Lighting dimmers provide variable light output; they are among the most effective lighting controls for home energy improvements.
Dimmers allow different lighting outputs, according to the needs: more lighting for your cooking and cleaning activities, for instance, and also low-level lighting for night-time navigation.
When you dim a CFL or a LED bulb you are reducing their wattage and their output, saving energy and extending its lifetime.
Dimmer switches add versatility to traditional switches; they can brighten or dim a room just by moving a control up or down, or by turning a knob, or by some other similar mechanism, allowing significant energy savings.
Dimming the lights 50% can save around 40% in electricity costs, besides extending the bulb life and setting the right light level for the moment (reading, entertaining, having dinner…)
Wall Dimmer Switches
The most typical dimmable wall switches comprise a large on/off switch and buttons to fine-tune the light levels. They are easy to operate and install.Wall switch dimmers are great to set the right light level for an activity (having dinner, reading, TV watching...) and to get energy savings.
Some wall dimmer switches have separate controllers, to allow the user to operate the switch from contiguous rooms.
Unless there is a way of commanding the switch from other parts of the room, you should install it at each end of hallways.
Dimmers for table and floor lamps are inexpensive: often less than $10. Wall dimmers are more expensive, typically around $30-$40. Prices depend largely on their exact features. Most dimmers have a LED locator light (to help you find the dimmer in the dark), and are of slide on/off type. Be aware to compatibility issues with other lighting controls.
Dimmers & Presets
Some dimmers offer "presets", that is, they allow users to set pre-programmed light levels. This concept can be quite sophisticated, with controllers operating multiple remote switches in a single or several rooms. See: Whole House Lighting Controls Energy Savings.
Fixtures with dimmers aren't necessarily more expensive than other fixtures, and can provide significant energy savings. Choose a qualified fixture: Energy Star qualified fixtures with dimmable ballasts are a good choice. See: Qualified and Dimmable Lighting Fixtures
New Dimmers and New Low-Energy Lamps
Consider dimmers for CFL (fluorescent bulbs) and LED light lamps; only they can provide significant energy savings. Incandescent and halogen lamps provide little power savings.
Only bulbs designed as "dimmable" should be used. Check the package or call the bulb manufacturer to ensure the light bulb is dimmable.
Prefer a certified (UL listed...) dimmer. That's important for quality and performance.