From: keating@rtsg.mot.com (Edward Keating)
Subject: Re: Cost of running my PC?
Organization: Motorola Inc., Cellular Infrastructure Group
Date: Wed, 30 Dec 1992 19:33:14 GMT

ldstern@rodan.acs.syr.edu (Larry Stern) writes:

>I'm sure this is a simple one, but somehow it eludes me.  I need to know how
>much electricity my pc is using.  It is a 386 AT clone with a 200W power
>supply.  I can find out the local rate for KW hours but how do I determine
>how much current the pc (and monitor) is drawing?

>						Thanks,
>						Larry

And after you`ve measured the power drain, you'll find out that the 
incandescent lights in the room draw more power than the PC.

I recently was chartered to determine power drain and the heat generated by
a network of up to 8 PC based machines used in a BBS by our local
computer group (Chicago Computer Society). Some individuals using the nameplate
ratings of equipment determined that we would need additional power lines in
order to run the 10K Btu Air conditioner that would be required.

Knowing that PC's do have switching power supplies, I decided to actually
measure the power drain of some typical machines.

An IBM PC (original PC) with 30Mb external powered hard disk, + monochrome
monitor (IBM) draws 117w. (with 1 floppy disk spinning)

An AT clone (1mb 10Mhz 286 + 2 Full height -power hungry- disk drives,
	NEC Multisync 1, HST 9600 and an external Alloy tape drive) draws
	only 150w.

The NEC Multisync 1 specifications state power draw at 76w. Actual measured
draw was 53w.

The AT tech reference manual states the power draw of an AT to be 5 amps
(600w!). I don't know what kind of cards need to be loaded into the machine
to draw that much, but I suspect that it involves aircraft landing lights.

My measurement setup involved measuring the AC current drawn by the device
under test. (Current calculated by AC voltage measurement across a series
.01 ohm resistor.) The power measured by this setup will be higher than
the actual power drawn as it includes both real and imaginary factors.
The goal was to find the maximum power so that the air conditioner and power
requirements could be determined.

Based on nameplate figures the other individuals calculated that without
air conditioning, the room would rise to 110 F. The thermodynamic equations
involved in their calculations included the roughness of the surface of the
room air ducts and the height above sea level. Truly a sight to behold!

Plugging in my measurements into the page and 1/2 of Thermodynamic equations
provided by this other group, I replied that the room should not get above
80 F. (Assuming a 72 F ambient.)

Well the BBS has been moved for about a month and the reports are that the
room is between 74 and 78 with the heating thermostat set to 78.
Just goes to show, nameplates are only a legal out for the manufacturer.
You have to measure the real power if you want to know.

Other trivial you might want to know:

A sixty watt light bulb does draw 60 watts.

Maximum power drain of a PC occurs within the first 10 seconds.
(I did not measure inrush current, but the disk drive spin up can
account for up to 40-50 watts extra on powerup)

A USR HST 9600 draws about 10 watts.

A 286 laptop with disk spinning draws about 19 watts.

External powered disks draw about 30 - 50 watts on startup, but
settle down to 15-20 watts after they have spun up.

The floppy disk read with motor spinning does not affect a PC-AT's power
draw. The original PC floppy disk draws 6 watts additional when active.
(63.5 W rated power supply)
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