I'm trying to determine if I've oversized my power supply for a brand
new build, and I'm using a kill-a-watt type meter to do so.
Anyone know just how accurate these meters are when it comes to power
Most power supply sizing webpages imply that I'd be using 500-600 watts
for my pc. I assume this is under 75 or 90% load or somesuch.
Yet when I measure with a kill-a-watt knockoff meter it's around 90-130
Power supply in question is a corsair TX750. PC in question is an I7
3770K with 1 ssd, 9 sata harddrives (all low RPM), 5 120mm fans, nVidia
650ti 1 gb vid card, 4 sticks of ram, 2 pci cards, a few usb 2 devices.
Now you know what Kill-a-Watt does. Tried Amprobe?
If your measurement is accurate, that means the PSU draws ~1 Ampere of
AC current. If you care to calculate the Total Wattage of major
componenta of your PC draw and compare it to the Kill-a-Watt reading
keeping in mind efficiency of the PSU. How does it look?
When it comes to non-linear loads (ie - switching power supplies) -
those kill-a-watt meters are crap at measuring the load correctly.
If this is a new build, then you probably aren't running the older
Pentium Extreme edition 90 or 135 watt CPU's from circa 7 years ago, so
I'd be surprised if your new build is using more than 200 watts.
9 sata drives?
What - are they all 250 gb?
If this is a server, then no need for a gaming video card.
The kill a watt meter measures the line side of the power supply. A
power supply is only around 70-80% efficient.
The web pages are closer to what you should be using, but they are set
up to err on the side of caution.
Nice setup. Do the lights dim when you turn it on? :)
It is a nice set-up and with 60 MBps from the ISP to boot. With all
these fans running it sure feels like a wind tunnel under the desk.
I always thought the power supplies were the most inefficient
(percentage of wasted power) at low power so I'm using a 750 watt supply
to a PC that wants 130?
I get that 130 watt draw from Kill-a-watt, but have no way to tell how
accurate that reading is at all. Personally I don't think it jives.
The video card spec sheet alone says it uses 100 watts and it has two
fans on it by itself...
The theory is I could go for a lower watt rating power supply and get
more efficiency during the idle times.
Run your test while performing a GPU-intensive benchmarking application,
one of the more recent ones. Do the same with prime95 (whatever version
I'll bet your system draw increases by a bit.
I agree. There is a significant difference between peak power usage
and idle. And I doubt the difference in power consumption having a
somewhat oversize power supply is worth worrying about. There is
some penalty, but I bet it's small. The fact that he's only measuring
120W with it running shows that.
The Kill-a-Watt (and most meters) won't tell you what's important in this
case, and that's how much *startup* current those 9 drives, etc. all draw
when the machine is first turned on. I'll bet it's far higher than the
running current. With that many drives it's probably always best to err on
the side of caution and make sure you have enough power to start up the
Just because it has a 500 or 700W rating, a huge switching PS doesn't
consume significantly more power than a smaller one when running the same
sized load. It just has the ability to run a *larger* load if need be and
to handle startup current draw better. I've been able to see the startup
draw of my refrigerator briefly on the Kill-a-Watt display and it's
substantially higher than where the reading settles at once it's up and
running. Maybe if you watch the Kill-a-Watt display as the computer starts
up you'll see the difference between starting current and run current.
Some of the earlier drives, like full-height Seagate 40MB "desk shakers" had
to be set up to "stagger start" when multiple drives were installed just to
keep the PS from overloading during bootup.
<<"When the jumper is not present and pin 11 is floating, or high, the drive
won't spin up until the PHY initialization is complete. When the jumper is
present, it doesn't matter whether pin 11 is grounded, floating, or pulled
high: the drive won't spin up until the PHY initialization is complete.">>
That means that the drive won't spin up until the drive controller receives
the appropriate request via a software or firmware command. Those were the
good old days. (-:
I would guess that hard drive technology has improved to the point where
your nine SATA drives don't draw as much current as that one huge 40MB
Seagate, especially when starting up. Still, with that many drives, your PS
has to be sized for the maximum startup current, which in a setup like yours
is going to be quite high.
Gaming video cards are also notorious for requiring substantial power to
start. So much so that most modern high-end video cards now have a direct
connection to the PS instead of drawing power through the card bus.
It only draws that much if you are using it for something that
requires it to WORK hard. Surfing the net is just loafing for the
video card and I may only be drawing 25 watts. A lot of time the
manufactures specs will show the range. It's when you are playing
video games and pushing it to high frame rates that make it draw lots
The unique thing here is that you have 9 drives. Are
they all active when you're doing your measurements?
They probably draw 20watts or so each.
That kind of PC would ship with a 350 to 500w power supply, without
all those drives. With it, I think you're
sized about right. But, what's the difference? As long
as it's big enough, which it obviously is, you're OK.
The PC is only going to use whatever power it needs.
Yeah, there is some small additional power penalty if
you have an oversized supply, but it's not that much.
On Sunday 17 March 2013 21:36 Hench wrote in alt.home.repair:
That's not implausible *once everything's running**at idle conditions*.
Stick an ammeter inline with the mains feed if you want a double check the
kill-a-watt. The power factor of modern computers is near as matters 1.0 so
a straight forward amps RMS times supply volts RMS will give you the power.
However what you should probably do is to put some artifical load on the PC
- the easiest way is to run some benchmarking type software that thrashes
the disks and runs the CPU cores flat out and measure during that.
Then you'll want to oversize anyway for startup (do all the disks spin up at
once or do they have staged start?).
Ballpark 500W PSU is a fairly safe bet for your sort of rig. If you are a
hardcore gamer and have a mental video card, that could add a bit. No harm
in going to a slightly bigger PSU apart from cost but I'd feel failry happy
with your sizing :)
Fileserver I take it?
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Fileserver and a gaming pc at the same time. I game, but three teevees
and other assorted devices will pull multimedia/music from the pc.
I'm gonna try encoding some video files tonight to test the kill-a-watt.
Then I'll try a couple of video games with the most intensive graphics
options set to test the same meter and see what happens.
The power supply has to handle the peak loading that might occur and a
Kill-A-Watt is just a cheap device that certainly is not intended for
measuring non-linear waveforms such as those produced by switching
power supplies. Almost every manufacturer of pc power supplies offers
an online calculator.
I wouldn't be so sure about the Kill-a-Watt meter being incapable of
measuring the power draw of a switching power supply. Measuring
and amps, multiplying the two, and doing it at a fast enough rate to
accurate reading isn't a difficult thing. That function can be done
a fairly simple and cheap chip, it's nothing exotic. It's less
that what's done in other cheap, consumer electronics.
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