Most of the time, production is simply slowed. You can disengage the turbine on an windmill or dam, and you can simply burn less coal/gas/etc. in a traditional plant. Balancing production vs. demand is a **huge** part of energy production.
Some energy can also be stored and used later. We can, for example, use excess energy to pump water back above a turbine, releasing it when we need more power.
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