Who makes the best Kill-A-Watt meter

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On 4/10/2012 5:48 PM, Home Guy wrote:

They only claim 2% accuracy but they do measure true RMS and PF. I don't know why you'd think any less; it's surely not difficult nor expensive in today's microprocessor world to sample and compute _very_ inexpensively. 2% is, of course, not billing accuracy, but certainly for the homeowner monitoring purpose adequate enough for virtually any purpose.
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dpb wrote:

I found some interesting material from a couple of forum threads.
The first one: http://forums.anandtech.com/showthread.php?t 7169
To quote from that thread:
==============I had a friend over today that wanted to show me his new purchase, a fluke 1735 , so I thought I would do a quick comparison with the killawatt. The killawatt isn't a bad meter, especially for the price. Its does have its flaws though. Comparing it with a fluke 1735 I found a few differences.
The killawatt is terrible at inductive loads, so don't use it for measuring those. Inductive loads would be things typically with motors, compressors. So not good for measuring a refirgertator or washing machine. It measures them , but its results are not .2% accurate.
Refrigerator with compressor running 921 watts, fluke 841.8 watts
Its not accurate at measurements of small wattages. Things like the power usage of a dvd player in standby mode are not accurate. Killawatt said dvd player was using about 5 watts, fluke 2.64 watts.
The sampling rate is very low compared to meters like the fluke, so it can miss quick spikes or surges in usage . When I used it to measure the power usage of a 51" hdtv at turn on, it constantly gave different readings. Range from 410 watts to 504 watts. Fluke 448 to 453 watts. ================ The second thread:
http://fuelzilla.com/homepower/Power-measurement-question-Kill-A-Watt-and-Watts-Up-Pro-33213-.htm
Quoting several posts from that thread:
================I have both a Kill-A-Watt and a Watts Up? Pro power meters. To check how close they're against each other, I connected them in series with the Kill-A-Watt going into the plug, then the Watts Up? Pro and the load.
They should read about the same or the Kill-A-Watt should read the power consumption of the load plus the Watts Up? (no more than a watt or two). Comparing the actual meaasured values, the Watts Up? Pro consistently give a value about 10% higher than the Kill-A-Watt with inductive and non-sinusoidal loads and not quite as much difference with resistive loads. With the computer I'm using to write this message connected as the load, Kill-A-Watt is reading 174W and Watts Up? Pro is reading 190W.
Both devices agrees within a reasonable degree against a known good DMM for voltage.
DMM: 120.3V W: 121V (does not resolve to 100mV) K: 120.4V
Current do not agree with each other: DMM: Unable to measure, my DMM is not true RMS capable W: 2.37A K: 2.19A (w/ no load, device reads 0.02A, 0.0W)
PF: both devices reads 0.66
Here are the differences in construction:
Voltage measurement:
Watts Up? Pro: An isolating transformer drops the voltage used for both measuring the voltage and powering the device.
Kill-A-Watt: It is directly powered from the AC line through a series R-C circuit and a separate resistive divider is used for voltage measurement.
Current measurement:
Watts Up?: Current transformer. Kill-A-Watt: Shunt
-----------------
Realistically speaking, high frequency load (say something that draws power in 25 15A spikes each half cycle, such as some copy machine/laser printer heater controller) rich in harmonics and high in crest factor would not give the meter same accuracy as measuring a plain resistive load.
Examples of highly harmonic loads:
Almost ALL IT equipments Most home electronics Residential electronic ballasts and CFLs These loads frequently have a THD greater than 60%.
-------------------
I purchase three kill-a-watt units last year and put them all into each other for comparisons, They all matched each other as close as the LSDigit would allow.
I also took one of the units and checked it against a lab standard traceable to the NBS standards and compared it for Voltage, Current, Power and Reactive Power and I can tell you this, you could use this unit interchangeably with our lab standard. No digit showing on the kill-a-watt unit to it finest resolution was out by even one count. Now our lab standard has a few more digits.
The all Vars (reactive power) and no real power (watts) comparison may be off a little on our lab standard and I did not compensate using known documented accuracy tables. The accuracy formulae is always divided by the PF which makes ???? accuracy but this lab standard is about as accurate as it gets in Canada without controlled environments etc.. etc..
I am really impressed with the Kill-a-Watt units. No tests on waveform distortion or harmonics were performed to date by me.
Waveform distortion form factor may be where the differences are found. OTOH the Watts Up may just not be calibrated properly or junk.
------------------
An update on Kill-A-Watt,
