Flip a switch and a light comes on, using electricity generated an instant earlier, perhaps hundreds of kilometers away. The power grid is among the largest machines ever built, and almost nobody thinks about it until it fails.

From one station to whole continents

In September 1882 Thomas Edison opened the Pearl Street Station in lower Manhattan. Pearl Street supplied direct current to customers within about a mile of the plant, at first lighting a few hundred lamps. Alternating current, which can be sent long distances at high voltage, soon won out. Today a grid has three main parts: power plants that generate electricity, high voltage transmission lines that carry it long distances, and local distribution lines that bring it to homes and businesses. Most of continental Europe now runs as one synchronized grid.

Balancing every second

The grid has one rule above all others. Supply and demand must match at every moment, because the grid's frequency drifts away from its set value as soon as they differ. In North America the current alternates sixty times a second, and in most of the rest of the world fifty. If demand outruns supply, the frequency falls; if supply outruns demand, it rises. Push it too far and protective devices disconnect power plants and customers to prevent damage, which can make the imbalance worse.

Operators are helped by a hidden property of traditional plants. The heavy spinning generators in coal, gas, nuclear and hydro plants give the grid inertia, resisting sudden changes in frequency and buying operators precious seconds. Solar panels and many wind turbines connect through electronic inverters instead, so they do not provide this naturally. Engineers are now deploying "grid forming" inverters and batteries that can imitate it.

Who runs first

Demand swings through the day and the seasons. Operators meet it by calling on plants in a set order. Under the merit order, plants are called on in order of their running cost, cheapest first, until demand is met. In many markets the price everyone receives is set by the last, most expensive plant needed. Because wind and solar burn no fuel, their running cost is close to zero, so when they are plentiful they push costlier plants out and lower wholesale prices. That is good for buyers but makes it hard for the plants pushed aside, which may still be needed on calm, dark days, to earn enough to stay open.

When grids fail

On August 14, 2003, about fifty million people in the northeastern United States and Ontario lost power. The blackout began when overloaded power lines in Ohio sagged into overgrown trees while a failed alarm system kept operators from seeing the trouble. As lines tripped, power rushed onto others, which tripped in turn, spreading the failure across eight states and a province within minutes. The event shows the double edge of interconnection. Linking grids lets regions share reserves and help each other, but in a tightly interconnected grid a local fault can cascade across a huge area unless it is contained quickly. The United States afterward made reliability standards mandatory rather than voluntary.

Wires as the bottleneck

The best wind and solar sites are often far from cities. Getting their power to customers needs new transmission lines, and in the United States and much of Europe a major line can take a decade or more to plan, permit and build. In many countries, a shortage of transmission lines now holds back more clean power than any shortage of projects to build. Thousands of proposed plants wait in queues to be connected. The objections are not all selfish: landowners lose views and land, communities may get little benefit from lines that pass through them, and someone must decide who pays.

Worked example: Texas, February 2021

Texas runs its own grid, largely separate from its neighbors, which spares it some federal oversight but also limits how much power it can import in an emergency. In mid February 2021 a severe winter storm brought days of freezing temperatures. Demand for heating soared to record levels just as power plants of every kind failed. Gas wells and pipelines froze, starving gas plants of fuel. Some wind turbines iced up. Coal piles froze, and a nuclear unit tripped when a sensor line froze.

With supply collapsing, the frequency fell dangerously. Operators ordered rotating blackouts, cutting millions of customers to stop the whole grid from crashing, and later said the system had come within minutes of a collapse that could have taken weeks to repair. Many homes went without power for days in freezing weather, and the state counted 246 deaths. Federal investigators found that frozen equipment and gas supply failures caused most of the lost power, and that earlier advice to winterize had not been required.

Apply the ideas. The balancing rule left operators no choice but to cut demand. The merit order did not matter, because the problem was plants that could not run at any price. Limited links to other grids meant little outside help. Claims that wind alone caused the crisis were wrong, but so were claims that fossil plants were blameless. The lesson is that reliability depends on preparing every source, and the grid itself, for the extremes.