Do Dimmer Switches Save Electricity

Posted on 2008-10-09
Last Modified: 2010-04-21
Is the same amount of electricity used regardless of the dimmer switch setting.

For example, a 100W lightbulb consumes a 100 watts of electricty per hour's use. If the dimmer switch is set midway and the light is dimmed, does it mean only 50 watts of electricity is used per hour?

Potentiometers come as linear and logorithmic. It would make sense to use linear pots in the manufacture of dimmer switches. Is the corrolation between the dimmer switching and the amount of electricity used a straight line graph?

The reason I ask is, by dimming lights, would it save on the use of electricity? A single 100W lightbulb can cost 15p over a 10-hour period (10h X 100W = 1KWh @ 15p/KW). That single lightbulb could add about £13.50 to an electricity bill per quarter alone!!
Question by:t0t0
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by:Thibault St john Cholmondeley-ffeatherstonehaugh the 2nd
Thibault St john Cholmondeley-ffeatherstonehaugh the 2nd earned 30 total points
ID: 22683454
There are two types, the old rheostat ones use the power in the switch instead of the bulb, so no saving. The newer type do save electricity but the dimming is greater than the energy saved ie. You are wasting power that would be better used on a lower wattage bulb.
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war1 earned 30 total points
ID: 22683467
Hello t0t0,

Yes, dimmer switches does save electricity.  Your example is like using a 50 watts lightbulb.

Hope this helps!
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ozo earned 30 total points
ID: 22683490
While it is possible to construct a dimmer that used  the same amount of electricity regardless of the dimmer switch setting, it would require shunting current past the bulb, and I doubt anyone would want to do something like that.
But although a dimmed bulb uses less electricity, it uses more electricity per unit of light
So one undimmed bulb uses less electricity than two dimmed bulbs set to produce the same total amount of light.
A more practical way to save energy could be to use compact florescent bulbs or even LED bulbs.

Assisted Solution

coflynn earned 30 total points
ID: 22683609
Dimmer switches do the dimming by chopping off part of the AC cycle. I doubt you'd run into the older rheostat type in any reasonably modern building.

For a detailed description of how dimmers work, see

The general stuff you need to know is that they cut off part of the AC cycle. Hence they will save some electricity, as they cut down on power used.

It's not quite as simple that a 50% dimmed bulb is taking 50% less power though. Due to how power is metered I you are going to be paying more than that. Your best bet is just to have lower-wattage of power-saving bulbs.
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rid earned 30 total points
ID: 22690059
Going into details of what a 50% setting on a consumer-grade dimmer actually means may quickly get quite complicated. The effective voltage will depend on "where you are" on the waveform envelope when the dimmer cuts out and the power will be really hard to calculate.

Also, incandescent bulbs take more current when the filament is "cold" than at  working temp, so the situation gets even more involved.  To top off, the actual visible light from such a bulb will not follow the power curve as you "undim" it. As the filament grows "whiter", the useful emission increases quicker than the power.

Dimming a bulb will give it longer life though. Generally, going down 5% on voltage will give about 4 times the normal life for a bulb.
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Accepted Solution

aleghart earned 50 total points
ID: 22690395
Dimmers do save electricity, and allow you to use appropriate bulbs for lighting where fluorescent and LED lights are terrible replacements.

I use dimmers where appropriate:  living room (with 6 can lights), bedrooms with 100W ceiling fixtures, central hallway.  In those locations, CFL are the wrong bulb for the job.

You cannot dim them.  They throw horrible light.  They take time to reach full brightness.  On illuminated switches, the CFL will flicker non-stop like a low-powered strobe.  Figured that out after two days of a baby and a 4-yr old not getting much sleep at night.

The dimmers allow us to use an appropriate amount of light.  Low lighting used most often at night in the living room, versus burning 6x26W CFLs at full power...the glare is terrible, and you can't watch TV.  In the bedrooms, using a dimmer allows us to check on the kids without waking them.  Also, one kid can be dressing and going to the bathroom in the morning without waking the baby.

Have not measured power consumption of full versus dimmed.  Not easy to do without measuring amps on the live circuit wire (not easy to do without opening the electrical panel).  Or, you could build a test rig with a Kill-a-Watt plug-in meter, cord wired to dimmer, wired to fixture with bulb.

Been thinking about building such a rig for personal edification.  Anyone have test results (including brand & model of dimmer)?
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Author Comment

ID: 22738171
Apologies for not responding sooner.... I'm still querying this one..... and my investigations are not complete.
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Author Comment

ID: 22847356
I have increased the points to 200.

