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

On Dec 31, 3:08 am, "amdx" <a...@xxxxxxxxxxx> wrote:
"David L. Jones" <altz...@xxxxxxxxx> wrote in messagenews: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?

Your average punter is not going to want to do this, and even I
couldn't be bothered. Also, that is not a solution for long term
consumption monitoring which is what these things are good at also.
I've got a silicon chip power meter kit (which I got ridiculously
cheap), and that is pretty good, but I wouldn't mind a one or two more
cheap commercial ones. They are dime-a-dozen in the US, but not here.

Dave.
.