A dim question.

February 27, 2008 7:46 AM

Let's say there are two identical light fixtures, on two identical dimmer switches. One fixture has two 15W dimmer-compatible compact fluorescent lights, the other has two standard 60W incandescent bulbs. The dimmer switches are turned just beyond the "off" position to the "dimmest" position (identical on both dimmers). What will the difference in power utilization be between the two fixtures?
posted by eschatfische to science & nature (8 comments total)

In my experience the CFL setup will use up far more resources, not just electric power, because so-called "dimmer-compatible" CFL's really are not and will fail far too soon.

I hope my experience has been atypical and so I will be watching this thread with interest.
posted by JimN2TAW at 8:19 AM on February 27, 2008

Yeah. Fluorescents and dimmers are not really compatible.
posted by Doohickie at 8:23 AM on February 27, 2008

Not my area of expertise, but I do know that fluorescents have something called a "power factor," which is different than the watts listed and more accurately reflects the electrical drain.
posted by StickyCarpet at 8:38 AM on February 27, 2008

I don't think that the CFLs will light at all if the dimmer switch is moved to just barely on. Wikipedia suggests that dimmable CFLs don't go below 20% of their maximum light output, though there is a product in the UK that goes down to 2.5% of the maximum light output. So, I'd say that the incandescent bulb will use more power, because the CFLs won't use any at all until you give them enough power to light up.
posted by ssg at 9:13 AM on February 27, 2008

Just FYI, this isn't a hypothetical question with some goofy catch. I've tried both "dimmable" CFLs and 60W incandescent bulbs in the same light fixture in our home with the same dimmer. The 60W incandescents put off a small but pleasing amount of light at the lowest setting, and provide bright light on the high setting. The 15W "dimmable" CFLs put off very bright light on the lowest setting, and extremely bright light at the highest setting.

My understanding is that power utilization will be fairly linear with the incandescents as the dimmer setting increases -- that at lower settings, the bulb is likely to only use 10-15W of power, which would then increase to all 60W at the highest. However, I don't understand the mechanism by which the CFLs regulate power -- if the CFLs are using roughly the same amount of power as a dim incandescent at the lower settings but provide brighter light, my personal preference would be to use the dim incandescent, both because dim incandescent light is more pleasing to me, and due to concerns about the environmental impact of producing and disposing of CFLs due to the mercury content.
posted by eschatfische at 9:55 AM on February 27, 2008

Since incandescent lights are only about 2% efficient and CFLs are only about 7% efficient you don't need to make any fancy electrical measurements. In both cases more than 90% of the energy is going into heat. To determine which is using the most power just put your fingers on both. The hottest one is using more power.
posted by JackFlash at 11:39 AM on February 27, 2008

According to this PDF, the Varilight dimmable bulb uses 60% of its rated wattage at half brightness, so, assuming your two CFLs are similar, they will use 18W at half brightness. Your incandescent bulbs will use that much power at 15% brightness (and presumably the CFLs would use quite a bit less than 18W at their minimum brightness).

Using your assumption that you use the incandescents at a minimum of 10W each, you are going to use less energy for the CFLs throughout the range of brightness.
posted by ssg at 11:48 AM on February 27, 2008

Depends on the type of dimmer too. And have you pre-warmed up the bulb? And how does the CFL make itself dimmable?

But yes, if you give a bulb less electricity, it will use less.

(I've got to get one of those kill-a-watt things, as I changed over significantly to fluorescent lights, and noticed nothing on the electric bill. I suspect there is something weird going on.)
posted by gjc at 11:14 PM on February 27, 2008