To save energy, should you turn lights off or leave them on?

Cecil’s classic post from 1980 has been revisted in the daily email.
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It occurs to me that he missed out one energy-saving aspect. This is that there is an energy and resources cost in manufacturing the bulbs/tubes in the first place, and in disposing of old bulbs.

So if you buy longer-lasting tubes or bulbs, you’ll be helping to save the world by reducing the need for raw materials and energy to make the blessed things.

Andrew Denny

Those 100 watt bulbs put out a lot of heat. My kids are forever leaving them on and my part time job is shutting the damn things off. My reason? Not that I care about the juice to power the bulbs.
We run our air conditioning nearly year round and my completely uneducated guess is that the extra work on the compressor to offset the heat created in the house by five bulbs burning in unoccupied rooms does make a noticeable difference on our household power consumption.

At least I think.


Seems to me that people sitting aroundpump out about 100 watts, also, so clearly a little work is being done to run the air conditioner.

What I’ve always wondered is if the reduced lifetime of the incandenscents from turning them on and off wastes more energy than leaving the bulb on. It’s got to take a fair amount of energy to make a light bulb and ship them to your store, and they almost always blow when you turn them on. (Anyone else remember stories of light bulbs at NY Precincts that had never been turned off, and stayed good for decades?). What is the break even point for leaving incandescents on?

I’m going to take a guess here that the dollar is a pretty good unit of energy here. That is to say, if you use the lights in such a way as to minimize your dollar costs, you’ll probably be close to minimizing your total energy useage, as well. After all, most of the materials in the bulb aren’t that expensive in their raw form; the cost of making the bulb is mostly the cost of the energy to process those materials. There’s a markup on the bulb between the factory and you, but the factory probably also gets its energy for a cheaper bulk rate, so it evens out. Given that, and given that a new 60 or 100 watt bulb costs in the neighborhood of 50 cents or a dollar, it’s obvious that the energy in using it is a lot more significant than in making it.

For the air conditioner, I don’t know exactly how efficient yours is, but it seems a reasonable approximation to say that the AC would need to consume about a hundred watts to offset a hundred watt light bulb. So when figuring the economy of your bulbs, consider them to use about twice the power listed on the box.

For ease of processing by brain cells, I suppose a 2:1 ratio might be alright for ballpark figuring of energy usage. But, let us not forget that these devices are not 100% efficient. Electrical motors range from around 70% up to around 90%. Then there is the efficiency of the other bits n’ pieces in the system like mechanics of the compressor, the fan blades (or squirrel cages), condenser and so forth. All of which accumulates to produce some over all efficiency.

Our goverment being efficient as always has invented something called SEER (Seasonal Energy Efficiency Rating as way to gauge how efficient your AC unit is. It is a ratio of the average number of BTU’s per an average number of Watt Hours. Nifty, eh? According to the poop at Trane’s web site if your AC unit is 10 years or older and you have a 19.5 SEER unit installed to replace it, you’ll cut your AC cost in half. In which case you can up the wattage of those light bulbs and install that sun lamp in your shower and still save $$. :smiley:

What about the extra use the light switch gets? I would think you could cut it’s working life in half by being anal about turning lights on and off all day long. If you need to hire an electrician to replace it you also have a very big bill that far exceeds the cost of the energy savings, add to taht the energy the electrician must use to drive to and from your house. Even if you replace it your self you still have to make a trip to home depot and waste fuel. Not to mention the energy cost of makign that switch.

Well, 50¢ to a dollar can get you 5-20 kW·hrs. This can burn a 60-100 W bulb for 50-330 hours. If a bulb lasts 750-1000 hours, then a light bulb costs between 5% and 50% as much to burn as to buy. The usage energy is definitely more significant, but I don’t know if the initial cost is completely insignificant.

I’m not an electrical engineer, but I hang around with a few. There are some EE types on SDMB, if we’re lucky, maybe one of them will take an interest in this thread. So I’m sort-of-guessing here, but here goes:

A conventional, el-cheapo, non-halogen light bulbs’ life is not affected much at all by the number of times they are switched on or off. This is part of the reason this very old technology is commonly selected for signals and flashers of all sorts.

