Re: Basic AC wattage question: am I doing my math right?


"David L. Jones" <altzone@xxxxxxxxx> wrote in message
news:225f59f5-6244-4384-b702-1aa6afbe5640@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Dec 30, 12:48 am, NoS...@xxxxxxxxxxx (Bob Masta) wrote:
On Sat, 29 Dec 2007 00:08:08 -0800 (PST), HC <hboo...@xxxxxxx> wrote:
On Dec 28, 11:54 pm, John Larkin
<jjlar...@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
Your math is right for a simple (ie, resistive) load, but might be off
by, say, 2:1 in either direction, depending on the actual load
waveform and/or power factor.

John

Hey, John, the more I've been reading the more I think my math and
measurements have been badly wrong because they're rooted in DC
methodology which seems to be quite different from the AC world (or
can be for equipment/circuits that are not 100% resistive). So, as
you say, I could be quite wrong by just simply not knowing the
intricacies of AC power flow. I found a formula for calculating AC
current consumption on single-phase which is P = V x I x cosine Theta
which is great except that Theta is the "power factor angle" of the
equipment which I don't currently know. I approached this problem
thinking like I do about DC and I've found that it is wrong and the
problem is considerably more complex. I'm going to do a lot more
reading about this and "reboot" my whole test.

Check out the "Kill-A-Watt", it does *exactly* what you want.
I bought mine on-line for about $20 (with free shipping!) for
uses like this. I tested 2 different TV models of the same size
(27") and similar age and found that one used nearly 16 watts
when off, while the other used less than 8.

The Kill-A-Watt shows RMS volts, amps, watts, KWH,
power factor, line frequency, and probably some others
I've forgotten. You plug the Kill-A-Watt into the wall, then
plug the thing you want to test into the Kill-A-Watt, and
push a button to select what you want to read.

I've wanted a watt-meter for *years* and always
figured I'd have to bite the bullet and build a crude
one. But the Kill-A-Watt does more than anything
I'd ever have built... and for only $20 !!! This is by
far my favorite toy of the year.

(PS: I have no affiliation with Kill-A-Watt, just
a delighted customer.)

Best regards,

Bob Masta

DAQARTA v3.50
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, FREE Signal Generator
Science with your sound card!

Sadly, the Australian market still does not have a low cost consumer
power meter like that one.
*sigh*
One was released recently, but the supplier removed it from sale
because it had accuracy issues on the low end of the scale.

Dave.
How about as a work around for the poor low end accuracy;
Put three 100 watt light bulbs on as a load, measure
the power then add the low power load he wants to measure and note the
difference. Would the power meter have the resolution to do this?
Mike
Mike



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