How much power is wasted with a light dimmer?
Dimmers traditionally involve a TRIAC that chops the ac waveform with a potentiometer controlling the duty cycle.  These circuits cause highly distorted currents leading to a low power factor.  Knowing this, you will easily find examples of such circuits online.

Varying current through an incandescent lamp with a series resistor (rheostat, potentiometer) wastes an unacceptable, even unmanageable, amount of power.

Note that this applies to incandescent lamps. There is no single standard solution to dimming LED lamps because of the varying drive methods used in the lamps.