unplugged: again
February 9, 2011 9:49 AM   Subscribe

How much current does an AC adaptor draw when it's plugged in but not powering anything, or the device to which the adaptor is attached is powered off?

The AC adaptor for my document camera says "AC input 100-240 VAC 1.2A Max 50-60Hz 90VA-150VA; DC Output:12Vdc 4.0A" on it. Is there a way to use this info to determine how many kilowatt hours of electricity this device uses on a daily/annual basis? Specifically, I'd like to know how much power the AC adaptor draws when the doc camera itself is powered off but the AC adaptor remains plugged in. Why? Because our in-house "Energy Team" disconnects the AC adaptor jack from my camera several times a week but, when they do this, they always leave the AC adaptor plugged into the wall jack. Isn't the adaptor still drawing a small amount of current? Therefore, isn't unplugging the jack from the powered-off camera accomplishing little-to-nothing?
posted by RockyChrysler to Technology (16 answers total) 2 users marked this as a favorite
AC input 100-240 VAC 1.2A Max 50-60Hz 90VA-150VA; DC Output:12Vdc 4.0A

None of that information has anything to do with how much power the supply draws when plugged in but not in in use. Still, you're right that most power supplies do draw some power whenever they are plugged in, regardless of whether another device is connected.
posted by jon1270 at 9:55 AM on February 9, 2011

Also, that gobbledygook printed on the power supply tells you how much power the supply is able to deliver, not how much it actually uses. The power consumed (aside from the no-load power) depends on the demand of whatever device it's plugged into.
posted by jon1270 at 9:58 AM on February 9, 2011

And since I haven't posted quite enough answers here yet --

You can probably find a model number printed somewhere on the power supply. With that and Google, you might dig up the manufacturer's data sheet, which in turn might tell you what the no-load power usage is.
posted by jon1270 at 10:03 AM on February 9, 2011

I measured this once for various AC adaptors (for phones, computers, etc) that I had lying around, and none of them registered on the meter; i.e. they definitely each use less than 1 watt when plugged in but not in use. I suspect that modern AC adaptors include circuitry to cut off the current when there's no load, but I can't find any evidence for that online, so don't quote me.
posted by Salvor Hardin at 10:08 AM on February 9, 2011

Is it warm? If so, there's at least a bit of waste energy. You could always check with a multimeter or kill-o-watt, too.
posted by glibhamdreck at 10:08 AM on February 9, 2011

You can buy a Kill-A-Watt and find out. I have tested a number of household items ((tv, computer, iPhone charger) but no heavy appliances). I didn't find any where the answer was more than "little or none" with the exception of projectors (but I only tested one projector). FWIW, the one projector I tested drew an astonishing 10 watts while off (but plugged in).
posted by Phredward at 10:10 AM on February 9, 2011 [1 favorite]

FWIW, the one projector I tested drew an astonishing 10 watts while off (but plugged in).

They keep the bulb warm so that you don't have to wait the full cold-off 5-10 minutes to preheat the bulb.
posted by Netzapper at 10:19 AM on February 9, 2011

There's a difference between standby power and power used by an AC adapter plugged into the wall, but not into a device with zero load. The former can be a real concern but the latter is going to be an inconsequential amount of power. Feel how warm they get. A typical adapter when just sitting there doesn't feel any warmer than room temperature. If its not getting very warm I wouldn't worry about it.
posted by damn dirty ape at 10:23 AM on February 9, 2011

You can buy a Kill-A-Watt and find out.

Like Phredward, I've tried this but was never able to come up with a value indistinguishable from zero for an unplugged adaptor brick. I did this for a period of a week or so with a couple of different ones, with little result. The Kill-A-Watt isn't terribly sensitive though, and I'm sure someone with better equipment could give you a better answer.
posted by bonehead at 10:40 AM on February 9, 2011

Despite the PG&E/Enron propaganda suggesting that there's any appreciable load from such things, the measurements on the wall warts I actually have have all been under the limits of the Kill-a-Watt meter to actually measure them. I have hooked up my multimeter to measure current at wall voltages once, and came up with numbers that, if I remember right, suggested at it cost me a few pennies ($.13 per kilowatt hour, 8766 hours per year) to leave them plugged in. Way under the error of my devices to measure them.

Back in the 80s when these were actual transformers that was different, but with modern switching power supplies the only current flow you see is basically leakage from the capacitor, which is miniscule fractions of your clock.
posted by straw at 10:41 AM on February 9, 2011

Sorry, that should be "not indistiguishable from zero", a hedgy, sciency way of saying nothing, zilch, or nada.
posted by bonehead at 10:43 AM on February 9, 2011

My local library has Kill-A-Watts to lend, maybe yours does too? nthing 'too small to be of consequence, but greater than zero.'
posted by fixedgear at 10:47 AM on February 9, 2011

I have at least a dozen of these in a windowless studio. I never worried too much about it, until last summer's heat wave, when the power was degraded. They seemed to feel hotter to the touch then, and I'll try to keep unused transformers disconnected from now on.

I'm plugging all those kinds of things into a series of power strips that can all be turned off together at the doorway threshold to that room.
posted by StickyCarpet at 10:48 AM on February 9, 2011

Best answer: The fact that your device specifies such a wide range of input voltages says to me that inside it is a nice efficient switching power supply, not a lossy laggy old mains transformer. I'd expect it to use rather less than one watt when plugged in and not powering anything, for a cost of less than 1W * 0.001kW/W * $0.25/kWh * 24h/day * 365day/year = $2.20/year.
posted by flabdablet at 11:32 AM on February 9, 2011 [1 favorite]

burhanistan, no matter how many people it's multiplied by it's still dwarfed by refrigerators, A/C, desktop computers, televisions, and even lighting.

If you have a decent sized TV and it's on for an hour a day, twenty wall warts are still just a little noise.
posted by straw at 8:36 PM on February 9, 2011

I've seen someone on the web connect multiple AC adapters (mobile device chargers from memory) to a Kill-A-Watt and it took 4-5 to register 1W. I thought it was at mefi's own dansdata but my google-fu fails me.
posted by markr at 9:56 PM on February 9, 2011

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