Has anyone ever told you that you shouldn’t turn off lights if you’ll need to turn them on again within a few minutes? That it takes more energy to turn the light back on than to leave it on?

I don’t know where this idea came from, but it just isn’t true. What’s the theory behind it? People think there’s a power surge when you turn on a light, and that this surge uses as much as fifteen minutes of what the bulb normally uses. So the misguided assumption is that it’s better to leave lights on than to turn off lights you’ll be needing in the near future.

To understand how this is a fallacy, let’s consider the lowly (and soon to be obsolete) incandescent light. A bright incandescent bulb burns 100 watts. Let’s assume we turn the light on, then off, then on again, at intervals one second apart. That means that every other second, we would be using the equivalent of five minutes of what that 100 watt bulb uses.

That 100 watt bulb uses 100 watts per second (110 watt-seconds) each second it is on. So if it were to burn five minutes worth of extra electricity each time it is switched on, it would burn 100 watts, times 5 minutes, times 60 seconds, in other words 30,000 watt-seconds, during the second when it was switched on. Since those 30,000 watt-seconds are all being burned in a single second, we cancel out the seconds and get the theoretical equivalent of 30,000 watts of power. And since watts = amps X volts, and we know volts is either 120 or 240 (more or less), that would mean the circuit the bulb is running on would be 250 or 125 amps, respectively. Since most household circuit breakers are in the 5-15 amp range, we can see that if this fallacy were in fact true, people would be blowing their circuit breakers every time they switched on a light! QED.

If I turn off lights too often, will they wear out

All right you say, but won’t you wear bulbs out if you turn off lights too often?

Yes,you will wear a bulb out sooner. All you have to do is watch a 5-year-old switch a light on and off five times a second for a few minutes, then watch the bulb burn out, to realize that frequent on-and-off action for a light bulb does indeed wear it out. But we are not talking about rapid on-and-off cycling. We are talking about a cycle that lasts a minute, five, or fifteen minutes or more.

You might like to know how much of a bulb’s life hours are used up each time it is turned on and off. Sorry I can’t tell you – haven’t been able to find a reliable answer to this. (There are plenty of places where it’s recommended you switch the lights off if you’re going to be gone more than 15 minutes, but no data to back this up.)

What I can tell you is this: even if it turns out that you shorten a bulb’s life by one hour each time you turn off lights, you’ll still save both energy and money if you turn off lights whenever you leave a room, even if you plan to come back a minute later.

Turning off incandescent lights

Let’s look at the lowly incandescent again. I can buy one for about 25 cents. They last for about 1,000 hours and they burn 0.1 kilowatt hours (kwh) for each hour they are left on. Let’s assume each incandescent has 1,000 on-and-off cycles before it burns out, and let’s assume electricity costs us $0.10 per kwh, which means it costs 1 cent to run that 100 watt bulb for one hour:

  • Every time you turn a bulb off you are using 1/1000 of the $0.25 you paid for the bulb, or 0.05 of a cent
  • Every time you turn a bulb off for 5 minutes you are saving 1/12 of $0.01, or 0.16 of a cent

So you are actually saving more than three times as much by turning the light off for five minutes, as you would by trying to extend the bulb life by leaving it on. And my assumption about it using one hour worth of the bulb’s light to turn it on and off again just isn’t true – it was just to prove a point.

An incandescent bulb converts about 90% of its energy to heat. So when you’re trying to keep the house cool, turning it off will also help save on cooling costs.

And don’t forget that by leaving the light on, you’ve also removed another five minutes from the life of the bulb, since the bulb was running during that time.

Turning out LED house lights

As for turning off LED house lights, we don’t need to do the calculation – because there actually is no extra wear and tear on an LED light bulb from turning it on and off.

And don’t be lulled into complacency by the low power consumption of LED house lights. While a replacement for a 100 watt incandescent bulb might only use 5 watts, if you forget you’ve left it on, you’ll still be wasting electricity. Not as much as with a 100-watt bulb, but there’s no point in spending money on more energy efficient technology and then abandoning your energy saving ways!