I ripped it apart and started probing around. The shunt's output is rather low.
The shunt is placed across the neutral and looks like a 12 gauge wire looped into a U-shape, but I'm not sure what its made of. It gives a 47mV voltage drop with a 12A 1.5kW space heater connected, which tells me the shunt is 3.917 miliohms. The signal from shunt is routed on the board for 3" or so to an LM2902N op-amp. With around 560mW of dissipation, the shunt gets hot to touch and I'm not sure how much the heating affects the resistance of the shunt.
The current resolution on the Kill-A-Watt is 0.01A and this translates to current signal input resolution of 39V, which might make the device suspecticle to noise considering the signal path is not shielded at all. With a one kilowatt resistive load, it jumps around few tens of watts.
================= (end)
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When you pull your fridge or your washing machine out to plug it into the Kilo meter just look at the nameplate.
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<<<<>For inductive loads like motors, I wouldn't trust those consumer-grade "kill-o-watt" meters any further than I could throw them. >>>>>
Hi All
I have an evaporative a/c which uses about 650W on the low speed fan setting. I use this sometimes up to 24 hours/day.
I have one of these consumer grade Kill-A-Watt type meters measuring the energy used by the evaporative a/c. The a/c is always plugged into the meter so I can measure the total energy consumption of the a/c.
I can also estimate the a/c energy consumption off my utility energy meter. Btw if I run this evaporative a/c for 24 hours/day it uses about 15KWHr of energy. (It pretty well doubles my daily electricity consumption.)
While I'm not checking the Kill-A-Watt meter to 2% accuracy the incremental increase in energy useage read from the utility wattmeter and the Kill-A-Watt style meter reading are pretty close. I'm happy with the Kill-A-Watt style meters results for the a/c.
Otoh trying to measure standby power of things like VCR's or PVR's is not so good. Better to measure 10 of them together if you can and divide the result by 10. It is more a display thing than any inherent measuring error with low powers. My Kill-A-Watt style meters don't have a decimal point when reading watts.
One way of estimating refrigerator consumption is just see how often it is running when you happen to walk past it. A refrigerator uses about 250W while running. If you walk past it 10 times an hour and it is (say) running on 3 of those walk-by's then it is using about 3/10 of 250W per hour or 75WHr. Now to get 24 hour consumption multiply the 75 by 24, giving you a daily energy consumption of 75 X 24/1000 1.8KWhr.
This is how I initially estimated my refrigerator/freezer consumption some years ago. I can't remember exact figures of how the rough estimate related to my subsequent Kill-A-Watt type meter readings, but I do remember there were no big surprises when I read the consumption more accurately.
In my house the real optional energy consumption was going in standby power. I cut my total consumption by 10% or so by changing high usage light bulbs to CFL's and switching off lots of stuff at the outlet. Mostly it was switching off stuff at the outlet which made the difference.
I measured everything I could with Kill-A-Watt style meters, including the electric hot water service, and adding it all up it pretty much agreed with the utilities watt-meter daily energy reading. Anyway, there were no big surprises, or a huge energy discrepancy.
I found the Kill-A-Watt style meters were good enough to tell me where my daily electrical energy consumption was going and what was using the most energy.
Did I save me the price of all the meters I bought and the time I spent measuring things?
No, not really.
In summer I'm using about $6 worth of electrical energy a day on average (including about $1.20 for hot water from the HWS and electric kettle, and about $1.80/day for the evaporative a/c.) There aren't enough daily dollars there to pay for anything much to replace it before it's time. Electricity here in SA is just about the most expensive in the world too. (We have lots of green wind power. :) )
But it interested me to measure my own personal electrical energy consumption patterns. Not everything has to pass an economic test.
Mostly the readings I made convinced me not to fix it if it wasn't broken.
i.e. One day I'll replace my electric hot water service with a gas unit, but there is no economic case on energy consumption gas vs electric for changing it early, even though the 5 star gas hot water services use a lot less heating $$$ than the old electric hot water services.
Otoh the low pressure electric hot water services like I have go for up to 25-30-35 years, and mine is now about 25 years old, whereas some new efficient gas mains pressure hot water services only last 7 years, so end up being more expensive.
I have to change the electric HWS to a gas HWS _by law_ once the electric HWS fails, but I'll wait until it fails thanks very much.
"If it ain't broke, don't fix it". :)
It's a good rule in my book.
Ross

