tiny magnetic domains inside the steel must realign. This realignment isn’t perfectly efficient; some energy is lost as heat with every cycle. Engineers select “soft” magnetic materials with narrow hysteresis loops, meaning the domains flip more easily and waste less energy per cycle. In most transformer applications, eddy currents are the larger source of loss, but hysteresis is always present alongside them.
How Temperature Affects the Lines
The resistance of a metal conductor isn’t fixed. It increases as the conductor heats up. At higher temperatures, the atoms in the metal vibrate more energetically, creating more collisions with the electrons carrying the current. This means that on a hot summer day, when air conditioning drives peak electricity demand, transmission lines are simultaneously carrying more current and offering more resistance per unit length. Both factors push losses higher at exactly the wrong time.
The relationship is roughly linear for typical operating ranges. Resistance at a given temperature equals the baseline resistance multiplied by a factor that accounts for the temperature change. For metals like aluminum and copper, which have positive temperature coefficients, any increase in ambient or self-generated heat raises resistance and increases waste.
Conductor Materials: Aluminum vs. Copper
Copper is the better electrical conductor, with a resistivity of about 1.68 × 10⁻⁸ ohm-meters compared to aluminum’s 2.65 × 10⁻⁸. That means an aluminum wire of the same size as a copper wire will lose roughly 60% more power to resistive heating. To match copper’s conductivity, aluminum conductors need a larger cross-sectional area.
Despite this disadvantage, nearly all long-distance overhead transmission lines use aluminum. The reason is weight. Aluminum is about one-third the density of copper, so even with the larger cross-section needed, an aluminum cable is significantly lighter. That reduces the load on transmission towers, lowers construction costs, and makes installation faster. For outdoor, long-distance lines where weight matters more than compactness, the tradeoff favors aluminum. Copper dominates in buildings and equipment where space is tight and maximum efficiency per unit of cable size matters most.
AC vs. DC for Long Distances
Most of the grid runs on alternating current, but AC introduces losses that direct current avoids. AC creates reactive power losses, where energy oscillates back and forth between the line’s magnetic and electric fields without doing useful work. AC also pushes current toward the outer surface of a conductor (a phenomenon called the skin effect), effectively reducing the usable cross-section of the wire and increasing resistance.
High-voltage direct current (HVDC) transmission eliminates both of these problems. For short distances, the cost of the converter stations needed at each end of an HVDC line outweighs the savings. But for very long distances, HVDC delivers less total power loss than an equivalent AC line. This makes it the preferred technology for undersea cables and for connecting remote renewable energy sources, like offshore wind farms or desert solar arrays, to distant population centers.
Where the 5% Goes
That national average of 5% loss is split across the entire journey from power plant to outlet. High-voltage, long-distance transmission lines account for a smaller share because they operate at the highest voltages and lowest relative currents. The distribution network, the lower-voltage lines running through neighborhoods, contributes a larger share because voltage is lower and current is higher relative to the power delivered. Transformers at substations and on utility poles each take a small cut. Corona discharge adds to the total, especially during storms or in humid climates.
In developing countries with older infrastructure, losses can reach 15% to 20% or more. The difference comes down to the same physics: aging conductors with higher resistance, fewer voltage-stepping stages, overloaded lines running hotter than designed, and less investment in modern low-loss transformer cores. The laws of physics don’t change, but how well the grid is engineered around them makes an enormous difference in how much power actually reaches the end of the line.