There is a difference between wear on the bulb filament and stress on the filament. Leaving the light burning causes wear - the filament very slowly gets thinner. Switching the light on causes sudden heating of (and thus mechanical stress on) the filament. It’s true that this is usually when the bulb “blows,” but it only happens when the filament has been made very thin by many hours of burning, and would have burned out soon anyway.

An office I used to work in installed a system of incandescent track-lights in a hallway. There were something like 10 bulbs in all. They were left on 24/7. After about 6 weeks (~1000 hours), half the lights were out, even though they’d never been switched off. The guy in charge of changing light bulbs was kept fairly busy by that hallway.

I don’t think so. A four-pack of 60W bulbs typically sells for around US$1.80, just $0.45 per bulb. Each bulb, however, will consume around (1000 hours * 60 watts / 1000 watt-hours/killowatt-hour * $0.12 per killowat-hour) = $7.20 worth of electricity, or $28.80 for the four-pack. The energy costs for manufacturing and shipping the bulbs are reflected in the price of the bulbs. While it’s true that the factories and major shippers get cheaper rates than you and I do, they aren’t that much cheaper. For incandescent lighting, the cost of the bulb itself is usually pretty insignificant.

An incandescent bulb can be made to last an almost arbitrarily long time, as long as you’re willing to sacrifice efficiency. A typical 100W bulb, for example, outputs ~1800 lumens. You can get long-life or rough-service bulbs (with longer, thicker filaments) that last much longer - 5,000, even 10,000 hours - but the output will be much lower - ~1000 lumens, maybe even less. The miracle bulbs at the police and fire stations are probably old, very low-efficiency designs, and have likely consumed thousands of dollars worth of electricity over the years, while producing very low amounts of light.

I’m pretty sure that the additional wear & tear on the light switch would have little effect – those switches are designed for many, many cycles.

A few years ago, I replaced a stairway light switch in my Grandmothers 130-year-old house. This was an antique 2 push-button switch, installed when the house was converted from gaslights. But this was the only one that had been replaced, the rest of those original switches were still working fine. And these were real old contact switches, with many more moving parts and more wear & tear than modern silent mercury light switches.

Most houses are probably torn down long before the light switches in them wear out.

This fear of turning off lights goes back to Thomas Edison, who stayed up all night with his first 24hour filiment test because he was afraid that if he turned it out it wouldn’t restart.

He was wrong, as the next few bulbs proved.

And that line has been wrong ever since. Even the “old style” fluorescents were not that subject to burnout. Part of it was salesman’s hype to get people to change all their bulbs at once, as “preventive maintenance” - saves manpower over waiting for each to fail, but is not really a shortening of bulb life.

The Lawrence Lab at University of California tested fluorescents in the 50’s and found the trade-off time for leaving on vs. turning off was a mere 1/20 of a second, for turning on-and-off once a minute over the life of the bulb, i.e., a fluorescent bulb turned off half the time would essentially last twice as long - life varied by on-time, not times-turned-on.

I am an electrical engineer, though I haven’t worked in the fields covered by these postings. Nonetheless I will comment a bit.

“I’m pretty sure that the additional wear & tear on the light switch would have little effect – those switches are designed for many, many cycles.”

It’s heartwarming to read of your faith in corporate America. Apparently just the companies that I have worked for and those I am otherwise familiar with manufacture equipment with other things in mind than making them last as long as possible.

“Most houses are probably torn down long before the light switches in them wear out.”

Perhaps I should turn my house into a unique museum and make a lot of money, as it’s only fifty years old but has had several of its switches replaced due to their having worn out. In fact, I recall replacing one, due to its physically having worn out and not just for snicks, over thirty years ago. Too bad I didn’t save that rare and valuable example of a light-switch that lasted less than twenty years.

As for use of energy, I notice that none of you have calculated in the energy expended as we walk over to turn off the lights thousands of times per year, let alone all the energy that we’re expending here on this discussion.

Yay, I win.