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
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
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:
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
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
W: 121V (does not resolve to 100mV)
Current do not agree with each other:
DMM: Unable to measure, my DMM is not true RMS capable
K: 2.19A (w/ no load, device reads 0.02A, 0.0W)
PF: both devices reads 0.66
Here are the differences in construction:
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
Watts Up?: Current transformer.
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
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..
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
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 39µV, 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.
<<<<>For inductive loads like motors, I wouldn't trust those
consumer-grade "kill-o-watt" meters any further than I could throw
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
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
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
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
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
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.
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
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.
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
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.
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.
On 4/10/2012 11:08 AM, firstname.lastname@example.org 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
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.
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
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.
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.
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?
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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