A good way to cope with forgetting to turn of the lights is automatic light switches. But they must be using some energy to do their job. What's the rate of energy use for a typical automatic/sensor light switch? How does this compare with, say, a typical CFL?


2 Answers 2


The most common automatic light switch is a PIR sensor. I've looked up technical parameters of several such sensors. The idle (sensing) power consumption of most of them is 0.1 W. Some of them have the idle power consumption of up to 0.3 W and I believe some older ones could consume even 1 W. The active power consumption of the sensor is higher, but it's still only a fraction of the CFL's power consumption.

Running them for 24 hours, one week, one month or a year is summed in the table:

  Appliance      | Nom. power | Wh per 24h | Wh a week | Wh a month | kWh a year
  Efficient PIR  |      0.1 W |     2.4 Wh |   16.8 Wh |      72 Wh |    0.9 kWh
  Worse PIR      |      0.3 W |     7.2 Wh |   50.4 Wh |     216 Wh |    2.6 kWh
  Rather old PIR |        1 W |      24 Wh |    168 Wh |     720 Wh |      9 kWh
  Mid light CFL  |       15 W |     360 Wh |   2.5 kWh |   10.8 kWh |
  Poweful CFL    |       27 W |     648 Wh |   4.5 kWh |   19.4 kWh |
  Mid light LED  |        7 W |     168 Wh |   1.2 kWh |      5 kWh |

The conclusion:

  • using an effective PIR sensor for a year compares to forgetting a CFL on for 3 days
  • using a not-so-good PIR for a year is equivalent to letting a CFL on for a week, or even less
  • even if not asked I included a LED light: its weekly usage compares to a year of activity of an effective PIR sensor.
  • the old & power hungry automatic switches should be avoided
  • our ability to forget the lights on differs but seems to be sustainable

The BEST lighting control is an old school switch operated by a very ethical person. You can often turn lights off or down depending on what your task is, and what the sun's doing. And you will turn lights off the moment you leave, not after a built-in delay of five or ten minutes.

  • 1
    Welcome to Sustainable Living! I agree with what you are saying, but you are not answering the question. The premise here is that people forget to turn the lights off and that an automatic switch can help.
    – THelper
    Commented Sep 4, 2019 at 12:21

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