My wife and I have several lights on dimmer switches throughout our home. We generally prefer the ambient lighting provided by dimmed lights. I am curious, though, if we are saving any electricity by dimming the lights. I feel like I read somewhere that dimmers work by rapidly turning on and off the current to a light, though it's just as likely I fundamentally don't understand how a dimmer switch works.
Yes. And here's why.
Old dimmers, used a variable resister to dim the light. Lets look at a simple example.
We can find total resistance (RT), by adding up all the resistance.
RT = R1 + R2 = 0 Ohms + 144 Ohms = 144 Ohms
Then we can find the total current (IT).
IT = ET / RT = 120V / 144 Ohms = .83A
We'll then calculate the voltage across each resistive load.
E1 = IT * R1 = .83A * 0 Ohms = 0V
E2 = IT * R2 = .83A * 144 Ohms = 120V
Finally, we'll calculate the total wattage (WT)
WT = V^2/R = 120V ^2 / 144 Ohms = 100 Watts
Lets see what happens when we increase the resistance of R1
RT = 200 Ohms + 144 Ohms = 344 Ohms
IT = 120V / 344 Ohms = .349A
E1 = .349A * 200 Ohms = 69.77V
E2 = .349A * 144 Ohms = 50.23V
WT = 120V ^2 / 344 = 41.86 Watts
As you can see, we've increased the resistance of R1 and effectively reduced the voltage across R2. And now we have a dim light.
Modern dimmers use a TRIAC, to reduce the amount of time the light is on. However, because of the circuitry in the dimmer, there is not a direct 1:1 energy savings. Dimming the light to 50%, will not equate to a 50% savings in electricity.
A typical waveform in an AC system would look like this.
A TRIAC prevents electricity from flowing every time voltage reaches 0, something like this.
So you end up with a waveform that looks like this.
With the TRIAC, the light is actually turning off and on 120 times per second. With every cycle, you're saving a small amount of power. Is it enough to actually see on your electric bill? I guess it would depend on how long the lights are on, and what percentage they are dimmed.
It can depend on the dimmer type - older ones just used to drop the load across a resistor, so you ended up dissipating the same power, just converting it to heat in a resistor rather than heat and light in a bulb.
Modern ones should save some power, they switch on and off rapidly, and just change the duty cycle to give more or less 'on' time.
I spent a fair amount of time researching this exact question recently, including paying an electrician to come to our home. He had NO understanding of the problem. Most dimmers that you buy are simply variable resistors. This means that if you have a 100 watt bulb on the circuit, but dimmed half way, you are sending out 50 watts to the bulb and 50 watts gets turned into heat in the switch box.
Dump too much heat into the switch box, and you may find you are cooking the dimmer. In our case, 300 watts of bulbs on a dimmer, dimmed down to a low ambient lighting was sufficient to cook a dimmer switch that was rated to handle 500-600 watts. (Our electrician saw that the dimmer switch was theoretically rated to handle the wattage, so it could not possibly be our problem.)
So, no, you are NOT saving electricity at all by dimming a bulb down, at least with a standard dimmer. You can buy LED bulbs, or CFL bulbs to help here. But beware that all LED bulbs do not seem to work on all dimmer switches. And CFL bulbs do not dim terribly well at all, even those that are designed to dim.
You can also buy an electronic dimmer. This is a dimmer that does its job by cutting the power off completely, many times per second. It does indeed save electricity, because the electrons which do not pass on to the light are not just shunted through a resistor to generate heat. Electronic dimmers are more expensive. Note that most dimmer switches you buy at the home store are still the resistor kind.
Finally, you can do one other thing. If you normally run the switch dimmed down quite far, then put fewer or smaller bulbs in the receptacles. For example, we had five 60 watt incandescent bulbs on a single circuit, that we normally ran dimmed way down for ambient lighting. While I plan on buying LED bulbs to replace them, dimmable LEDs are far too expensive now to justify this. Simpler was just to back out 3 of the 5 bulbs. Two 60 watt bulbs, still dimmed down half way are entirely adequate to light the area as we wanted it to be lit.
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Short answer is yes, you will save in electrical cost. Probably any dimmer made in the past 20 year has the technology to save you money. This is from Lutron, one of the largest dimmer manufactures in the world.
As you see, not only will you save electricity but your lamps will last longer. Thats why 130 volt lamps last longer than 120 volt lamps.
Dimming LED are easy but to get the best dimming experience you will need a dimmer designed for LED's. These have what I used to call a trim screw so you can adjust the dimmer to use the whole dimming range. Trim screws were used for fan speed controls and you would adjust the trim screw down to where the fan is spinning when the speed control is turned to the lower setting.
While rheostats were used as dimmers in theatrical lighting back in the early days, this has not been common since the 1950s or earlier and I have never seen a domestic light dimmer that didn't use a thyristor. You don't save a lot of energy dimming incandescent lamps due to the highly nonlinear relationship of efficiency to filament temperature (brightness) but it is NOT due to the extra voltage being burned up in the dimmer. Rather as the lamp is dimmed, the light output shifts towards infrared with a larger percentage of the power turning straight into heat in the bulb than visible light. You still save some, but not as much as you might think.
Some of the modern LED bulbs that can be dimmed really do save a lot of power though. On several I've measured, a 10-13W "bulb" dimmed down to what looks roughly half as bright to my eyeballs draws only 2-3 Watts.
While it is true that adding a variable resistance in series with a light bulb will lower the current and therefore lower the power (as the resistance increases) the fact remains that power is wasted in the form of heat through the variable resistor. I don't think there are any of these older "rheostat" (variable resistor) types being sold today. The newer design on the market modifies the AC waveform to be on only a portion of each cycle. This design is more efficient as it does not waste unused power however the silicon component used in the dimmer must also dissipate heat and does this through its mounting flange (usually aluminum). This is one reason why only a certain number of switches and wires are permitted to be in the box.
With incandescents (which are, as others have noticed, the only reliably dimmable bulbs), even in the best case scenario, their light output goes down faster than their power usage - so for example (made up numbers, but the principle holds), if you dim them to 75% of normal brightness, you're still using maybe, 80-90% of the original power. The brighter they are, the more efficient they are.