[vox-tech] [OT] Electrical Engineering Question

[Jeff Newmiller]

On Fri, 31 Jan 2003, Rod Roark wrote:

> I got my first electric bill at the new house; looks too
> high.  So I decided to do an experiment.
> 
> Outside the house is an electric meter.  It reads KWH
> accumulated on 5 dials, and has a horizontal platter that
> appears to spin about 100 revolutions per KWH (anyone know
> if this is exactly true for a standard meter?).
> 
> So I figure that means 10 watt-hours per rev, or 36,000
> watt-seconds per rev.
> 
> I timed one revolution with most things in the house turned
> off.  45 seconds.  Then I turned on a 250W light bulb and
> timed it again.  32 seconds.  So:
> 
>   36,000 watt-secs / 45 secs =  800 watts
>   36,000 watt-secs / 32 secs = 1125 watts
> 
>   1125 - 800 = 325 watts -- for a 250W bulb.
> 
> How come?  Should I complain to PG&E, or is there some
> gotcha that I'm missing?

Go look on your meter face for a number labelled Kh.  A typical value is
7.2.  Units are Watt-hours per revolution.

Count the number of seconds (S) it takes for the disk to revolve once. (I
usually time two or three turns and average. Total measurement intervals
between 40 and 90 seconds will usually give good results (though if it
spins three times you will need to remember to divide the total interval
by three before using the answer).

Power = Kh * 3600 / S

The only difference between your equation and mine is your use of Kh=10.

Assuming you have a regular meter like mine, the calculation should yield
810W - 576W = 234W, which could be reasonable. I don't know what kind of
light bulb you used... torchiers output is often adjustable. Beware of
"hidden loads" too... a refrigerator kicking off or on toward the end of
your measurement interval can reduce or increase respectively your
apparent load.

For those looking to check these answers: the energy meter is almost
certainly more accurate than whatever test equipment the average computer
geek is likely to have laying around for measuring energy consumption
(energy meters have a tested accuracy of <0.3%).  P=V*I always for
simultaneous/instantaneous measurements, but the root-mean-square readings
typically reported by DVMs ONLY hold for _sinusiodal waveforms that are in
phase_.  Those conditions are often not valid in the real world, so beware
of getting out your DVM to double-check your energy meter. :)

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