measuring power usage

Dave Lane dlane at
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

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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