Kill A Watt = Hunk of junk?

[Muse]

Has anyone tested their Kill A Watt? I mean tried to calibrate it? I have a Model P4400 SN: SI02883 bought 02/20/2008 off Newegg. I just did some testing. I have an extension cord with banana plugs running off from the middle of one of the wires, that go to a digital multimeter, where I get the amps. Multiplied by 120 volts, this gives me the true watts. I've used this for years before I got my Kill A Watt. I always suspected that the Kill A Watt was useless at low watts, but I'm finding it's not so hot at higher watts too. Here's what I just came up with:

My multimeter is a Metex M-3800 I've had for over 15 years, cost me around $90 I think. Decent piece of equipment AFAIK.

Metex.... Kill A Watt
------------------
76.3.... 67
50.2.... 44
38.9.... 30
26.6.... 26
20.1.... 22
14.8.... 13
11.0.... 3
6.4.... 2
3.6.... 3
3.5.... 1
2.9.... 1


I did have a problem with my Kill A Watt. I left my desktop plugged into it for several months and it stopped working at all. After a while I tried it again and it was again functional. YMMV. Has anyone else tested theirs? Can you report the results here, assuming you have a way to determine actual (i.e. reasonably accurate) watts?

 

[Fritzo]

I find Kill A Watt reVolting.

 

[ussfletcher]

You also need to measure V(rms) of the wall, its not 120, probably more like 114-116 V

 

[Miramonti]

Hmmm...may just have to try this at some point.

 

[Muse]

You also need to measure V(rms) of the wall, I'm guess its not 120, probably more like 114

The Kill A Watt reports it at 119.8 or so. My Metex reports it at 120.4.

 

[Muse]

Hmmm...may just have to try this at some point.

Cool, please report the results here.

 

[ussfletcher]

The Kill A Watt reports it at 119.8 or so. My Metex reports it at 120.4.

Then I believe you have found your problem! Although, thats well within tolerance... its like .5% off

 

[PottedMeat]

is the Metex measuring true RMS?


try comparing the currents and voltages measured with a resistive load like a hair dryer or heater. they should be about the same.

 

[thecoolnessrune]

Wait Wait.. You are simply multiplying whatever amps you get by 120? Additionally, you're using a 15 year old multimeter device that requires periodic, professional calibration (which I assume you've never done?)

I have no doubt the the kill-a-watt is hardly an example of state-of-the-art measurement technology, but what you are stating based off of how you're testing is ridiculous. It's like saying that the new exhaust on my truck is adding definitely 20HP because my "butt-dyno" feels about 20HP more from last time.

 

[Muse]

Wait Wait.. You are simply multiplying whatever amps you get by 120? Additionally, you're using a 15 year old multimeter device that requires periodic, professional calibration (which I assume you've never done?)

I have no doubt the the kill-a-watt is hardly an example of state-of-the-art measurement technology, but what you are stating based off of how you're testing is ridiculous. It's like saying that the new exhaust on my truck is adding definitely 20HP because my "butt-dyno" feels about 20HP more from last time.

Now you're telling me what? I need to calibrate or junk my Metex? I can't trust it in determining the amps drawn? I do have a 1/2 dozen other multimeters around, most of them cheapo Harbor Freight. I also have an even older analog Radio Shack MM. I can run tests with those, but I would suppose the Metex is the best. No, I don't know what I'm talking about, I'm not an engineer. Like I say, YMMV. My money's on the Hunk of Junk idea.

 

[mugs]

I don't expect it to be absolutely accurate, but it sounds like it at least gets you in the right ballpark. That's good enough for my uses.

 

[Muse]

is the Metex measuring true RMS?


try comparing the currents and voltages measured with a resistive load like a hair dryer or heater. they should be about the same.

Why would it matter what the nature of the power draw is? I have a hair dryer, and will test and report below in a few minutes...

Hair dryer on low:

Metex.... Kill A Watt
------------------------------
262.... 257

The KAW reported 120.0 volts before turning on the dryer, 117.5 volts when the dryer was on, so I presume there was a voltage drop.

 

[frostedflakes]

Your multimeter is measuring apparent power, not true power. It doesn't measure and account for power factor like a wattmeter does.