Turning out compact fluorescent lights

Let’s apply the same logic we applied to an incandescent bulb, to a CFL. For a compact fluorescent, turning the light off does actually cause a bit of wear and tear on the bulb. But the bulb lasts about eight times longer than an incandescent bulb. Let’s assume the same 1 hour shortening of the bulb’s life when you turn the light off. A typical CFL has a life of about 8,000 hour life. A CFL of 23 watts, available on the web for $3.95, emits about the same amount of light as a 100 watt incandescent bulb. For electricity at $0.10 per kwh, it costs you 0.23 cents ($0.0023) to run that bulb for one hour.

  • Every time you turn a CFL off you are using 1/8000 of the $3.95 you paid for the bulb, or 0.05 of a cent
  • Every time you turn a CFL off for 5 minutes you are saving 1/12 of $0.0023, or 0.02 of a cent

Here, the economics of leaving the light on if you’re gone for less than five minutes seems to make more sense. Except for one thing: the one hour life shortening is made up. We just don’t know how much of a bulb’s life is eaten up by switching it on once. (At least, I don’t know, and I’ve looked around a fair bit!)

And except for one other thing: distraction!

Distractions when you don’t turn off lights?

If you make it a habit to turn off lights whenever you no longer need them, they’ll stay off until you come back. You might think you’re coming back in two minutes, or five or fifteen. But what if something comes up? What if a long phone call, a visit from a neighbor, or a quick run to the grocery store keeps you away longer than the five minutes you planned?

And what if you just plain forget about the light you left on? Then the five minutes can become five hours (for a bedroom or bathroom light, for example), or even five days for a light in a room you rarely visit such as the furnace room.

Using motion or infrared sensors

If you just can’t remembering to turn off lights when you leave a room, you might consider installing a motion sensor plug (for a floor or table lamp) or a motion sensor light switch or ceiling light adapter. These devices work as follows:

  • You can turn on the light from the switch, or have it go on when motion or the heat of a human body is detected.
  • A Manual-On device is only activated when you flick the switch; a Sensor-On device can also be activated by motion or heat.
  • A few minutes after the device stops detecting movement or body heat, the power to the light is shut off.

The sensors in these devices do consume a small amount of power, but far less than you’ll consume if you keep forgetting to turn off lights!

See the motion or infrared sensor light bulb connections, switches, and plug-in devices featured on this page – they are all available through Amazon.com.

Retrofitting existing switches

For those who want a motion sensor light switch but aren’t electrically inclined, you can install a special sensor panel over top of a standard light switch that will turn the switch off after a set period of inactivity.

This switch mounts easily with just a screw driver. It requires three AA batteries, and can be set to turn off after 1, 5, 15, or 30 minutes of inactivity. Once you’ve set it up, it monitors motion in the room, and shuts the light switch off when there’s been no motion during the timer period.

This is a great way to save money if someone in your house is prone to forgetting to turn off a light, especially one that’s in an out-of-the-way room where the light might otherwise get left on for hours at a time.

Turn off lights – make it your philosophy

Make it your philosophy to turn off lights. Not only will you save electricity when you turn off lights, and save money overall, but it will remind you to be an energy saver in other ways. And you’ll be setting a visible example to others, who will become more conservation conscious as well.

Once you’ve gotten into the habit of turning off lights (and convinced others in your household to do the same) don’t forget to put some of the savings into adding more energy saving lightbulbs to your home – CFLs, dimmable CFLs, LEDs, fluorescent tube lighting.

3 replies
  1. Tiny
    Tiny says:

    Strange, but years ago I read in the newsletter of my electric bill from COMED that I shouldn’t turn the lights off when leaving a room if I plan to go back in within the next 15 minutes or so because it used more electricity than if I left that light on.
    When did this change?

    • Robin
      Robin says:

      The problem is an idea takes hold, and seems sensible, and so countless people (including people who should know better) repeat it until it becomes general knowledge, even if it is wrong.

      As I explain in the article, if it were true that turning it off and back on uses more energy than leaving it on for 15 minutes, you would blow the circuit the light is on.

  2. SL701
    SL701 says:

    I think the idea originated with old-fashioned fluorescent lights, which required a prolonged surge of current over, sometimes, five or ten seconds, before they finally “caught” and turned on, and whose life reportedly was significantly shortened by cycling on and off. It may never have made much difference to old-style incandescent lights, and as you point out, new technologies have made the whole argument moot.


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