What really is the power consumption of a PC ?

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Popped into Maplin recently and saw a 'power meter thingy' - you plug it
into the wall socket and the appliance
plugs into the meter. The meter provides displays of minimum and maximum
wattage used - timed usage and if you
input the price of your kwh it'll tell you the cost of running said

I've been meaning to do this for ages so I bought it  and tried it with my
PC and got a real surprise. The box consumes between
120 idle and 168 watts when gaming (STALKER, Crysis etc). The LCD screen is
25w and speakers another 5w.

Like Tony (see post below this) I've been looking at power consumption of
graphics card and imagining I've been
using 300w to 400w regularly but this meter clearly shows a different

my system is C2D E7200, 2x1gb DDR2 800, 8800GT, 2x SATA II HDDs, DVDRW, SB
Audigy and couple of 92mm case fans for
airflow. use that only PSU calculator (Extreme) and it says my power usage
is close to 400w which is why I have a 550w.

I ran this meter from dawn til dusk and did everything on the PC - gaming,
scanning and printing  (scanner runs off the USB)
and burned a DVD too. Max wattage was only 192w so now I'm really puzzled.

Can anyone enlighten me?


Re: What really is the power consumption of a PC ?

On Tue, 26 Aug 2008 20:30:56 +0100, Sleepy wrote:

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The 300-400 watt probably comes from the capacityof common power supplies.
but as you have found out, it varies depending upon what components you
are actually using. Also, assuming that the meter is correct.

Re: What really is the power consumption of a PC ?

On Tue, 26 Aug 2008 20:30:56 +0100, "Sleepy"

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Your system might be able to run fine from a very high
quality 300W PSU, but that might've cost near as much as
what you have and not everyone rates wattage equally even if
it seems like they ought to.  The typical PSU calculators
online are basically wrong in that they tend to assume it's
possible for every single part to put the maximum load on a
PSU all at once, plus they can only make generalizations
about some things as it would be a daunting task to keep
trace of actual power consumption of every specific tech
change that came along like different motherboards and
chipsets, whether certain power management modes were
enabled and whether system was ever at full load (in at
least the primary, CPU or video subsystems).

Years ago you could get pretty high quality but low capacity
power supplies (as well as junk).  Eventually someone wised
up that if the rest of the PSU was very high quality, they
only had to make a minor further change in components to
have one that has a significantly higher market value
because of it's higher wattage rating.

It's not so much that you system needed a 400W PSU, it's
that it needs a decent quality PSU expected to last long
term and with enough capacity on each power rail.

Plus, total PSU wattage doesn't tell exactly how much
current there is on each rail.  Since all systems consume a
little more or less power, manufacturers have to shoot for
some kind of average and provide more current on each rail
than that average so their product is highly compatible,

Another factor with newer PCs is the components using the
most power on the 12V rail tend to be CPU and video card.
These are very fast parts today and can cause significant
changes in current which the PSU must respond to quickly.
Unfortunately many PSU reviwers today no longer test for
this kind of load and rather than cutting it too close and
having instability, it is better to overshoot the mark.

Re: What really is the power consumption of a PC ?

Sleepy wrote:
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Is the power supply a high efficiency one, or a regular one. I can try my
crude calculation method, and see what number it gives.

Processor 65W / 0.90 =  72.2W      (includes waste heat from 90% efficient Vcore)
Motherboard_memory   =  50W        (covers the low power of modern RAM, plus the
hot chipset.)
                                    (with an Intel chipset, there is data
available for this.)
Hard drive 12W x 2   =  24W        (some modern drives do much better than this
CDROM                =  25W        (only if media present, 5W otherwise perhaps)
8800GT               =  78 to 86W  (Xbitlabs 3D peak measurement, two different
Fans                 =  6W         (say, up to three fans)
Standby              = 10W         (covers some USB loading on +5VSB)
Add-in cards         =  0W         (low power and not worth estimating)
Total                  265W        (at output side of ATX PSU)
AC input power = 265/0.80 = 331W   (AC input to computer, assuming an 80%
efficient ATX supply)

And you got 192W.


Re: What really is the power consumption of a PC ?

Sleepy wrote:

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What is the hysteresis in measurement?  That is, how much lag is there
when there is big jump in wattage and then a big drop?  An averaging
power meter won't help you figure out how much you need to handle the
peak loads, like when you first power on or when you load an application
that pumps up the load on the CPU, memory, mobo, video card, and hard
disks all at once.  I've seen folks actually try to use their house
power meter to figure out how much an appliance consumes but that never
shows peak consumption over a short interval.  Did you see a spike when
you powered on your computer?  If not, the hysteresis is too long so all
you could ever measure is the typical operational power consumption
where there isn't much change in what components are powered on and how
hard the host is working.  Along a similar vein, a window air
conditioner may read low enough when it is running to be on a 15A
circuit but the spike when you first activate the compressor would blow
the breaker if a couple of lamps were also on at the time.

Unless you are getting very high quality power supplies and have some
independent testing to show they actually support their rated maximum
load for indefinite time intervals (and actually the best ones are
underrated in that they supply more power than they are rated for by
anywhere from 10% to 25%), figure on getting maybe 65% to 75% of the
rated load (for a constant load at that value).

Did you have every IDE, SATA, SCSI, or other port connected to a hard
drive?  Was every tap on the PSU connected to something?  Was every
memory slot filled?  Was every PCI slot filled?  Were you running games
or Prime95 during testing of power consumption?  While your load isn't
close to what you may have expected, remember that you probably want
expandability later without having to buy a bigger PSU when you later
add drives, add more memory, play a super-resource intensive game or
video editing app, etc.  Of course, without an oscilloscope, you can't
see how much the ripple voltage increases as you approach the maximum
load capacity of the PSU, so you really want to make sure you never come
close to its max.

By the way, did you measure the wall outlet wattage and then remember to
multiply by the PSU's efficiency rating to figure out what the PSU was
delivering for power level?

Re: What really is the power consumption of a PC ?

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Thanks to Terry, Kony, Paul and Vanguard. It is a cheap meter - 8 from
isn't going to get me a professional quality meter I know but it does seem
to be accurate.
I watched the small LCD screen on booting up my PC and it did show
in wattage before settling down to around 120w at desktop. For gaming I
STALKER because it uses the PC pretty thoroughly including the HDDs as it
loads data as you move around. It never went above 170w when gaming.

Yes I am aware of the need to check the individual amps on the rails in
particular the 12v.

I'm not about to dump my 550w PSU for a 300w - no fear !!

I was just pleasantly surprised to see my general power consumption was less
than I feared and
less than most hardware reviews indicate. Also by switching off the
multi-sockets that feed my
PC, TV and hi-fi I found I was saving 18w at night.

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