Power measurement question. Kill-A-Watt and Watts Up? Pro ?

[ITSME.ULTIMATE]

Hello all,

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 eachother:
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


Which setup is likely to have an inherently better accuracy?


Product information:

Kill-A-Watt:
http://www.p3international.com/products/special/P4400/P4400-CE.html

Watts Up? Pro
https://www.doubleed.com/products.html

 

[John Gilmer]

Good question(s).

I definitely will pay attention for the answer because I was thinking of
getting such a product. I went to Radio Shack but the local store never
heard of it.

When this is resolved, I suppose I will go "on line" for my purchase.

 

[Martin Riddle]

A shunt style would give better accuracy, but the CT may as well under resistive loads.
The CT may be adding inductance or filtering out the AC line.
It depends on what the designer did. And how they are calculating current. The effects of the CT can be compensated for.

I would go with the Shunt style.

Cheers

 

[m Ransley]

Do you mean you plugged the Watts Up into the Kill a Watt, Then the
kill a watt consumes the power.

 

[TimPerry]

Hello all,

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 eachother:
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


Which setup is likely to have an inherently better accuracy?


Product information:

Kill-A-Watt:
http://www.p3international.com/products/special/P4400/P4400-CE.html

Watts Up? Pro
https://www.doubleed.com/products.html

part of your answer can be found in the specs given for the Watts UP?

* 120 VAC, 60 Hz, 15 amps continuous

* True power, RMS volts, RMS current measured and displayed

* +/- 3%, +/- 2 counts of the displayed value for loads above 10 watts

* +/- 5%, +/- 3 counts of the displayed value for loads below 10 watts

Accuracy is of the displayed value, not the range. Some devices claim a
smaller number for accuracy but it refers to the range. For instance, a
specification of 0.2% of the range sounds good, but it is actually 3.6% of
the display (.02 * 1800 = 3.6), which is a worse accuracy

consider that if one unit reads 3% high and the other reads 3.6% low the
difference will be 6.6% and yet both units will be within spec.

as with any meter accuracy will depend on several factors including:
calibration, stability within an environmental range, methodology of use.

i own one of these units but would not consider the reading to be
sufficiently reliable to use if something serious (like my job) was at
stake. they are fine for go/no-go applications like: is my AC circuit near
max? is my UPS overloaded?

 

[ITSME.ULTIMATE]

part of your answer can be found in the specs given for the Watts UP?

Yes and no. I consider it marketing type presentation for the naive
type. 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%.

* 120 VAC, 60 Hz, 15 amps continuous

* True power, RMS volts, RMS current measured and displayed

* +/- 3%, +/- 2 counts of the displayed value for loads above 10 watts

* +/- 5%, +/- 3 counts of the displayed value for loads below 10 watts

Which doesn't include current crest factor or frequency bandwidth or how
it will behave with other peculiar current waveforms.

Accuracy is of the displayed value, not the range. Some devices claim a
smaller number for accuracy but it refers to the range. For instance, a
specification of 0.2% of the range sounds good, but it is actually 3.6% of
the display (.02 * 1800 = 3.6), which is a worse accuracy

And Watts Up? claim maybe based on resistive load.

consider that if one unit reads 3% high and the other reads 3.6% low the
difference will be 6.6% and yet both units will be within spec.

Watts Up? reads 10% higher than Kill-A-Watt.

 

[John Gilmer]

Do you mean you plugged the Watts Up into the Kill a Watt, Then the
kill a watt consumes the power.

Sounds like it, doesn't it!

Back in my student days our EE lab only had ANCIENT instruments. In most
of our "experiments" the voltmeter always drew significant current and the
ampmeter had a significant voltage drop.

One would hope that the electronic stuff would be better.

It's likely that the designers made one or two many ASSumptions about the
nature of the load.

 

[John Gilmer]

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.

It "might be fun" to see how much of a difference putting a reasonable
sized motor run capacitor across the load would make. One would expect it
to decrease the kVA a bit and have a small effect on the kW.

 

[ITSME.ULTIMATE]

It "might be fun" to see how much of a difference putting a reasonable
sized motor run capacitor across the load would make. One would expect it
to decrease the kVA a bit and have a small effect on the kW.

It would only affect inductive loads. It would not affect non-
sinusoidal distortion.

 

[John Gilmer]

It would only affect inductive loads. It would not affect non-
sinusoidal distortion.

Not so fast there!

It would tend to "soak up" the higher harmonics. An inductance (or even a
small resistor) before the cap would convert some of the current peaks to
voltage peaks and thus permit the cap to supply more of the harmonic
currents.

Determining the power consumed by a VERY non-linear load isn't an easy
problem. Putting an efficient low pass filter between the load and the
power meter will reduce the complexity and increase the accuracy.

 

[[email protected]]

The KAW is a fine unit, but when I plug it into a Vector Maxx mod.
square wave inverter and try to find the Hz, it just reads dC.

Perhaps your Maxx is making DC (?) A lot of appliances would work fine
with 0 V, peak/2, peak, peak/2, 0, and so on.

Nick

 

[Beachcomber]

Hello all,

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.

But how does each one compare to the mechanical wattmeter that is
likely spinning around outside your house?

