[K12OSN] electricity use

"Terrell Prudé Jr." microman at cmosnetworks.com
Wed Mar 14 05:50:26 UTC 2007

Robert Arkiletian wrote:
> On 3/13/07, Brad Thomas <bthomas at bhbl.org> wrote:
>> I am a social studies teacher and I've been building a lab in my
>> classroom
>> of old, discarded computers over the last two years (I was up to 20). I
>> have been using small distros like DSL (DamnSmallLinux) to make them
>> work,
>> but was planning to switch to a k12ltsp setup before the end of the
>> year.
>> However, my principal just sent an e-mail last week instructing me to
>> remove all but 6 of the computers from my room implying that they were
>> using too much electricity. I just got back from a school planning
>> council
>> meeting where she and an assistant principal said that they called Dell
>> (we buy all our new fat machines from Dell) and Dell said there should
>> only be one computer per 20 amp circuit (which translates into one per
>> room I think). As far as I can tell (using a Watts Up meter) one
>> computer-and-monitor use a little more than 1 amp of power, so I
>> don't get
>> this. Can anyone out there give me some guidelines they go by? Or
>> steer me
> I am not an electrician but I do teach basic electric circuits in
> physics. The basic equation for power is P=IV (Power=VoltagexCurrent)
> So 120 Volts (which is a north american standard) x 20 A of current
> equals 2400 Watts of power.  Now if you remember those old boxes
> probably have 200W power supplies max + monitor ~75W. Add another 25
> for safety. So I would say 300W/box is reasonable. Although less would
> probably work because I doubt they would draw the max of the PS unit.
> So with a 20A circuit (2400W) that equals 8 machines working at full
> tilt.
> Now compare that to an ebox 2300 + and a 19in lcd monitor consumes.
> 15W + 40W
> Plus the server, don't forget.

I actually was an electronics technician in the military for several
years, so I'll chime in.

What that power rating means is what the power supply is *capable* of
putting out.  That doesn't mean that it actually *is* putting that out
all the time.  The only time you'll see that max power rating is if you
stick a load on that power supply that draws that much power. 
Regardless of your voltage, in order to have a load, you must draw
current.  The less current you draw, in accordance with P=IV, the less
power you draw.

What that means is that you can disconnect the hard disk drive and
CD-ROM drive, thus lowering the load that the computer's power supply
sees.  Now, there's only so much you can do without un-soldering parts
from the motherboard.  :-)  But any time you can power down a mechanical
device like a disk drive, do so.  Oh, if you're netbooting off the NIC's
boot rom, you might as well unplug the floppy disk drive, too.

Furthermore, that 300W rating on the power supply *is* accurate.  That's
the total output that it can handle at any one time, out of all outputs,
combined.  Sure, if you add up all the power ratings from the 5v, 12v,
etc. outputs, you might get a figure higher than 300W.  But if you
actually try sucking max power out of each input simultaneously, you
*will* blow up that power supply.  What happens is that if you want to
take the entire 5v current rating, you must decrease your 12v current
draw by an amount to keep your total power usage under, in this example,
300W.  That's why folks with these multiple CPU cores, super-duper-hot
video boards, multiple hard disks, light kits, etc. in their computers
need these insanely huge power supplies.  Along with their 5v needs,
their 12v needs also just went sky-high, and they can't borrow from the
5v side w/o shutting down the motherboard...or blowing up their 300W
power supplies, thus probably frying said motherboard and expensive
CPU(s).  That's why they go buy those 550+ watt monsters.

Now, that said, it is true that there is some inefficiency with power
supplies.  Gotta keep those capacitors charged up, you know (there are
also inductive and resistive losses).  The bigger ones, even if they're
very efficient, do indeed lose more juice than a little one of equal
efficiency; they're just bigger.  But what really matters most is the
load you put on it.  So, start pullin' those disk drives!

If you want to further reduce your electrical requirements, look into
LCD screens.  They use about a third of that which an active CRT of the
same screen size uses.

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