system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What happens when a power company generates too much power?

When a power generation station produces too much power what do they do with the excess?  Who pays for, what I'm assuming, is wasted output?
gpt
When you take electricity from a generator, it becomes stiffer to turn. If the engine turning the generator keeps producing the same amount of power, then if the electricity taken from the generator reduces, then the generator (and engine) will speed up - like how if you are driving a car at steady speed with the accelerator pedal still, but then reach a downhill slope, the car will speed up, unless you release the accelerator.

If you connect two generators together, then they will lock their rotation together. So all the rotation of all the generators in a grid are locked together in perfect sync. This means all the generators start speeding up.

The grid manager will give instructions to all the power plants telling them how much power to produce, but they will also give out instructions telling the power plants what to do if the speed of the generator changes. In the US, power plants aim for 1800 rpm generator speed, but the grid manager might tell some power plants that they should automatically start reducing power if the speed goes over 1802 rpm, and, for example, drop down to 50% by 1806 rpm. The exact instructions depend on what the power plant is capable of doing, and whether the power plant operator wants some compensation payments for reducing power.

The power plant operators will try to sell power in advance to utilities in an auction system. Then, they'll tell the grid manager, how much power generation they have booked, and whether they are fully booked, and what price their spare capacity is, as well as whether they are willing to reduce power, and how much compensation they they want if they have to reduce power.

The job of the grid manager is then to try to work out which power plants give the best value for handling sudden unexpected power shortages or power excesses, and let them handle the fluctuations.

Just be glad you don't have to work out all the billing between these different companies, trying to match what was pre-arranged, with what actually happened, and all the billing for the automatic controls, compensation for reduced output, etc.