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My co-workers (who worked with the company longer than I have) loves to remove their laptop's power cord when they need to move around the office.

I frown on this practice since they usually don't remove the power brick from the power socket. I've told them numerous times that its dangerous to leave the brick unattached to the laptop and that they should remove it from the socket if they need to move around.

I think I need a better argument on why they should stop this practice. Does anyone has any experience to share to discourage this behavior? Is there a research or article somewhere that shows how much power is wasted?

Edit: If you think I'm wrong and leaving the power brick connected is not dangerous, go ahead and prove me wrong. :)

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migrated from Nov 13 '09 at 23:41

This question came from our site for system and network administrators.

Sometimes the people in question are disorganized, and always moving away the power cord is a risk to lose it. So, I would argue that if I lose my power cord it would cost more than an entire year plugged alone (I do remove my power cords, but I know some people would argue this). – GmonC Nov 14 '09 at 2:07
up vote 13 down vote accepted

Can I flip this around and ask why you think it is dangerous?

Is it the danger of it bursting into flames? (Miniscule) Is it the heat given off by the brick perhaps melting nearby items? (Maybe - chocolate?) Is it the trip hazard from the cables? (On a desk?) Is it the untidiness? (People are messy...) Is it the energy waste? (~1W from this Coding Horror post)

None of those strike me as 'dangerous'. Sorry.

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Don't apologize. Its a legitimate answer. Yes, Energy waste and bursting into flames are what I perceived as dangerous. I'm going to accept your answer since you've proven me wrong (although, I might unaccept if someone else proves this answer is wrong. ;) – MrValdez May 19 '09 at 2:36
Oh, can you edit your answer to link to the energy waste [citation needed]? This would be helpful to people who might see this post. – MrValdez May 19 '09 at 2:40 perhaps? – warren Sep 25 '09 at 12:02
Of course! Thanks, warren. – fourstar Oct 1 '09 at 14:46
Perfectly safe to leave it plugged in. It won't be drawing much power when disconnected from the laptop but connected to the mains so heat is minuscule. – Matt H Aug 23 '11 at 0:46

The amount of power consumed by the adapter when the laptop is not drawing any load is minimal. There's the parasitic power losses involved in having the voltage converter produce its output voltage, but without the laptop there's no real current being drawn from the wall.

The only things drawing power when the laptop is not plugged in are the resistive elements and diodes in the adapter itself. What about leaving the adapters plugged in do you think is dangerous?

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I could be wrong about the adapter being dangerous if left unplugged to the laptop. Anyways, its alright to prove me wrong on this. – MrValdez May 18 '09 at 9:23

as long as the power supply gets warm/hot despite unused, you've got a brilliant argument for energy saving.

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Does it? I thought it gets hot only when doing actual power conversion. – vartec May 18 '09 at 10:52
the thing is: IF it gets hot, it DOES waste energy. And then: Why not save this minuscule power waste ? – lImbus May 19 '09 at 13:33
"I thought it gets hot only when doing actual power conversion" - that is what should happen, and does for a 100% efficient power supply. Switching brick-style power supplies are very efficient, but some can still leak small amounts of energy, and rarely they can leak enough to warm up a bit. – thomasrutter Dec 11 '11 at 23:40

I must admit, that very often I do it. If I'm going to unplug laptop for short time, I don't unplug the PSU. AFAIK, when not used, it doesn't consume any electricity. It gets hot, because power conversion is never 100% efficient. So some 20-30% of electricity drawn gets converted to heat. But when laptop is unplugged, 20-30% of zero is zero.

The risks I see is cable lying on the floor or in the case of power surge.

