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Old 05-09-2012, 12:19 PM   #1
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Power consumption of PIR sensor switch


I installed a passive infrared sensor switch (Enerlites WOS15) in my kids' bathroom to turn on a night light over the toilet (my young boys are content to pee in the dark, with predictable results). So far it works great.

As I was installing the switch, it suddenly occured to me that it uses energy. The nightlight is only a 4 watt incandescent. Does the PIR switch use more than 4 watts? Am I actually wasting energy by using a PIR sensor rather than leaving the nightlight on 24/7?

I cannot seem to find any energy consumption specs for PIR sensors. Anyone know?

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Old 05-09-2012, 02:19 PM   #2
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Power consumption of PIR sensor switch


well, you might be disappointed with your payback scenario.

A 4 watt light bulb left on 24 hrs a day 365 days a year will cost you $2.80/year in energy... If you can save 80% with a PIR switch then you save $2.25/year assuming the PIR draws nothing when off. The more it draws the less the savings.

My guess the PIR will take 10-15 years to payback. It will probably fail before then.

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Old 05-09-2012, 03:18 PM   #3
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Power consumption of PIR sensor switch


Given the limited wattage and low dollars involved, I'm not that concerned about payback. Just curious to know whether the PIR actually wastes energy in my application.

The bulbs last 3000 hours, so if I ran them 24 hours I'd use three bulbs a year at $1 apiece. I'm guessing they will last around 4-6 years at their current use, so that's about $2.75 a year I'm saving by not running it 24/7.

As for energy use, 24 x 365 = 8760 hours per year. 4 watts x 8760 hours / 1000 = 35 kilowatt-hours per year. SCE has a tiered rate structure that ranges from .05 to .23 per kilowatt hour (5, 8, 16, 20 and 23 cents per kWh). Let's just say it's about .15 for my average.

35 x .15 = $5.25 per year if I left the light on 24/7. Even at .10 per kWh, that's still $3.50. Your $2.80 number is only .08 per kWh, which is way less than my average, given my pool pump and central A/C.

Last edited by M3 Pete; 05-09-2012 at 03:32 PM.
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