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July 14, 2008

Living With Less – Are Dimmers Better than CFLs?

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Have you ever wondered if you installed a dimmer whether you’d save any energy in your home?  I have tons of networked dimmers installed throughout our house on every incandescent light bulb we have - including floor rope lights used for night time lighting.  "Why?" you might ask.  Besides being completely crazy about controlling and monitoring things around my house, it makes good sense to adapt the energy consumption of a particular light to the current requirements.  The interesting argument is, "how much do I save and are they better than CFLs?"

I’m going to propose a standard house.  One scenario will use Incandescent bulbs and no dimmers, one will use Compact Florescent Lights (CFLs) without dimming, and the last one will use Dimmers.  We will then run a simulation of a standard day usage pattern to find out which one of these makes the most sense in reducing a household’s lighting energy consumption.

OK, so we need a standard house.  The US average power consumption for homes is around 900 kW-hrs per month.  The US Department of Energy (DoE) states that around 8% of that is consumed by lighting (on the average).  The rest is HVAC, refrigeration, water heating, TVs, electric appliances, pumps, etc.  So the amount of power consumed by the lights in a month would be around 72 kW-hrs.  So that provides us roughly 2.4 kW-hrs per day for lighting.

Figures 1 and 2 below show two separate scenarios based on probable usage pattern of a family of 3 (i.e. husband, wife and teenager) during a normal weekday for a house with 16 bulbs.  The blocks indicate 30 minute periods to simplify the charts.  Figure 1 was filled to roughly 2400 W-hrs for 65W incandescent bulbs and no dimmers which is roughly our standard US household.  By replacing all 16 bulbs with CFLs, the total energy consumption drops to 622.5 W-hrs.  This is a 75% savings in energy for the lighting.  Figure 2 shows the effect of adding dimmers to the same lights and lowering the brightness according to various tasks.  Many times lights are left on simply to navigate through a house and rarely need to be at full brightness.  Also, while watching TV, lowering the lights to a comfortable viewing level makes it easier to see.  TV’s with adaptive brightness may also lower their backlight or projection brightness to adapt, saving power as well.  The calculation with the dimmers drops the energy consumption to 1837.5 W-hrs.  This is a 26% savings which is much less than the 75% savings of the CFLs.

You can download the Excel spreadsheet I created by clicking here so you can run your own scenarios.

So in a year’s time how much money does that save?  For a normal year of days like those above (365.25 of them), 2490 W-hrs * 365.25 equals 909.5 kW-hrs.  At an average rate of $0.10 per kW-hr, the power would cost approximately US$90.  The CFLs’ power would only cost US$23 per year and if we had installed dimmers, we’d spend US$67.  To convert the incandescent bulbs to CFL units would probably cost around US$64 which would pay for itself in the first year.  Dimmers could cost anywhere from US$4 to over $US100 each, so the payback (best case) would be 2.78 years...

Additional considerations would be the environmental factors of CFLs - they all contain mercury which is extremely toxic.  Newer versions use less, but the mercury is required for the bulb to operate.  Non-toxic LED bulbs will eventually emerge and drop in price enabling cost savings as well as an environmentally safe solution.

Got a comment?  Drop be me an email or comment here on the blog.  Till next time...

Figure 1 - No Dimming, Incandescent and CFL bulbs
Figure 2 - Dimming, incandescent bulbs


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wayne edwards

The third paragraph illustrates the biggest confusion in promoting CFLs - lighting is typically a very small portion of electricity consumption in homes. If light was free, electricity consumption within the home would only decrease 8%. It's a start, but only a start, and a pretty small one at that. Also, energy costs for manufacturing and disposal of CPLs seem to be ignored - does anyone know whether they more or less energy to produce and dispose of than other light sources? I don't. More significant savings are likely to come from tackling the larger energy users in the home.

Petri Pihkala

Somewhere was mentioned that mercury of CFL's is more than enough compensated by their smaller energy usage. This is because electricity is for large part made by coal burning installations. And every ton of coal has impurities in it, including mercury. So each saved kWh by CFL's means less burned coal and therefore less airborne mercury coming from smokestack.


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