Lately, there's been a fair amount of discussion on the topic of lighting, how much it costs and how important it is to replace the photo sensors that are supposed to turn them off and on with the sun. One person told me that we could save $500 every year by replacing the broken sensors.
This topic started coming up a few years ago when the sensors started failing. Our sense then was that it might not make financial sense to replace them.
So, I decided to look into it today. In short, we could replace them, and maybe we should, but it's not a big deal if we don't. In long, here's what I found.
Based on our last few bills, the cost of electricity is about 9 cents a kWh, which means that we could save $1.97* every year for each 5 watt light bulb that we turned off during the day. If you have a 7 watt bulb in your front yard, the annual savings for you increase to $2.76.
If we assume the cost to repair a broken sensor is $12 for the sensor and $100 for the service call, it would take you 41 years of savings from the repair to pay for the cost of it. I'm not saying it isn't a good idea - maybe you'll sleep better knowing you aren't wasting the electricity, or maybe it just annoys you to see the light on during the day - just don't expect it to make financial sense.
The garages are a slightly different story, but not much. Three of our garages have as many as three 5-watt lights operated by a single sensor, so we get some economies of scale. In that case, we stand to save as much as much as $5.91 per year, so the payback period is 19 years - a lot better than 41 years, but still not quite enough to get me off the couch and on the phone with the electrician yet.
With enough coordination and planning, this thing could still make sense. Altogether, we have 15 lights on our garages, operated by a total of 7 sensors. Assuming we could get them all fixed for $150 service call plus the cost of the sensors, the payback period goes down to just 8 years, which is now slightly better than the expected life of a new photo-sensor, which is 10 years. With this approach, we'd see a net savings of about $13 per year. That's 80 cents per house.
You might think that we'd save even more money if we coordinated everyone's private sensors with the garages, but because the payback period is so long on individual homes, that makes even less sense. We wouldn't make our money back for 12 years, even with the lower cost of doing all the work at once.
I suppose we could also consider the metals and environmental impact of manufacturing the sensors and the fact that they will go into a landfill once they fail, and consider the fact that much of the electricity we use in this area is sourced from hydro-electric and wind, so feel free to do that if the topic really interests you, but I'm already starting to feel silly for doing this much research over thirteen bucks.
Still reading? Post a comment and let me know.
*5 watts * 24 hours a day * 365 days a year / 1000 *$.09/kWh / 2 = $1.97