Back-of-the-Envelope: Motion Sensors

September 8th, 2014 by Potato

In the name of efficiency, many places are moving towards using motion detectors to control the lights, which can be annoying when the decision circuits decide the room is empty and turn the lights off on you. On the whole I find sensor-controlled lights more of an inconvenience than a labour saver. Still, if the lights are off more that’s going to save power. Advances in lighting efficiency means it’s not quite as bad as it used to be to leave the lights on, but unless there’s a next-generation LED technology coming, turning them off when you’re not in the room is still going to be a necessity as always-on lighting just isn’t realistic.

However, motion detection isn’t free, either: the sensor uses some electricity, and of course has some capital costs. So the question is how bad do you have to be at turning off the lights for a motion-controlled light system to make sense?

Doing some brief research (I googled it), the sensor is not energetically expensive: drawing roughly half a Watt, that’s only 4 kWh/year. If the sensor is controlling four 100 W incandescents or eight 50 W halogens, that’s only ten hours of accidentally leaving the lights on, less if it’s an even larger room or hallway. Of course with lighting getting more efficient, even setting aside LEDs and using four CFLs of 13 W each, it would take 77 hours of accidental usage to break even, or about 12 minutes per day. And you have to be especially negligent to make it worthwhile to put a sensor on a circuit with only one or two bulbs.

And on the flip side, sensors can lead to more light usage if you rely on the timer to turn the lights off rather than turning them off yourself. If you leave a room long enough for the lights to turn off say 3 times per day, and each time the lights burn for 2 minutes longer than they would have if you just hit the switch on the way out, then that’s an extra 36.5 hours of light caused by the switch. More if there’s more in-and-out traffic through the area, less for more rarely visited spots.

It might be because I’ve got investments on the mind, but this sounds like it’s going to shape up to be an analysis focused on risk: if the room is a place you go to infrequently with your hands full (so less likely to turn the lights off, and more likely to have them burn for a long time if your forget), with many high-consumption lights, then the risk of having the lights burning all weekend may outweigh the drain (and capital cost) of the sensor.

There are some other benefits to motion sensor controlled lights, such as infection control. There are also drawbacks, such as the existential crisis that happens every time the sensor fails to see you: are you a ghost and just don’t realize it yet, or is the sensor on the fritz?; and DEAR GOD TURN THE LIGHTS ON ALREADY I’M JUST TRYING TO POOP AND WHY IS IT DARK?

In the end though it doesn’t look like we’re talking about large sums of money either way. 4 kWh/yr will work out to about a dollar in electricity, and the sensor-powered light switches are only a few dollars more than a regular one. For your own private dwelling it may be a toss-up, but for a lightly used commercial washroom the math may make more sense, when the lights could be left on unnecessarily for hours every day. Which is a shame, because those are the same places where the malfunctions are most annoying.

5 Responses to “Back-of-the-Envelope: Motion Sensors”

  1. Steve Says:

    I have absolutely been in a situation where I was pooping in a washroom and the motion sensor turned the lights off and it was…. a little frightening!

  2. Potato Says:

    It is just the worst. You’re in an unfamiliar environment at your most vulnerable, only to be plunged into the depth of one of humanity’s primordial fears by capricious technology.

  3. Alicia Says:

    Oh man, that is quite the inspired post… As I said on twitter, I am constantly flailing at the motion detector in my work building. Though I was in there last night and I felt like god walking through a dark corridor and the lights flicking on as I went. Now that’s power!

  4. Wayfare Says:


  5. Potato Says:

    I still haven’t found an answer to the other question: are there LEDs that don’t flicker? How do I identify them? I know from xmas lights that there are some that are better than others for that… I always figured the flicker was due to rectification of the 60 Hz power, but I’ve seen it on the taillights of some cars which should just be DC powered…