What happens to the electricity output when new users of electricity are added to the power grid with limited energy input?

That's when you begin to have occasional 'blackouts' ... when the system fails and
shuts down because of excessive load ... or occasional 'brownouts' ... when the utility
dispatcher/controllers reduce the voltage of the mains supply by a few volts, causing
every device to consume slightly less energy, and run slightly cooler, dimmer, slower.
That's when you begin to have occasional 'blackouts' ... when the system fails and
shuts down because of excessive load ... or occasional 'brownouts' ... when the utility
dispatcher/controllers reduce the voltage of the mains supply by a few volts, causing
every device to consume slightly less energy, and run slightly cooler, dimmer, slower.