AC Power and the Electric Grid

A battery pushes charge one way around a circuit: direct current. The outlets in your home supply alternating current, where the voltage swings positive, then negative, then positive again, and the current sloshes back and forth with it.

AC comes naturally from a generator. As a coil turns in a magnetic field, the voltage it makes rises, falls, reverses and returns every turn, tracing out a sine wave. The Magnetism unit explains how induction makes that voltage; here we follow what happens to it next.

In the 1880s Thomas Edison's companies built DC systems, while George Westinghouse, using Nikola Tesla's AC motor patents, backed AC. AC won the "war of the currents" for one big reason: its voltage can be changed easily with transformers, which made long-distance transmission practical.

The grid in the US, Canada and much of the Americas runs at 60 Hz: 60 full cycles every second, so each cycle lasts 1/60 s ≈ 16.7 ms. Most of Europe, Africa, Asia and Australia uses 50 Hz, a 20 ms cycle. Every generator on a grid must spin in step at exactly that frequency.

Because AC keeps changing, we describe it with a kind of average, the root-mean-square (RMS) voltage. It's the DC voltage that would heat a resistor at the same average rate. For a sine wave,

V_rms = V_peak ÷ √2 ≈ 0.707 V_peak

A US outlet is labeled 120 V, which is its RMS value. The voltage actually peaks at about 120 × √2 ≈ 170 V. In Europe, 230 V RMS peaks at about 325 V. Power formulas such as P = IV work for AC as long as you use RMS values.

A transformer is two coils wound on one iron core. The changing current in the primary coil makes a changing magnetic field, which induces a voltage in the secondary coil. (See Magnetism, Idea 7 for how that works.) The voltages are in the same ratio as the turns:

V_s ÷ V_p = N_s ÷ N_p

A step-up transformer has more turns on the secondary and raises the voltage; a step-down transformer lowers it. An ideal transformer doesn't create energy, so if the voltage goes up 10 times, the current goes down 10 times. Transformers only work with AC, because a steady DC current makes no changing field.

Power lines are long, so even thick aluminum cable has some resistance. Current through that resistance heats the wire and wastes energy. The power sent is P = IV, and the power lost in the line is

P_loss = I²R

To send the same power at 10 times the voltage you need only 1/10 the current, and because the loss depends on current squared, the loss falls to 1/100. That's why long-distance lines run at hundreds of thousands of volts, commonly 115 kV to 765 kV in the US. Overall, about 5% of US electricity is lost in transmission and distribution.

(The Electricity unit introduces P = IV and P = I²R for simple circuits.)

A large generator typically produces around 20 kV. A step-up transformer at the plant raises that for the transmission network, for example to 345 kV. Near towns, substations step it down for local distribution lines, often around 12 kV. Finally, a transformer on a pole or a green box in the yard steps it down to the 120 V and 240 V used in homes (240 V for large appliances like dryers and ovens).

The grid has almost no storage. At every moment, power plants must generate as much as everyone is using. Grid operators watch the frequency: if demand outruns supply, generators slow slightly and the frequency dips below 60 Hz, so more generation is switched on.

Most power plants spin a generator with a turbine. In natural gas, coal and nuclear plants, heat boils water into high-pressure steam that drives the turbine (many gas plants also burn the gas in a jet-like turbine). Hydropower uses falling water and wind turbines use moving air. Solar panels are the exception: they turn light directly into DC electricity, and an inverter changes it to AC for the grid.

In 2023, according to the US Energy Information Administration, about 60% of US utility-scale electricity came from fossil fuels, 19% from nuclear and 21% from renewables. Wind and solar output varies with the weather, which is one reason batteries and other grid storage are growing fast.

Power companies charge for energy, measured in kilowatt-hours: energy (kWh) = power (kW) × time (h). A 1.5 kW space heater running for 4 hours uses 6 kWh. Multiply by the price per kWh to get the cost. The Electricity unit has a table of typical appliances and their monthly costs.

A bill usually also has a fixed monthly charge, and some utilities charge more per kWh at peak times, such as late afternoons, when the grid is working hardest.

The first electric companies sold light, not power, and they fought hard over which kind of current to use.