I can see the logic in using a triac or some other semiconductor-type high-speed switching device which merely cuts off the supply for short periods of time and therefore this would indeed draw less current than a light bulb connected directly to a supply would normally as it only draws current during the 'on' times.

Going back to my original questio and being more specific, if a suitable resistor is placed in series with a light bulb so that a circuit consists of only the light bulb and the resistor. Am I correct in assuming the light bulb's brightness can be effected by the value of the resistor and if so, is less current being drawn from the supply or is the current drawn the same?
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Expert Comment

ID: 22848094
A series resistor will naturally decrease the current and the power consumption will also decrease, as power will be voltageXcurrent. Decreasing (average) current with a chopper device is less wasteful, though.
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Expert Comment

ID: 22848104
> Am I correct in assuming the light bulb's brightness can be effected by the value of the resistor
> and if so, is less current being drawn from the supply or is the current drawn the same?
increasing the resistance means less current
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Expert Comment

ID: 22848209
reducing the current does save electricity, but the electricity that is used is less efficiently turned into light because
the resistor will heat up dissipating energy without producing light,
and if the bulb is incandescent, it would get less hot, which would shift the radiation peak furthur into the infrared, so less ofthe energy goes into visible light.
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Author Closing Comment

ID: 31523092
Thank you for your replies.

Rheostat-based dimmers absorb energy = no saving. New types = saving. Low Watt bulb = greater saving.

Dimmers = saving. 50% dimness = 50% energy used.

Dimmers chop AC cycle. 50% setting does not equal 50% energy used. However = saving. Low Watt bulbs = greater efficiency.

1) Non-linear correlation. Dimming extends life of bulb ie, 5% = 4 times life.
2) Resistor-based = less current & less power. AC chopping = greater efficiency.

Dimmers = saving. Dimmers = some advantages. Test using Kill-a-Watt plug-in meter in test rig.

1) 1 undimmed bulb uses less enegry that 2 dimmed bulbs. Not as efficient as flourescent or LED lights.
2) Increasing resistance decreases current consumed. Resistors effect brightness.
3) Reducing current used = saving. Resistor absorbs energy dissipated as heat without even producing light. Less energy goes into visible light.

I have now closed this question.

What I really wanted to know was, if a suitable resistor is placed in series with a light bulb where only 50% of the power is reaching the bulb, does the resistor 'consume' the other 50% of the 'matching' enregy OR does the circuit draw only 50% (or so) energy from the supply? But that is based on my earlier assumption that dimmer switches use resistors to govern light output.

AC chopping is self-explanatory and there is no actual 'saving' as only as much energy is drawn from the supply as is needed during the 'on' states of the AC cycle. To clarify this, during the 'on' states, a 100W bulb will draw 100Ws of juice. The light will be dimmer however, I accept the 100W of juice is spread over a greater period of time.

In a resistor-based circuit, there is no AC chopping and therefore, a 100W bulb places a continuous 100W load on the supply. What happens to this energy ,or power, or whatever?

Overall, dimmer switches DO save on electricity used by delivering only as much electricity as is required to fire up the bulb's filament during the 'on' times of the AC cycle. The dimmer the light, the LESS electricity used (as opposed to the greater saving made). One could arguably say rather than saving electricity, one is using less light. But light costs money so in real terms, using less light costs less money.

The resistor in the resistor-triac circuit is purely there to control the 'chopping' intervals of the AC cycle.

I may have to go down to physics level to fully understand the resistor-only-based circuit as a 100W load across two terminals is STILL 100Ws whether you chuck a resistor at it or not..... or am I wrong?
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Expert Comment

ID: 23105372
Just thought I'd comment this:

"What I really wanted to know was, if a suitable resistor is placed in series with a light bulb where only 50% of the power is reaching the bulb, does the resistor 'consume' the other 50% of the 'matching' enregy OR does the circuit draw only 50% (or so) energy from the supply? But that is based on my earlier assumption that dimmer switches use resistors to govern light output."

For a given circuit you can of course match the resistance of the series resistor so that it takes 50% and the bulb takes 50%. Since incandescent bulbs are non-linear components, you may have to adjust the resistor empirically; there is no simple way to calculate what value to choose for the resistor.

For the entire assembly (bulb + resistor) the total power will be less than it was for the bulb alone, given constant voltage feed, whatever value is chosen for the resistor (as the TOTAL resistance will be higher, but again, it's very difficult to compute, as the filament resistance will decrease as it cools.

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