[K12OSN] electricity use

John Lucas mrjohnlucas at gmail.com
Wed Mar 14 11:31:06 UTC 2007

On Tuesday 13 March 2007 22:50, 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 have some actual measurements for power consumption that I did some time ago 
comparing a "fat" PC to a diskless terminal. The PC was a Gateway 450Mhz PII 
(this was some time ago) with a 20GB HD and a 15" LCD monitor. The terminal 
was a Disklessworkstations LTSP 150e with the *same* 15"LCD monitor. The 
measurements were made using "apcupsd" with and APC Back-UPS XS 1000. This 
gives power draw on the AC side in percent of capacity resulting in 
"Volt-AMP" (voltage x amps) units:

	PC & LCD 		= 120VA
	Terminal & LCD		= 60VA

The PC measurements were with an "idle" HD, if it were seeking/reading/writing 
it would draw slightly more and system startup would draw significantly more. 
So the PC draws about 1A (when idle) and the terminal draws about 1/2A. More 
modern PCs will likely vary from this, but the old PC I used is typical of 
the "recycled" PCs used as terminals with LTSP.

        "History doesn't repeat itself; at best it rhymes."
                        - Mark Twain

| John Lucas                          MrJohnLucas at gmail.com               |
| St. Thomas, VI 00802                http://mrjohnlucas.googlepages.com/ |
| 18.3°N, 65°W                        AST (UTC-4)                         |

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