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Re: [K12OSN] electricity use

Robert Arkiletian wrote:
On 3/13/07, Brad Thomas <bthomas 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

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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