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There are different types of generators: some supplement, and others replace, the grid.

Cogeneration is designed to produce heat (in chillier climates) and the process makes electricity as well while doing so, because why not?

* https://en.wikipedia.org/wiki/Cogeneration

Emergency / standby generators do nothing most of the time, and only kick in when needed:

* https://en.wikipedia.org/wiki/Standby_generator

* https://en.wikipedia.org/wiki/Emergency_power_system