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Electrical load when equipment not in use

  1. Mar 7, 2016 #1
    I have a question related to electrical load.

    Electrical/electronic devices such as laptop, tablets often come with SMPS. Say my laptop is rated to consume 90 watts at full load and 10 watts at idle. But my question is related to how much electricity will it consume if laptop is completely turned off, but it's walwart is still hooked to the AC line? Does it still consume electricity if it's just connected to the line without being switched on?

    Thanks for your help in advance.
  2. jcsd
  3. Mar 8, 2016 #2


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    The wallwart itself will draw a little current, whether or not it is connected to the laptop. The laptop will charge its battery if needed, and perhaps show one or more LED indicators. I suspect, but don't know for sure, that as long as the wallwart is connected, the charging ckt. will be active and drawing a tiny bit of current.
  4. Mar 8, 2016 #3

    jim hardy

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    Mine stays cool to the touch when the computer is off. I estimate it consumes around a half to one watt.
    There is a green LED on my wallwart that probably consumes not more than 1/10 watt and another green one on the laptop indicating power is reaching it. So that's 2/10 watt, the rest is the largely due to SMPS idling at very low current. It still has to switch to light those LEDS

    Be aware something inside the computer knows the state of charge of its internal battery and will draw more power until it's finished charging the battery. Afterward the consumption drops to a small but nonzero value.

    Feel of it and compare its warmth to an incandescent nightlight which is around 7 watts.

    At 14 cents per kwh which is typical power cost here in Arkansas , 5 watts costs you 1.68 cents per day.
  5. Mar 9, 2016 #4
    Thanks gentlemen for your participation. I did suspect the same. LED indicators and related circuitry isn't turned off so long as it's connected to mains. That must consume some power. I had the same thought as well.

    The objective of my question is to discuss/understand three aspect of this.

    (1) Does it do any good for environmental health for the planet? The answer seems to be "Yes". It reduces the power draw as the circuit runs at a "smaller idle current" as against it's usual rated current.

    (2) Does it reduce the life of the device? I mean keeping the device always connected? For example a phone charger. Okay, in this case the power draw is not huge and economically or environmentally it may not be so damaging. But how about the life of components? Do electrical/electronic devices age? Is it good for them or bad to have them always connected?

    (3) Does it affect their electrical/electronic properties? Several manufacturers advise keeping their equipment always connected or always disconnected. "Audio amplifiers" for instance; many manufacturers advise keeping the amplifier always on for best performance. OTOH, some equipment such as Microwave ovens are recommended to be kept disconnected while not in use. What could be the underlying logic?
  6. Mar 9, 2016 #5


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    It would be even better if you switched it off at the wall so that there was zero power being used ( no standby)

    yes components have life times ... the component with the shortest life span is the electrolytic capacitor
    These have limited life spans even just sitting doing nothing, having small AC currents around / in them so that they are running warm
    only shortens that life span even more

    I have heard that, and have never seen good evidence to support it
    Hence I refer to it as a myth

    most are always connected as most have a clock in them and people want to see that clock
    otherwise there's no real advantage to having it on or off other than what was discussed above for component life spans

  7. Mar 10, 2016 #6
    Yes and No...
    Thermal fatigue can produce failure with the mechanical flexing of components as they cycle through differences in temperature from the On/Off switching. Although that would becomes more of a problem when the temperatures of hot and cold are widely spaced.
    Sometimes it is preferable to keep some device always on to increase its life span.
    For walwarts, I wouldn't expect that life span is a critical issue if it is always plugged in, or unplugged when not in use.
  8. Mar 10, 2016 #7

    jim hardy

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    There's diminishing returns at play.
    A watt is such a feeble amount of power i can't see trying to save that drip of electricity.
    You'd be energywise ahead , in winter, to expend your effort opening the drapes on your south facing windows to admit light , avoiding a ten watt LED bulb and maybe a thousand watt space heater. And in summer watering your roof to cool it, easing the load on your airconditioner. In August I hose mine a couple times every afternoon and can feel the difference inside. I'll be adding insulation to my attic as soon as finish some electrical work up there.
    Capacitors have a lifespan and it is reduced by heat. So, if at idle the device stays cool, you're not shortening its life. In my power plant I encountered electrolytic capacitors that had lasted thirty years because they were operated a a fraction of their rated current, effectively they were idling.
    Plugging in a SMPS causes a surge of current as the capacitors charge. That surge can be hard on the rectifiers.
    So, my opinion is as long as it stays cool i see no downside to leaving a device on.

    old jim
  9. Mar 11, 2016 #8


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    I'm afraid that the answer to that is kind of fuzzy.
    Widely circulated estimates say that these AC adapter/standby power losses account for 2-6% of USA electric energy consumption. I think that's high, but I only have an opinion. I know of no authoritative source that provides peer-reviewed data on this subject.

    Modern wall warts are better today than in 2001, but there are still lots of bad devices (and old devices) around. On a personal level, I have a five year old AM/FM car radio on my boat that draws 1 amp @12V, 24x7 when turned off. I must use the circuit breaker to turn it all the way off, and then it looses its memory of preset stations. If I left it "off" with the breaker on, that one device alone would increase my energy use 40%.

    So the issue of adapter/standby power energy consumption remains unsettled. Devices grow more efficient but the number of such devices that most of us own grows, and most people leave many of them plugged in 24x7. Any number in the range 0.5-5.0% of USA electric energy consumption could be credible.

    A comprehensive survey producing hard data would be very welcome. Utilities with smart meters could look for houses with zero HVAC running, no lights on, and nobody home, and measure the base power use, ignoring on/off transient loads such as refrigerators and water pumps. It would be one of those trendy "big data" projects. I think it would be a fun study and I'd do it myself if I were not retired.
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