# Re: [K12OSN] electricity use

• From: Vince Callaway <vince totalsense com>
• To: "Support list for open source software in schools." <k12osn redhat com>
• Subject: Re: [K12OSN] electricity use
• Date: Tue, 13 Mar 2007 20:12:00 -0700

```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.

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).

```