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Thursday, October 22, 2015

Energy efficient motion sensing night lights

I am revisiting the topic of night lights.  Last month I wrote about "Low Cost to Operate Night Lights" and discussed electroluminescent and LED nightlights that provide a sufficient glow for navigating a dark home at night.  These lights stay on 24/7 and thus they are on in the daytime when they are not really needed, however since they only cost around $.02-$.10 per year to operate this is of little concern.
Motion sensing night light
Recently I decided to try out some motion sensing LED night lights that only illuminate when motion is detected in a darkened room.  These well-designed units are available in packs of two for about $20, and come in cool white and warm white options.  I placed one in my front hallway and another in the bathroom and have found that I now no longer need to turn on the lights when going to the bathroom at night or when coming into the house after dark since they provide sufficient illumination to see.  This is significantly more light than the always-on LED lights I reviewed recently.  It is quite pleasant to have a light come on automatically just when and where you need it.  They have an impressive range and turn on instantly when they detect someone moving around at night and remain on for 60 seconds.  If you stand or sit perfectly still, they will go off which can be sometimes disconcerting in the bathroom, but the moment you move again they come back on.  For this reason I wish there was a switch to enable them to stay on longer.


Motion sensing night light only activates in the dark

As an engineer I naturally needed to quantify how much energy these lights use compared to the LED lights I wrote about recently.   I found that they consume about 3 Watts in both the off and on state - about half the power of an old-style night light lamp.  I was actually a little surprised to find that the unit uses slightly less power when the light comes on!   Clearly the motion sensor is using power all the time to  detect motion and ambient light.  This means that their cost to operate if you are paying $.10 per kilowatt hour for electricity (which is the US national average) is around $.21 per month or $2.60 per year.  While this is more than the $.10/year cost to operate an always-on LED nightlight I find the trade-off quite amenable.

The sensor is a Passive Infrared Sensor (PIR) that works by detecting infrared light emitted by warm bodies.   The lens in the dome of these sensors focuses IR light onto a device that triggers when it sees warm objects passing across its field-of-view.  There is also a photocell that detects ambient light level and disables the light when the ambient light is above a certain threshold.

I am so pleased with these lights that I plan to order a couple more.  While they do use more energy than other types, I have enough surplus power from my solar array that I can spare a few Watts.  I have not paid for grid power since May (just payed my Sept-Oct bill and still have a credit on my account thanks to net metering).