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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.

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If you prefer dimmed lights and would never want the lights to be brighter you could always install lower output bulbs instead. –  ChrisF Jan 26 '12 at 10:17
    
It is better to have less light on, then all the lights dimmed. –  Walker Jan 26 '12 at 12:57
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For both answers: It's not true that a simple resistor does not save power. As Power = Voltage² / Resistance, and Voltage is always 230V (or 110V depending on country), the consumed power actually drops. –  Nikodemus Jan 26 '12 at 13:45
    
I guess @Nikodemus comment comes from eliminating I (current) in P = V*I and V = I*R. But to understand this better it helps me to think about how, as the resistance is increased, current (and hence power) must drop, because the job of mains is to keep V from sagging at all under load. –  wim Jan 26 '12 at 14:02

8 Answers 8

up vote 27 down vote accepted

Yes. And here's why.

Rheostat dimmers

Old dimmers, used a variable resister to dim the light. Lets look at a simple example.

enter image description here

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

enter image description here

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.

Thyristor dimmer

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.

enter image description here

A TRIAC prevents electricity from flowing every time voltage reaches 0, something like this.

enter image description here

So you end up with a waveform that looks like this.

enter image description here

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.

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Some of the newer ones have nice fancy PWM circuits that can start/stop the flow thousands of times a second. –  Brian Knoblauch Jan 27 '12 at 13:16
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Yes, those are generally for CFL/LED light bulbs, which take advantage of the circuit design of the bulbs themselves to allow the bulb to dim when a TRIAC wouldn't "trip" the bulb to turn off, and wouldn't provide the "spike" needed to charge the ballast of a CFL to turn it back on. An incandescent normally couldn't care less how you turned it on or off; it responds more to the RMS power in the line than the exact on-off pattern. –  KeithS Jan 27 '12 at 19:07
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Even for a pure rheostat, increasing the total resistance lowers the total power delivered. P = V^2 / R. –  Brad Nov 6 '12 at 21:51
    
"In these dimmers power is actually consumed by the resistor, so there is no power saved when the lights are dimmed." This is untrue and for a highly voted question/answer it should be corrected. –  whatsisname Feb 12 '13 at 16:18
    
@whatsisname You are correct, thanks. –  Tester101 Feb 12 '13 at 16:53

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.

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For a constant voltage, increasing the total resistance lowers the power. P = V^2 / R. –  Brad Nov 6 '12 at 21:51

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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To work the math - on a 120 V system, a 100 W bulb is 144 Ω. For the bulb to dissipate 50 W, the voltage across it must be 85 V. This means the voltage across the resistor is 35 V, which means the resistor itself must be 60 Ω, which means the resistor dissipates 20 W. This may be affected by the fact that the light bulb will run at a lower temperature. (and a 100 W bulb may have to dissipate a different amount than 50 W to match the visible light brightness of a 50 W bulb) –  Random832 Jan 26 '12 at 14:59
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The basic point, though, is - as Nikodemus mentioned - that the overall lightbulb + dimmer system has a higher resistance than the lightbulb alone, and thus has less total current/power. (For an extreme example, for the light bulb to get 0 W of power, the resistor must have an infinite resistance, and therefore also dissipates no power) –  Random832 Jan 26 '12 at 15:02
    
If the bulbs have a weak buzzing sound, then you have a non-resistor dimmer, operating at the line frequency, chopping at 100 or 120 Hz. –  Skaperen Jan 26 '12 at 20:30
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@woodchips: Are you saying that if I put a 1000W lamp a dimmer, and turn it all the way down to the lowest setting, the dimmer is putting out 1000W of heat? –  Jay Bazuzi Feb 1 '12 at 17:35
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Every dimmer I have bought is the "chopper" type in one form or another. I can hear the "buzz" or "singing" in the bulb sometimes. –  Skaperen Mar 15 '13 at 6:26

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.

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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.

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How does a bulb designed for lower wattage compare to a higher-wattage bulb dimmed to the same light level? –  Random832 Jan 26 '12 at 19:03
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A 100W bulb dimmed to be as bright as a 60W will use more energy than a 60W bulb running at full brightness. –  Aric TenEyck Jan 26 '12 at 19:59
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Bulbs that operate with that more yellow color are less efficient. That color is the tell-tale sign they are emitting a greater portion of their output in the useless infrared range (unless you are using them for the heat). –  Skaperen Jan 26 '12 at 20:32
    
@Skaperen, or IR photography :) –  auujay Jan 26 '12 at 21:33

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.

enter image description here

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.

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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.

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I tested a $5 Leviton rotary dimmer, rated 600 watts, by dimming Christmas lights. The load totaled 520 watts. The dimmer worked by attenuating the AC voltage supplied. What I found is the dimmer temperature increased when the AC voltage is maxed and the temperature decreased when the voltage was attenuated. I initially thought the heat dissipated goes up when dimming, but now it looks like the majority of the heat generated is due to the inefficiency of the transistors inside. The higher the voltage and more current flowing through the hotter the unit got. At 520 watts load it got hot enough that I could not touch the heatsink longer than a few seconds.

The lesson here is don't use dimmers if you're gonna leave the light on at max most of the time. Energy not converted to light is wasted as heat. Dimming the light will use less electricity and save you money. Whether the light is dimmed or not the dimmer wastes some energy in the form of heat. The wasted energy goes up proportionally with the load.

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