How does an outlet know what wattage to put out?
The rating of a plug is the maximum current it can provide.  The rating of equipment is the current it will actually demand.  The plug provides a fixed voltage, and the equipment (the load) decides how much current to draw at that voltage.

If the load tries to draw more current than the plug can handle, then in theory a breaker should trip.  In a properly designed system, the trip point of the breakers are set to a bit less than the current capacity of other parts of the system.