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Actually the power supplies on these things are called "switched mode" . They don't contain a big transformer which makes them small. And they are quite efficient. Around 85-90%. That's why they don't get that warm. – Matt H Aug 23 '11 at 0:49
"But when laptop is unplugged, 20-30% of zero is zero." That would be true if power supply efficiency was entirely linear, but it isn't. A power supply's efficiency only applies over most of its range (say, 20% through to 100%), but once you get significantly below this, efficiency drops away. Even when there is no load at all, there will still be some energy used in the power supply. As others have said, in a properly functioning power brick it should be about one Watt, which would be nothing to worry about. – thomasrutter Dec 11 '11 at 23:43

A laptop power supply has to convert 110V AC to (say) 12V DC. It does this usually with a switching power supply. A switching power supply is quite efficient (around 90%), so it only takes say 110 watts to produce 100 watts of output.

Under no load (disconnected) they consume a small amount of power to operate the electronics. A power supply should not be hot under no load. The only hazard is if you apply a short circuit across the terminal (the business end that goes in the laptop), you can destroy the power supply. Good quality switchers have output protection which will sense the short circuit and shutdown the output while the short condition is in effect.

Yes, you can also trip over the cord.

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Hypotheticaly it can be dangerous in some cases:

Some one walks over a cord and falls down and breaks something.
There is a water leakage somwhere near and you risk a short circuit.
Somebody can plug it in some other non-compatible device and break it.

But there is no any threat to adapter itself in keeping ac adapter plugged into wall outlet while laptop is unplugged.

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Why is it not a trip hazard when plugged in and suddenly a trip hazard when not? – Matt H Aug 23 '11 at 0:51

I disagree with the power load being minimal. I was trying to track down some powere consumption issues, and discovered that the Dell bricks (for monitors as well as laptops) draw a fair amount of power even when the monitor is switched off. If you want to test this lazily just watch it remain warm even when not connected.

Dangerous - probably not, except as a trip hazard.

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I've noticed warm unconnected power bricks of many devices large and small. It is best to leave unplugged unless your electricity is free. – spoulson May 18 '09 at 12:30
Unless the brick is malfunctioning, it's drawing less than 1W. Easily tested with a kill-a-watt. I agree with unplugging it regardless, but I think that qualifies as a minimal draw. – Kara Marfia May 18 '09 at 14:52
@Kara Marfia: it's not that easy to measure power consumption in that low range with devices that are supposed to meter higher currents and "cleaner" draw. The null-power-draw can be very "nasty" electronically spoken, so hard to measure. – lImbus May 19 '09 at 13:35
He didn't ask about power draw when the brick was connected to a device that's off but about power draw when the brick is disconnected. – David Schwartz Sep 30 '15 at 6:59

I don't like leaving a laptop supply plugged in, because some power connectors have exposed contacts and if they hit a metal desk or chair leg, they may short out. It's not likely to start a fire, but it will usually kill the adapter.

The good adapters will be wired with a coaxial connector with the hot end on the inside (most Dells). Some will not. USB power connectors, more common now, are relatively safe as well.

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From my knowledge a transformer works by sending current through a coil which induces a current in a coil thats located very close to the initial coil. This is how you reduce the power from 120v to 5v, or whatever. So, even though you disconnect your notebook, there is still current running through the block. There just is no current running through the secondary circuit, which the primary (house) circuit is creating.

At least, that's what I remember.

My point is, that the brick probably is using the same amount of power, always.

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I think there must be something missing in my analysis, and all I could think is there is some sort of variable device in the brick that allows for more energy to flow through if necessary, since the power brick usually get's hotter or colder depending on usage. I know my 360's power brick gets very hot during use, but isn't hot when it's just sitting there. – kshaeta May 11 '11 at 22:36
power bricks don't use transformers- they use switching regulators – Journeyman Geek Aug 22 '11 at 23:47
@kshaeta Even if it was using a transformer, it wouldn't use the same amount of power always. If there was no draw from the output, most of the power used to build up the magnetic field in the transformer would just be returned to the circuit when the magnetic field was torn down. The transformer would basically ring like a bell, requiring only enough power input to compensate for inductive loss. Of course transformers are designed for low inductive loss and the amount of inductive loss has nothing to do with the power used by the output. – David Schwartz Sep 30 '15 at 7:02

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