Modern electrical dimmer switches do save energy. A dimmer that is used to
control a frequently used light will almost certainly save enough in energy
costs to pay for itself.
Dimmer switches save energy by switching the light on and off 120 times per
second, which reduces the energy moving through the circuit. The longer the
switch is off, the lower the light output and the greater the energy
savings. To accomplish this, the switch uses a semiconductor device called a
triac (triode alternating-current switch). It's the same as turning down the
thermostat on a heater or applying less pressure to the accelerator pedal
while driving a car.
Although switches are comparatively simple devices, there is more to dimmer
switches than meets the eye. Be sure to match the dimmer's wattage rating to
the load. For example, control no more than 600 watts of lighting with a
600-watt dimmer. Also, match the dimmer to the type of load, such as
incandescent or low-voltage lighting.