In other words, if you are measing a complex load with complex
impedance and harmonics to let's say, determine the precise amount of
power consumption per hour. One of the electronic units may give you
a figure of precise accuracy. but if that measurement doesn't match
the recording characteristics of the simple meter that is used to
actually used to bill you per kWh, then it doesn't make much sense
unless you are doing some sort of precision lab experiment.

Beachcomber

 

[Derek Broughton]

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.

I remember when you did that. I've had no reason to doubt the accuracy of
mine either except for the no-load - or very light load - situation.

OP reported:
Mine, for some reason, shows 0.04A and 0W with no load. More surprisingly,
it showed the same thing plugged into a single strand of LED Xmas lights
(which were rated at 4W).

 

[Pop]

....

: OP reported:
: >> K: 2.19A (w/ no load, device reads 0.02A, 0.0W)
:
: Mine, for some reason, shows 0.04A and 0W with no load. More
surprisingly,
: it showed the same thing plugged into a single strand of LED
Xmas lights
: (which were rated at 4W).
: --
: derek

That's odd: Mine shows no current with no load. By "no load",
do you mean nothing plugged into it? Or whatever's plugged in is
just turned off?
I can't see a 4W load directly at first, but ... if I leave it
plugged in for a few hours, I'll get a fairly accurate watt-hour
number. The PF goes wild though, and shows about 40%; don't
think it's quite THAT low <g>! The VA was high, too, but ...
guess nothing's perfect and sometimes confusing (as in, how VA
can be off, and kWH still be close to calcs. ANYway, it's 400 &
4000 watts I'm concerened with!

Pop

 

[Derek Broughton]

...

: OP reported:
: >> K: 2.19A (w/ no load, device reads 0.02A, 0.0W)
:
: Mine, for some reason, shows 0.04A and 0W with no load. More
: surprisingly,
: it showed the same thing plugged into a single strand of LED
: Xmas lights
: (which were rated at 4W).

That's odd: Mine shows no current with no load.

I doubt it's that odd. If yours and mine are at opposite ends of the
possible range shown by kill-a-watts, then OP's is right in the middle

By "no load",
do you mean nothing plugged into it? Or whatever's plugged in is
just turned off?

Nothing plugged in. My initial thought was that there was actually a
phantom load in my new washer, so I unplugged it - and found it was still
reading 0.04A.

I can't see a 4W load directly at first, but ... if I leave it
plugged in for a few hours, I'll get a fairly accurate watt-hour
number.

Same here.

ANYway, it's 400 & 4000 watts I'm concerned with!

Being off-grid with a small energy budget, it's all the 5-10W loads that
really bug me. I never pay attention to the A or VA values, anyway. It's
the watts that I worry about.

 

[ITSME.ULTIMATE]

Perhaps your Maxx is making DC (?) A lot of appliances would work fine
with 0 V, peak/2, peak, peak/2, 0, and so on.

Nick

Well, I contacted the manufacturer of Watts Up? Pro regarding the
influence of high crest factor non sinusoidal loads on its accuracy. I
will keep you guys posted with what they have to say.

Crest factor and bandwidth limits are both important specifications for
a power meter and it is usually included in a higher grade power meter
such as this:
http://www.valhallascientific.com/data-sheets/pdfs/2100_DS.pdf

 

[Steve Spence]

The KAW is a fine unit, but when I plug it into a Vector Maxx mod.
square wave inverter and try to find the Hz, it just reads dC.
Everything has limitations.

It works ok on a AC Delco 2500 watt MSW inverter.

 

[Derek Broughton]

I think you will find that the current or VA is really
all you have to worry about. If a load is 50% pf with
100va to produce 50watts then the source has to produce
that 100va even if you only use 10 watts of it.

but....

VAs do not add up exactly. The phase angle has to be
prorated into the addition.

Argh! We've been around this a number of times. Ordinary humans don't
understand the whole power factor thing. I swear last time power factors
were discussed someone asked which number really mattered, and was told
"watts". If you're on the grid, you're paying for Watts (or at least
watt-hours). I'm not on the grid, but I still think that what I'm
concerned about is Watts. aiui (which I admit, I really don't!), volt-amps
only really matter to me if I'm anywhere close to using the maximum
available current on my system. I've never done that.

 

[ITSME.ULTIMATE]

Current measurement:

Watts Up?: Current transformer.

Kill-A-Watt: Shunt

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 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.

 

[daestrom]

Argh! We've been around this a number of times. Ordinary humans don't
understand the whole power factor thing. I swear last time power factors
were discussed someone asked which number really mattered, and was told
"watts". If you're on the grid, you're paying for Watts (or at least
watt-hours). I'm not on the grid, but I still think that what I'm
concerned about is Watts. aiui (which I admit, I really don't!),
volt-amps
only really matter to me if I'm anywhere close to using the maximum
available current on my system. I've never done that.
--

The problem is you have to worry about both. But for different reasons.

If you're off-grid, it's watt-hours that drains your batteries. And it's
watt-hours that you get from your PV or wind or wattever (pun intended).

But the inverter has to supply the VA, and the wiring has to carry the VA.
If the inverter shutdown at 30 amps, it will shutdown at 30A with pf of 0.3,
or 0.7, or 1.0.

So you need to be 'aware' of both, and know when each one comes into play.

daestrom