measuring power usage

Dave Lane dlane at ap.stmarys.ca
Tue Jun 3 18:50:38 EDT 2003


At 06:40 PM 6/3/2003 -0400, Robert G. Brown wrote:
>On Tue, 3 Jun 2003, Selva Nair wrote:
>
> > On Tue, 3 Jun 2003, Trent Piepho wrote:
> >
> > > I seem to recall reading in a mailing list, I think this one, about a
> > > relatively inexpensive device that would measure the real wattage and 
> power
> > > factor of electronics.  I think that someone on the list bought one 
> and posted
> > > a review.  Does this ring a bell with anyone?  I'd like to get some real
> > > numbers about what our cluster is drawing and how much we capacity we 
> have for
> > > new hardware.
> >
> > google "kill a watt"
>
>Yes, very lovely thingie.  There are links to at least one source on the
>brahma vendors page, and I >>have<< pictures of a kill-a-watt in action,
>if I ever overcome my innate inertia and post them on the brahma photo
>tour...

There is nothing in the flier that indicates that it measures rms amps and 
hence rms watts. Computers are not resistive loads and they draw current in 
a switching fashion. As a result they usually produce a lower current 
reading on a non-rms meter than they are actually using. From memory the 
measurements I did produced about a 20% difference in current measurements 
between Fluke rms and ordinary meters.

... Dave

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