# Re: [K12OSN] electricity use

• From: "Robert Arkiletian" <robark gmail com>
• To: "Support list for open source software in schools." <k12osn redhat com>
• Subject: Re: [K12OSN] electricity use
• Date: Tue, 13 Mar 2007 21:09:44 -0700

On 3/13/07, Vince Callaway <vince totalsense com> wrote:
On Tue, 2007-03-13 at 19:50 -0700, Robert Arkiletian wrote:
> I am not an electrician but I do teach basic electric circuits in
> physics. The basic equation for power is P=IV (Power=VoltagexCurrent)

This is why things get confusing.  Power supplies in computers are rated
by the DC side of the power supply.

Good point! I didn't consider this. So the efficiency of the PS unit
is a factor. Nothing is 100% efficient.  I think if my memory serves
me right Europe has a law which requires a certain kind of more
efficient PS than here in North America. I can't remember the
technical details.

The power supply on my workbench is rated at 150 watts.  Here is the
real data from the sticker on the side:

AC Input: 110v 3A

DC Output: +3.3v 9.5A
+5V 10A
+12V 7A
+5V 1A
-12V .2A

Max +5V + 3.3V = 123W Combined
Max Total Output = 150W

So as you can see it can potentially pull a lot more than the 150W
rating implies.  A supply rated at 330W can actually pull up to 5 amps.
It all depends on how much hardware is in the box.

A diskless client should have as small of a power supply you can find.
Bigger supplies will have more parasitic loss (heat).

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