How a power grid stays balanced

A power grid cannot store the product it sells, except in special cases you can count. The electricity being used in a city right now was generated moments ago. If generation and use drift apart, the system does not politely run down. It changes speed.

In North America the speed everyone agreed on is 60 cycles a second, 60 hertz. Generators spinning in step with one another are what hold that number. When people turn on more load than the generators are producing, the machines slow a little and frequency falls. When generation runs ahead of load, frequency rises. Operators do not wait for the lights to brown. They watch that number, because a small drift is the earliest honest signal that the balance is off.

The old picture of the grid was a stack. Nuclear and large coal plants ran almost all the time, called baseload, because they were cheap to keep on and slow to start. Gas plants and hydro filled the afternoon peak. A few expensive units sat ready for the hour everyone came home and turned on a stove. That picture is less true than it was, because wind, solar, and cheap gas have shoved coal aside in many regions, but the underlying job did not change. Something has to be producing, every second, what is being used, plus a reserve for the generator that might trip.

Reserve is the part of the bill nobody sees. A grid that is using every plant it owns has no answer when one of them fails. Operators keep a margin: plants already running below their maximum, or plants that can start within minutes. That spare capacity looks like waste if you only count the electricity sold. It is the difference between a tripped plant becoming a flicker and a tripped plant becoming a cascade. The plants that almost never run are sometimes the ones a summer afternoon is paying for.

Transmission is not a detail. A region can have plenty of generation and still go dark if the lines that carry it are full, down, or pointed the wrong way. Electricity follows the physics of the network, not a contract. A contract says who pays. The wires say where the power actually flows. Congestion is when the cheap plant cannot reach the city that wants it, so a more expensive plant closer in has to run instead. The price spike is often a map of full wires, not a shortage of fuel.