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On Wed, 11 Apr 2012 06:25:56 GMT, snipped-for-privacy@invalid.invalid (RMD) wrote:

For an electric hot water heater, it may pay to put in a on/off switch in the house so you can shut it off when you have no need for immediate hot water. I once did this and it was nice.
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On 4/11/2012 6:49 AM, Doug wrote:

I'd like to hear more about this. "nice"??? There's already a switch in the breaker box, but I understand that you'd not want to use that frequently. Where did you put the added switch? What kind of switch did you use that could reliably switch 4KW? What did it cost for the switch/wire/installation/permits/inspections? How much did you actually save using it?
Last time I looked into it, the thermal time constant of a well- insulated electric water heater was on the order of days.
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I don't know about newer units, but my defrost goes for 45 minutes at 1500 watts. That's at least an extra kWh per day.
Greg

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Every frost free fridge I've serviced has a "defrost termination thermostat". If yours runs 45 mins at 1500 watts, you totally need that thing repaired. That's not normal.
Christopher A. Young Learn more about Jesus www.lds.org .
I don't know about newer units, but my defrost goes for 45 minutes at 1500 watts.
That's at least an extra kWh per day.
Greg
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I don't know if it sucks up power for that time, but it does stay on defrost for that time. I put a data logger on once to check temps. The freezer goes way up in temp, but fridge moderates. But DON'T OPEN ANYTHING during that period. I keep stressing we need smart units that defrost when we tell it to.
Greg
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On 4/10/2012 11:08 AM, snipped-for-privacy@toyotamail.com wrote:

I've messed with this a LOT. First thing to ask yourself is, "what am I trying to accomplish?" It's unproductive to ask a question if you're not gonna do anything with the answer.
If you want to save money, you already know how to do that. Just use less of everything. It costs half as much to shower every other day. But are you going to shower every third day based on the reading on your meter? A bucket, stopwatch and a thermometer will give you all the tools you need to calculate what a shower is costing you.
Are you going to raise the internal temperature of your fridge? Are you gonna forgo that cold can of pop 'cause it costs you $.0002 to open the fridge door? Are you going to drink cold coffee or quit toasting your bagel? If your welds are too strong, weld faster. If you have electric rates that differ over the course of a day, you can wash clothes at 4AM.
Bottom line is that we use as little as we can stand. Using less is not practical or we'd be doing it.
But, it is fun to look at the numbers. Some may surprise you and need to be dealt with. I'd vote against long-term monitoring. It's not worth the expense, 'cause you're not likely to pay any attention after the first month. The wife is gonna' suggest that bathing more often might help your love life...and you know what you're gonna do.
The Kill-A-Watt measures Watts and Volt-amps. Watts is what most utilities charge for. It's great for learning how much money your cable box is costing you in electricity. But, again, are you gonna unplug the cable box that's costing you $50/month to save a buck in electricity?
They are marketed under several brand names. Google will find 'em for you. If google doesn't know, you won't likely find it for sale anyway. I paid $2 for mine at a garage sale. If I add up all the electricity I saved using it, I think it'll be sometime in 2015 by the time I get my $2 back.
A clamp-on amp meter is a useful tool for 240V devices. It has no knowledge of power factor, so you'll only read volt-amps. But the water heater, stove, the heater part of an electric dryer all have a power factor of 1, so watts == volt-amps. Isn't gonna help much with your welder, or CFL lamps, or motors.
The simplest thing to do is use the utility meter on the house. You're monitoring exactly what you're being billed for. A stopwatch to measure how fast the wheel goes around as you turn stuff on/off will tell you exactly what you're paying for. Once you get the number, it probably won't change much. Then, all you need is to time how long it runs. An electric clock on the load side of the switch will tell you that. Shorter showers make the water heater run less. But you didn't need ANY measurements to know that.
Blue-Line Innovations distributes a device that clamps on the utility meter and watches the disk go around. Transmits wirelessly to the readout. They also have a computer interface. I got mine for cheap at a garage sale. I wouldn't pay the retail price for one. Again, marketed by several vendors.
If you have a digital utility meter, it likely has an infrared light that blinks. Mine blinks once for every watt-hour of use. I wrote a little program that runs on a Palm III. Point the IR sensor at the meter and it graphs usage. Kinda interesting to watch the water heater go on and off. Pretty soon, you recognize the power signature of the water heater, microwave, furnace, etc. I was so fascinated that it took over a week to become bored.
While I was at it, I hooked up a switch with a flapper over the vent to log the run-time of the gas furnace. Guess what...turning down the thermostat saves money.
One thing you will discover is how much power is wasted by stuff that's turned off. Common term is "vampire" devices. I could save about 35Watts of wasted power 24/7 by turning off all the devices related to watching TV. So, I put in a power strip and switch it off when not in use. Every time I wanted to watch TV, I had to turn on the power, reprogram the clock on two VCR's, wait for the devices to boot and figure out what they are...then I could watch TV. That lasted about a week.
IF I took all the time I've spent on measuring stuff and spent it working at minimum wage, the money I'd earned would more than pay for all the energy I'm ever gonna save over using common sense.
Conservation is a good thing. Use as little as possible, but no less. You don't need much real-time data to do that.
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Yesterday I was thinking of buying this K-A-W meter but I began to realize, I'm not going to change anything even after I see the meter's results so why bother getting one. I mean I always try to save electricity in my home using age old advice so I probably have little practical reason for this device except for the fun factor. I'm old enough to know by now how to save electricity without the meter and as you pointed out, saving electricity in some cases isn't always practical.
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On 4/11/2012 12:23 PM, Doug wrote:

A lot depends on you. I am a retired engineer. Actually, engineers never retire, we only bug our spouses with engineering-type stuff all over the house. I've had my Kill-A-Watt for about 5 years now and probably use it once a month, on average for generally testing, etc. I also had a voltage problem where the power company was jacking up the voltage, apparently to relieve low voltage problems in other parts of our local grid. I was up to 126 volts and sometimes higher. It did cause problems with at lease one piece of electronic equipment in my house. When I called them and gave them the results, they initially brushed me off. But when I got to talk to an engineer, he was interested. They called me back about 1/2 hour later and said the problem was theirs and it would be fixed immediately. I actually watched the voltage go down about an hour later.
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How would they do that ???
Greg
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On 4/11/2012 2:39 PM, gregz wrote:

I don't know, but it involved going to some site, probably a substation. BTW, the voltage, after they corrected it, was about 112 ... sometimes lower sometimes a little bit higher. However, over the months that follow, it has crept up to about 120 on average. I don't know if it goes at high as before (126 or higher) because I changed out the piece of electronic equipment that seemed to be responding to the problem. It was a set of computer speakers. I could duplicate the problem on the bench with a Variac. When the voltage would go to about 125, the speakers would start buzzing. I have several pairs of these speakers and all exhibit the same problem. So, I changed out the wallwart to a different 12 volt wallwart. Now those speakers have no problem.
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I currently have about 122 volts. Where I used to work, 125 was the norm.
Greg
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gregz wrote:

Are you aware that you quoted 83 lines of previously-posted material just to add a single line to this thread?
You, Art Todesco and "Doug" are responsible for quoting most or all of this entire thread in each of your respective replies.
Is there any reason why you people are doing this?
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It takes a bit more time using this iPad, but I usually do NOT delete lines. I figured someone would bitch.
Greg
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gregz wrote:

That's assinine thinking.

Why?
Why drag the entire expanding thread into each and every response?
The entire thread (all previous posts) are easily visible and accessible. If someone is reading a thread, one post at a time, there's no need to repeat the material with each successive response.
I know that people got into the bad habbit of doing that with e-mail 10+ years ago, but usenet is not an e-mail conversation.
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gregz posted for all of us...
And I know how to SNIP

I thought EVERYTHING was flawless with Apples products. You burst my bubble.
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Tekkie

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On 4/16/2012 12:20 PM, Tekkie wrote:

only the mac-in-toy owners think that. It's their fantasy.
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Steve Barker
remove the "not" from my address to email
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