How does the power grid stay balanced second by second?

A power grid has almost no buffer: generation and consumption must be balanced across the entire grid, because energy is consumed as it is produced. Every time you switch on a kettle, a power plant somewhere has to supply that much more, within moments. The signal that keeps everyone in step is the grid's frequency: the alternating current swings back and forth 50 or 60 times a second, and every generator on the grid turns in step with it.

In a synchronous grid, all the generators run at the same frequency and stay very nearly in phase. The nominal frequency is 50 Hz in large parts of the world and typically 60 Hz in the Americas and a handful of countries in Asia. Because generators can be connected in parallel only if they share frequency and wave shape, that number is shared by everything on the grid.

Frequency also doubles as a pressure gauge. When there is more demand than supply, the heavy spinning generators slow down; when there is a surplus, they speed up. In the first seconds, the rotational kinetic energy of the generators is the store that answers first. A heavily loaded grid slows, and governors raise output. A lightly loaded grid runs fast, and control systems cut output.

Small errors do show up. In 2018, Kosovo used more power than it generated. The frequency across the whole of continental Europe dropped to 49.996 Hz, and because some electric clocks keep time from the grid's frequency, the missing cycles added up until certain clocks were six minutes slow. A tiny dip, but a whole continent shared it.