When I turn down the dimmer switch on my lights, do I actually use less electricity?

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8

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.

ahsteele

Posted 2012-01-26T06:34:06.820

Reputation: 470

2If you prefer dimmed lights and would never want the lights to be brighter you could always install lower output bulbs instead.ChrisF 2012-01-26T10:17:30.420

It is better to have less light on, then all the lights dimmed.Walker 2012-01-26T12:57:27.757

4For 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 2012-01-26T13:45:28.377

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 2012-01-26T14:02:46.867

Answers

43

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.

Tester101

Posted 2012-01-26T06:34:06.820

Reputation: 110 876

3Some of the newer ones have nice fancy PWM circuits that can start/stop the flow thousands of times a second.Brian Knoblauch 2012-01-27T13:16:00.440

1Yes, 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 2012-01-27T19:07:41.500

Many dimmers use scr's cheaper than triacs and they advertise a 50% savings because they only use the positive cycle (or negative) and dim similar to a triac from there. Basicly a triac is 2 scr's facing opposite directions in parallel.Ed Beal 2017-11-15T00:32:05.913

1Even for a pure rheostat, increasing the total resistance lowers the total power delivered. P = V^2 / R.Brad 2012-11-06T21:51:49.663

19

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.

lqlarry

Posted 2012-01-26T06:34:06.820

Reputation: 7 137

17

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.

Rory Alsop

Posted 2012-01-26T06:34:06.820

Reputation: 2 790

6For a constant voltage, increasing the total resistance lowers the power. P = V^2 / R.Brad 2012-11-06T21:51:25.220

11

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.

user558

Posted 2012-01-26T06:34:06.820

Reputation:

6To 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 2012-01-26T14:59:26.460

3The 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 2012-01-26T15:02:19.090

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 2012-01-26T20:30:23.990

1@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 2012-02-01T17:35:13.070

@JayBazuzi - the fact is, the LOWER you set a dimmer switch the more energy is pushed through the resistors inside that dimmer, thereby generating heat. What I have not seen is an actual statement that says the relation is an EXACTLY as you state it. And a dimmer switch will not go down all the way to zero anyway. But I have tested my claim. Drop the setting on a dimmer, and it heats up dramatically. This ONLY applies to the old style dimmers that use this scheme. – None – 2013-03-03T14:40:19.980

@JayBazuzi - I just remembered that Random832 did write out the math. Read his response, which agrees with my statement (in principle) though I won't claim to work out exactly what happens with a 1000 watt bulb. – None – 2013-03-03T14:56:59.430

1Every 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 2013-03-15T06:26:38.647

9

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.

James

Posted 2012-01-26T06:34:06.820

Reputation: 91

Most theaters used variable transformers or variacs they look like a big wire wound resistor but are not.Ed Beal 2017-11-15T00:35:00.767

3

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.

Aric TenEyck

Posted 2012-01-26T06:34:06.820

Reputation: 435

How does a bulb designed for lower wattage compare to a higher-wattage bulb dimmed to the same light level?Random832 2012-01-26T19:03:10.707

4A 100W bulb dimmed to be as bright as a 60W will use more energy than a 60W bulb running at full brightness.Aric TenEyck 2012-01-26T19:59:48.613

2Bulbs 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 2012-01-26T20:32:26.803

@Skaperen, or IR photography :)auujay 2012-01-26T21:33:07.977

2

Dimming your lights will reduce power use. Some people have been suggesting that any power not going out the bulb is instead making heat at the switch. While some power will make heat, you will not be using nearly as much power as at the bulb.

Here is some simple electric math Power Equations: P=IE Power is equal to current times voltage total voltage across the circuit is essentially constant ~120V AC.

If your dimmer is a simple Rheostat resistor, as you raise resistance your current will lower according to Ohms Law V=IR Voltage is equal to current times resistance, since voltage is constant we can rearrange by dividing both sides by "R" to I=V/R to prove that current lowers as resistance increases with a constant voltage.

another way of writing the power law is: P=(V^2)/R with voltage held constant and resistance increasing the output power will lower. Power has a negative correlation with resistance.

if you had a 100W bulb and dimmed to 50W output, you would NOT be producing 50W of heat at the Dimmer. That would burn your house down.

The other type of dimmer you are likely to see is a TRIAC dimmer. This dimmer essentially turns the power on and off over 100 times per second, thus will use less power as the lights are off a greater amount of time out of every second the more that they are dimmed.

Handyman Matt

Posted 2012-01-26T06:34:06.820

Reputation: 21

2

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.

SteveR

Posted 2012-01-26T06:34:06.820

Reputation: 3 337

They used variacs not resistors they look like resistors but are not.Ed Beal 2017-11-15T00:36:54.300

2

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.

Jessica Money

Posted 2012-01-26T06:34:06.820

Reputation: 21

0

Electricity to me is a black science and I have no expertise on the subject. But it seems to me that when using a resistor type dimmer you are pulling power through the meter to the switch and then restricting delivery to the appliance or globe by changing some of that unwanted power to heat. That would mean saving power by using a resistor type dimmer switch is an illusion. Sure the result you see is less light. But what you pay for is not what you see but what is registered at the meter. Newer dimmers that turn the power on/off 120 times a second are a different story.

John Dee

Posted 2012-01-26T06:34:06.820

Reputation: 1

Resistor dimmers are antiques -- any dimmer you're going to run into these days is going to be a triac type (or use some equivalent control means)ThreePhaseEel 2017-11-03T11:41:18.640

-1

I don't know much about this stuff, but here's how i understand it..in simple terms, the more resistance there is to the current flow, the less current will be used...so if you put the same amount of voltage across something that doesn't conduct electricity very well, like a resistor, you will use less current than if you had a thick copper wire instead of the resistor...the word resistance kind of sums it up...the resistor is resisting the flow of electricity, therefore you will use less of it...the current that does pass thru a resistor will generate heat, but there is less current flowing thru the circuit with the resistor in place. If you used a thick copper wire to join the 2 prongs of an electrical outlet, (don't do that) you would trip a circuit breaker because there would be close to zero resistance to the current flow in that circuit, which would allow a huge amount of current to flow, and the current would quickly go over the max allowed by the circuit breaker....so resistance limits the current flow...whatever amount of current that does flow thru a resistor will generate heat, but with less current flow, there will be less heat generated...so a large resister that limits current flow to a trickle will not heat up very much because there is less current flowing thru the resistor, because the resistor is resisting the current flow...if you put a smaller sized resistor in place of the larger resistor, which had the same amount of resistance as the larger resistance, it would feel warmer than the larger resistor, but it wouldn't be using more current...it's just that the heat would be dissipated over a smaller area, so the resistor would feel warmer...if the smaller resister also had less resistance, then the resistor would get even warmer because a smaller resistance would let more current thru...current that you would be paying for....I know i over explained that, but just trying to make it clear.

IronicMan

Posted 2012-01-26T06:34:06.820

Reputation: 1

Hello, and welcome to Stack Exchange. This long block of text is hard to read, and doesn't really add to the discussion. You might check out the Help center to see how best to contribute here. Thanks!Daniel Griscom 2017-11-15T22:14:11.740

-1

Just put a dimmer knob switch, basic $4 one at home depot, on a desk lamp with 25 watt bulb. When I put my hand over the fully lit bulb, about 2cm away from the glass, it burns my hand severly in 5 seconds. When dimmed to about twice as bright as a nightlight, I cannot feel much heat at all. It is significantly cooler, and significantly less bright. And the resistor is cold, and remains cold. Physical, easily testable facts.

Hakunamatata man

Posted 2012-01-26T06:34:06.820

Reputation: 1