Surface Pro Power Consumption (Through the AC Adapter)

Discussion in 'Microsoft' started by Steve S, Mar 23, 2013.

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  1. Steve S

    Steve S Pen Pro - Senior Member Super Moderator

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    I spent some time a couple of evenings ago measuring the power consumption of my Surface Pro in a couple of operating states. The data below were measured with a Kill A Watt EZ power line monitor that advertises a 2% (if memory serves) measurement accuracy. The data were taken at the wall plug (so-called "wall plug efficiency") and are a measure of the total power draw of the Pro and its AC adapter. Therefore, the measurements include power conversion efficiencies and other losses. These data are only intended to be general indicators of power draw. Three Pro operating states were measured with both the Pro AC adapter and then with the RT AC adapter:

    <> Pro in Sleep
    <> Pro on at Start Screen
    <> Pro running a video (as a measure of typical usage; not intended to represent heavy usage or CPU / graphics saturation)

    The Pro was measured with the battery charging (starting from about 40%) and with the battery fully charged. The latter measurement presumably gives some insight into the system power consumption since the battery is no longer drawing charging current.

    Using the Pro AC Adapter:

    AC adapter rating: Input: 100V - 240V @ 1.3A Output: 12V @ 3.6A + 5V @ 1A ==> 48.2 W total power draw (Also, assuming ~94% conversion efficiency and PF = 1, at 121VAC wall plug voltage, current ==> 0.4 A. Because the PF is not 1, the actual AC current draw is higher.)

    Battery at ~40% charge
    <> Pro in sleep - 27.2W (PF = 0.53, VA = 51.1)
    <> Pro on at Start Screen - 37.3W (PF = 0.52, VA = 70.0)
    <> Pro running video - 38.7W (PF = 0.52, VA = 73)

    Battery at ~90% charge
    <> Pro in sleep - 13.2W
    <> Pro on at Start Screen - 22.8W

    Battery at ~96% charge
    <> Pro in sleep - 8.1W
    <> Pro on at Start Screen - 18.1W

    Battery at 100% charge
    <> Pro in sleep - 1.3W
    <> Pro on at Start Screen - 11.0W
    <> Pro running video - 13.1W


    Using the RT AC Adapter:

    AC adapter rating: Input: 100V - 240V @ 0.65A Output: 12V @ 2.0A ==> 24.0 W total power draw (Also, assuming ~94% conversion efficiency and PF = 1, at 121VAC wall plug voltage, current ==> 0.2 A. Because the PF is not 1, the actual AC current draw is higher.)

    Battery at ~40% charge
    <> Pro in sleep - 25.4W (PF = 0.53, VA = (didn't bother to measure))
    <> Pro on at Start Screen - 25.3W
    <> Pro running video - 23.5W

    So, what's happening here? Simple; the RT AC adapter is max'ed out at ~2 amps and simply can't provide any more power! Exactly how the Pro allocates input power internally when that power is limited is not known; it's possible that the CPU is throttled or that the battery charges at a much slower rate or some combination of these and other effects.

    I hope these measurements provide some insight into the two Microsoft AC adapters and their relative capabilities...
     
    Last edited: Mar 23, 2013
  2. Deathangel

    Deathangel Scribbler - Standard Member

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    Very cool! I have one small problem with your summary of the RT adapter though:
    You show that fully charged, the SP needs ~13W for typical usage. Since the RT can provide ~24W, the statement above sounds a little wrong. In the scenarios you show, the SP would use 13W of the RT adapter to run itself while still having 9W over to recharge the battery. Only if the SP starts being used in heavier duties, exceeding the 24W would the battery start to be drained, even with the RT adapter plugged in. Did the battery actually drain in the 40%/idle/video scenario? :)
     
  3. Steve S

    Steve S Pen Pro - Senior Member Super Moderator

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    <<...Since the RT can provide ~24W, the statement above sounds a little wrong...>>

    ...The statement is a generalization, but I see your point; I was rushing to finish the post so I could go to dinner. Under the stated conditions (battery charging and Pro operating), the Pro wants to draw about 39W, which is well beyond the power that the RT AC adapter can provide. The question is, how does the Pro allocate power internally when it is supplied with less than 39W? You are probably correct that the Pro consumes 13W to operate and the battery is relegated to charge more slowly with the ~9W that are left, but there may be other possibilities.

    I have modified the root post accordingly...
     
  4. Deathangel

    Deathangel Scribbler - Standard Member

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    Well, it is rated for 2A @ 12V output, which according to my math is 24W :p.
    The SP may be doing other things in the electronics, what I wrote was just what seemed most likely. :) Thanks for modifying it!
     
  5. MSGaldenzi

    MSGaldenzi Scribbler - Standard Member

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    So hold up... the Pro can use the surface RT charger to charge? If so... I gotta buy one asap... I hate the larger brick of the pro.
     
  6. Deathangel

    Deathangel Scribbler - Standard Member

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    Unless MS is doing something weird in the charging electronics for some reason, as long as the SP is off or being used lightly, the RT adapter will charge the SP. Albeit more slowly than the real thing. :D
     
  7. jhoff80

    jhoff80 Pen Pro - Senior Member Senior Member

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    It does work. I use my RT charger at my office.
     
  8. MSGaldenzi

    MSGaldenzi Scribbler - Standard Member

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    I actually sold my RT while I had the Pro and NEVER tried it... I read somewhere that the connector was different... Thanks for all that commented! I'm happy i get my little brick back (now I just gotta buy one). Has anyone tried those knock off ones on amazon/ebay?
     
    Last edited: Mar 24, 2013
  9. Steve S

    Steve S Pen Pro - Senior Member Super Moderator

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    <<...Has anyone tried those knock off ones on amazon/ebay?..>>

    ...There have been a couple of comments to the effect that they work, but I haven't seen many details.

    <<...I hate the larger brick of the pro...>>

    ...If you hate the Pro adapter because of the extra cord, there are ways to fix that (see Posts #15 & #16):

    http://forum.tabletpcreview.com/microsoft/55363-helpful-hints-tips-5.html
     
  10. MSGaldenzi

    MSGaldenzi Scribbler - Standard Member

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    Very nice, Thanks for the heads up. I like that L shaped connector and thats exactly what I need, but that site you referenced is trying to charge me 8 bucks for shipping... kinda crazy if you ask me. I am going to shop around, but thanks for putting that idea out there.

    *Edit* - Found this guy http://www.amazon.com/SF-Cable-adap...r=8-1&keywords=NEMA+1-15P+Plug+to+IEC+C7+Plug - Ordered it and free 2 day shipping with my prime. Should do the trick. Thanks again!
     
    Last edited: Mar 25, 2013
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