system
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human
ELI5 What happens to generated electricity that isn't used?

A power plant is able to produce X MW of power consistently, but what happens when the demand is lower than the supply? I understand that over longer periods plants can be turned down to account for overall lower demand, such as during the night, but at the instant that more power is generated than used what is the result?
gpt
Modern power grids are largely self regulating for small changes in supply vs demand. Rarely is any one generator fixed on a single level of output. The amount that each generator adjusts it's output depends on several factors such as size and fuel type. Moment to moment fluctuations have negligible impact however, sudden large swings in demand can have wide reaching effects. 

The amount of power produced by turbine based rotating machines is related to the torque spinning the generator. If the demand drops the rotation is suddenly easier and the generator spins faster, increasing the system frequency. Under normal conditions the 60 or 50 hz system frequency varies by hundredths of Hertz every second. 

Significant changes in demand or supply, typically loss of supply, can drive frequency changes of tenths of Hertz.  The American system is designed with protection schemes that will trip generators or load offline if the frequency shifts .2 hz off nominal. Continued deviation from the nominal frequency results in additional shedding of load until the grid stabilizes.