MAILBOX: Motion Sensors?

Motion sensor light switch
Is this motion-sensor light switch appropriate for your home?
Motion sensor light switch

Jeff Siegel’s letter on “Automatic Energy Savings” (“Mailbox” in HP159) raises expectations too high for room-occupancy sensor switches. To do their job, these switches must have heat or motion and light sensors, plus a decision-making circuit. Parasitic energy consumption for various switches runs between 0.9 and 5.2 watts—just for the switch. That’s as much as 3.7 kWh a month wasted on having a sensor acting as your electricity nanny. It’s cheaper just to use LED lights and hope you remember to turn them off more than half the time.

Worse, most light switches are wired to the AC “hot” leg only, with no “neutral” present in the switch-box. That means the switch must be designed to either pass a nonstop small current to the load, or else to allow a “load” current in the equipment ground wire, which used to be a safety violation.

Jeff wrote a letter, not a full article, so there was no space to distinguish between switches that are “manual, on/auto, off” and the “auto on/auto off” type. There was no room to discuss the value of prime-time energy savings versus off-peak consumption. The whole question is way too complicated to allow a simple answer. My preference is to skip the sensor switches: “It’s better manually!”

Joel Chinkes • Photon Harvest Company

Comments (5)

David Mackiewicz's picture

I am completely off grid and far, far away from any street lighting. So, to make it easy to walk from my car to the house, I have added battery powered LED with motion sensors. The batteries last months between charges. These motion sensor lights are only about $10 at CostCo.

Michael Welch's picture

Interesting. What brand and model number are you using?

David Mackiewicz's picture

Capstone Industries. Model: CT-M316. Here is a link to the one that I have.

rdhoore108's picture

Motion sensors are great in halls and corridors. The light goes on as soon as you enter, and goes off after a while when you leave it. Yes, they consume power, that should be clear. So they are used for comfort, not for saving on electricity. They also add to security, since they make it clear to potential burglars that someone is home. My family members are very happy with the automatic lights, and also guests love it.

And actually, I'm not even sure if my present setup uses more electricity than before. When there were two traditional switches in my stairwell, one upstairs and one downstairs, the LED lamps would always stay on very dimly, even when the light was switched off, due to impedance on the wires. Then I replaced the switches with motion sensors, and needed only one phase wire, one switched wire, and one neutral between the floors. Since then, the LED lights are really off when no one is around. I have never tried to measure the watts consumed by the previous "ghost light" though, so maybe I'm just trying to pacify my conscience :)

Michael Welch's picture

Yes, thanks for that. You are correct, and it is a matter of personal priority. Like hundreds of things in life, we sacrifice some energy for convenience. For example, we do it on a larger, more important scale every time we decide to drive our car to the store instead of riding a bike. Rare, practically nonexistent, is the American who does not use energy strictly for convenience.

I am not an electrician, so I don't really understand why your lights were dim when they should have been off, but it seems something was wrong -- that there should not be any energy flowing to them.

But it was important for us to set the record straight on the energy costs of using motion sensor switches.

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