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Real Watts???

Discussion in 'Photovoltaics' started by East-of-lake, Jan 14, 2004.

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  1. East-of-lake

    East-of-lake Guest

    Since I haven't popped the $100+ for a Watts Up meter I was wondering if
    anyone could answer the following:

    If I have a PC with a 350 Watt power supply will is draw 350 watts or should
    I add up the draw of each piece (motherboard, floppy, CD/DVD, hard drive,
    etc)? It's the etc that's particularly difficult as many peripheral cards
    don't bother giving you power consumption figures.

    Also I have a UPS and was wondering what it's "parasite" load is once it's
    fully charged and just passing on the power to the PC.

    Thanks for your help.
  2. daestrom

    daestrom Guest

    I bought a 'Kill A Watt' meter for <$40
    The 350 Watt power supply is rated for a *maximum* of 350 Watts. I doubt
    that any 'average' PC draws anything like that. My Dell draws about 80
    (measured with the aforementioned Kill-A-Watt). But the monitor draws
    another 70 when its on. Don't forget about that. And cable modem, network
    hub, whatever else you might want to use ;-)
    Sorry, can't help you here.

  3. Well, _my_ wife loves me, so I got one for Xmas! 8*)
    No, it'll be much lower than that, unless you are near or at the limit
    of the supply, in which case it may draw slightly more than that...
    Well, that's a good starting point (except as you note the number is
    hard to come by), but then you need to divide by the efficiency of the
    power supply, which will give you a slightly higher number.
    My system, with an Antec True 380S
    1G memory
    GigaByte GA-8KNXP motherboard
    2 35G Raptor drives
    Radeon 9800 Pro
    Floppy, dual optical drives, fans, etc

    draws about 3W in "off" mode,
    hits a startup surge of 457W,
    stays pretty much under 200W while booting,
    sits idle at about 135W
    sits barely under 200W when playing HalfLife
    is about 4W when in S3 (suspend to RAM) mode

    My APCC BackUPS Pro 650 draws about 11W by itself while in
    float-charge mode.
  4. Redmondite

    Redmondite Guest

    "Watts" the difference between the Watts Up and the Kill a Watt?
  5. steve

    steve Guest

    Load drawn by the computer power supply is dependent upon the power
    demands of the cards in it plus some overhead due to less than 100%
    efficiency of the power supply.

    Typical computer power supplies are around 90% efficient, so at most,
    with a full 350 watt load your computer power supply would draw 350/.9
    or 389 watts. It will be less because the cards inside the computer are
    not likely to draw all 350 watts.

    The ups will draw only slightly more power than the power drawn out of
    it -- UPS efficiencies are higher than 90% at full load.

    So assuming a 90% efficient ups (at 389 watts output load) the total
    would be 389/.9 or about 432 watts.

    Don't forget the monitor and other peripherals too.

    Yes, the manufacturers of the cards inside your PC don't do a very good
    job of reporting the power required most of the time. You may find the
    current required on each power supply voltage if you look carefully. I
    have found that hard drive manufacturers often do a good job of
    reporting total power required.

  6. Actually, when the power is on, the UPS isn't drawing any power except
    what it takes to float charge the batteries.
    Plus their numbers are probably the maximum power consumption of the
    card, not what it'll do under real-world conditions. Hard drives will
    draw a lot less when just spinning than they will when starting or
    working hard...
  7. I got the Watts Up Pro, which has a small (1K points) data memory and
    a serial connection for downloading data logs. Good box for the
    price, though a bit kludgey in the software department, which reminds
    me, I've got to get the latest software... [Nope, still doesn't save
    graph background or landscape print settings, oh well...]

    Watts Up will lose it's data if unplugged or the power fails (though
    the Pro will keep it's data log), and it has a cord instead of
    plugging into the wall directly.

    Essentially the Watts Up is a little nicer and more expensive.
    Imagine, you get what you pay for! 8*)
  8. Redmondite

    Redmondite Guest

    Well my original question is pretty much answered. But I've realized that
    there's a second question that the answers raise:

    If (for example) the components of a system (PC, stereo, whatever) draw 170
    watts are you better off using a 200 watt power supply running near it's
    rated load or a 350 watt supply loafing along? What about the power
    supply's life expectancy?

    The question's not theoretical since I have to upgrade my PC and had been
    planning on a new case/PS because the current one is a 200 watt box. I'll
    be running MB, FD, HD, CD/DVD
  9. Well, there's a tradeoff here. From an efficiency standpoint you are
    better off matching your power supply capability to your load, but
    many supplies are (ahem) agressively rated, and every supply will have
    a significantly longer lifespan if run at a fraction of its' rating.
    RUnning close to the rating also means you'll have trouble adding
    peripherals afterwards. I'd load a power supply at about 1/2 to 3/4
    if I were to plan ahead. [I didn't, but ended up at about 1/2
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