That said, as far as wattmeters go, the Kill-a-Watt is a pretty cheap devices. It probably isn't very accurate.

 

[jlee]

Past inflation indicates that a $90 item from 1995 would've been $125 in 2009. I think my kill-o-watt was less than 20 bucks.

Just sayin'.

I don't really have a way to test mine (other than a cheap multimeter), though.

 

[thecoolnessrune]

Now you're telling me what? I need to calibrate or junk my Metex? I can't trust it in determining the amps drawn? I do have a 1/2 dozen other multimeters around, most of them cheapo Harbor Freight. I also have an even older analog Radio Shack MM. I can run tests with those, but I would suppose the Metex is the best. No, I don't know what I'm talking about, I'm not an engineer. Like I say, YMMV. My money's on the Hunk of Junk idea.

See frosted flakes post above mine. No, you don't know what you're talking about. You can determine the amps drawn all day long but without power factor and TRUE voltage you're merely guessing at numbers. My guess is that the real number is likely somewhere in the middle.

 

[ShawnD1]

If you want accurate numbers, buy an analog meter. No math bullcrap involved. It measures real RMS voltage and current because that's what's moving the needle.

My $10 digital meter is horrible. It says my wall voltage is 134V.

 

[LTC8K6]

Fluke True RMS...

 

[Red Squirrel]

Anyone own a TED? I've been wanting to buy one of those, just can't find much reviews anywhere.

 

[Red Squirrel]

If you want accurate numbers, buy an analog meter. No math bullcrap involved. It measures real RMS voltage and current because that's what's moving the needle.

My $10 digital meter is horrible. It says my wall voltage is 134V.

Can you even get those for high voltage though? What's nice about those is you don't get phantom voltage. Phantom voltage can be real weird sometimes, though usually it indicates a problem somewhere so it may be good to be able to see it.

 

[Muse]

Past inflation indicates that a $90 item from 1995 would've been $125 in 2009. I think my kill-o-watt was less than 20 bucks.

Just sayin'.

I don't really have a way to test mine (other than a cheap multimeter), though.

$20 is a good deal on the KAW, but like I say, I'm less than confident in the results, particularly for low power draw devices. I just determined that my other multimeters (including my ~30 year old Radio Shack analog Micronta Cat. No. 22-204c) don't measure AC amps, so I only have my Metex from ~1993. It's not been used a lot. I tend to think it's pretty accurate but people are saying that it's not giving me reliable readings in terms of what power is actually being used. I don't know why that is, but I'm admitting my simpleton status in this arena. I bought the Metex from an engineer on the east coast in tandem with a magnetic field detection coil in my effort to measure EMFs "around the house." AFAIK, it does a great job of that as long as you average 3 orthogonal measurments.

 

[BlahBlahYouToo]

it's because you forgot to factor in the Vdroop of the KAW.

 

[So]

You also need to measure V(rms) of the wall, its not 120, probably more like 114-116 V

This. You have to measure your plug voltage at the time you measure the amperage. Your utility regulates frequency extremely precisely, but voltage is only nominally +/-5% and is probably measured at the nearest regulating device which may be miles away from your plug.

Edit: And other posters are right, you have to factor in power factor as well.
P = I * V is a simplification of S = I * V where I and S are complex numbers.

 

[The Stigenator]

agree. teh KAW isnt as accurate. I have a Watts Up Pro and it is WAY more accurate. Infact the KAW powerstrip is more accurate than the KAW. However my most accurate meter is the Watts Up Pro.

It retails for 199 (when I bought it) and can do data logging over USB.

 

[ShawnD1]

Can you even get those for high voltage though? What's nice about those is you don't get phantom voltage. Phantom voltage can be real weird sometimes, though usually it indicates a problem somewhere so it may be good to be able to see it.

The worst is when you're trying to measure a wave that is severely deformed such as a the AC side of a rectifier. The wave is so broken up and screwy that a digital meter will jump around and I can't even guess which displayed value is close.

Computers have rectifiers and horribly broken waveforms. Things like a $20 wattmeter mayve have troubles measuring it.

 

[geno]

it's because you forgot to factor in the Vdroop of the KAW.

Vdroop or Vdrop?