"Vampire" wattmeter

[KarlB]

Hi...

I saw a piece on CNN about how much power household electronics can consume
on "standby". In it, Alan Meier of the Lawrence Berkeley National Lab was
using a really neat little digital wattmeter that you could simply plug into
an A/C socket, then plug a device into it to read power consumption.

Anyone know where I might find a meter like that, OR a good schematic?
Haven't found much with a Google search.

 

[CJT]

KarlB wrote:

Hi...

I saw a piece on CNN about how much power household electronics can consume
on "standby". In it, Alan Meier of the Lawrence Berkeley National Lab was
using a really neat little digital wattmeter that you could simply plug into
an A/C socket, then plug a device into it to read power consumption.

Anyone know where I might find a meter like that, OR a good schematic?
Haven't found much with a Google search.


That sounds like the Kill-A-Watt, available at Harbor Freight or

www.jr.com (and many others)

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[KarlB]

Yes, when I changed my search to consumption meter instead of wattmeter I
started getting results, including the one you mentioned. Thanks.

"CJT" <abujlehc@prodigy.net> wrote in message
news:457C91E7.7000500@prodigy.net...

KarlB wrote:
Hi...

I saw a piece on CNN about how much power household electronics can
consume on "standby". In it, Alan Meier of the Lawrence Berkeley National
Lab was using a really neat little digital wattmeter that you could
simply plug into an A/C socket, then plug a device into it to read power
consumption.

Anyone know where I might find a meter like that, OR a good schematic?
Haven't found much with a Google search.
That sounds like the Kill-A-Watt, available at Harbor Freight or
www.jr.com (and many others)

--
The e-mail address in our reply-to line is reversed in an attempt to
minimize spam. Our true address is of the form che...@prodigy.net.

 

[Caesar Valenti]

While I'm sure a "Kill-a-Watt" meter is much more convenient, I just use
an accurate 10 or 1 ohm resistor (switchable) in series with the neutral
side of the line; all enclosed in a nice box. I then read the voltage
drop across the resistor with my AC voltmeter. To minimize voltage drop
issues, I use the 1-ohm resistor only for items that draw more than
about 1/2 amp of current. While this is very accurate for most devices,
I am not sure how well it works for items like switching power supplies
(e.g. a computer.) I am also not sure how this would correspond to what
a standard house watt-hour meter would record.
Preferably one would use a true RMS reading AC voltmeter to read the
voltage drop, although that's not what I am using.

And of course, all these unknowns (and probably more) may also apply
with the kill-a-watt.
cv

KarlB wrote:

Hi...

I saw a piece on CNN about how much power household electronics can consume
on "standby". In it, Alan Meier of the Lawrence Berkeley National Lab was
using a really neat little digital wattmeter that you could simply plug into
an A/C socket, then plug a device into it to read power consumption.

Anyone know where I might find a meter like that, OR a good schematic?
Haven't found much with a Google search.

 

[CJT]

Caesar Valenti wrote:

While I'm sure a "Kill-a-Watt" meter is much more convenient, I just use
an accurate 10 or 1 ohm resistor (switchable) in series with the neutral
side of the line; all enclosed in a nice box. I then read the voltage
drop across the resistor with my AC voltmeter. To minimize voltage drop
issues, I use the 1-ohm resistor only for items that draw more than
about 1/2 amp of current. While this is very accurate for most devices,
I am not sure how well it works for items like switching power supplies
(e.g. a computer.) I am also not sure how this would correspond to what
a standard house watt-hour meter would record.
Preferably one would use a true RMS reading AC voltmeter to read the
voltage drop, although that's not what I am using.

And of course, all these unknowns (and probably more) may also apply
with the kill-a-watt.
cv

The problem with that is that it gives Volt-Amps rather than Watts,

ignoring power factor.

KarlB wrote:

Hi...

I saw a piece on CNN about how much power household electronics can
consume on "standby". In it, Alan Meier of the Lawrence Berkeley
National Lab was using a really neat little digital wattmeter that you
could simply plug into an A/C socket, then plug a device into it to
read power consumption.

Anyone know where I might find a meter like that, OR a good schematic?
Haven't found much with a Google search.

--
The e-mail address in our reply-to line is reversed in an attempt to
minimize spam. Our true address is of the form che...@prodigy.net.

 

[Caesar Valenti]

CJT wrote:

The problem with that is that it gives Volt-Amps rather than Watts,
ignoring power factor.
Sorry, I guess I should have been more specific. This will just provide

the current in amps (or 10x amps if using the 10-ohm resistor). To get
watts, you must multiply amps by the voltage (typically 115). I don't
think power factor comes into play here, but am not sure. If it does,
would the "kill-a-watt" deal with it? Does a watt-hour meter deal with
power factor? Does power factor really come (significantly) into play
for devices found in a normal household?

 

[CJT]

Caesar Valenti wrote:

CJT wrote:

The problem with that is that it gives Volt-Amps rather than Watts,
ignoring power factor.

Sorry, I guess I should have been more specific. This will just provide
the current in amps (or 10x amps if using the 10-ohm resistor). To get
watts, you must multiply amps by the voltage (typically 115).

No, that gives volt-amps, which only equal watts for resistive loads.
For reactive loads, voltage and current are not precisely in phase.

I don't

think power factor comes into play here, but am not sure.

You are incorrect.

If it does,

would the "kill-a-watt" deal with it?

Yes. In fact, it will display power-factor as an option.

Does a watt-hour meter deal with

power factor?

A watt-hour meter measures watt-hours.

Does power factor really come (significantly) into play

for devices found in a normal household?

It can. For instance, I have a computer here that has a power factor
of about 0.65.

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[Caesar Valenti]

CJT wrote:

Caesar Valenti wrote:
CJT wrote:

The problem with that is that it gives Volt-Amps rather than Watts,
ignoring power factor.

Sorry, I guess I should have been more specific. This will just
provide the current in amps (or 10x amps if using the 10-ohm
resistor). To get watts, you must multiply amps by the voltage
(typically 115).

No, that gives volt-amps, which only equal watts for resistive loads.
For reactive loads, voltage and current are not precisely in phase.

You are right. I just never think about about the difference since
most of my stuff is resistive (switching power supplies excepted, and
I almost never measure motors)
think power factor comes into play here, but am not sure.

You are incorrect.
If it does,
would the "kill-a-watt" deal with it?
Yes. In fact, it will display power-factor as an option.

hmmmm.....for the $25-30 cost of this, I may have to get one. Sounds

like a bargain....if it is indeed accurate.
At one time I read that, for a typical household, trying to compensate
for power factor was just not worth it. However, that was before
switching power supplies became so common. Does the kill-a-watt tell
you if the power factor is due to inductive vs capacitive loading?

 

[CJT]

Caesar Valenti wrote:

CJT wrote:

Caesar Valenti wrote:

CJT wrote:

The problem with that is that it gives Volt-Amps rather than Watts,
ignoring power factor.


Sorry, I guess I should have been more specific. This will just
provide the current in amps (or 10x amps if using the 10-ohm
resistor). To get watts, you must multiply amps by the voltage
(typically 115).


No, that gives volt-amps, which only equal watts for resistive loads.
For reactive loads, voltage and current are not precisely in phase.

You are right. I just never think about about the difference since
most of my stuff is resistive (switching power supplies excepted, and
I almost never measure motors)

think power factor comes into play here, but am not sure.


You are incorrect.
If it does,

would the "kill-a-watt" deal with it?

Yes. In fact, it will display power-factor as an option.

hmmmm.....for the $25-30 cost of this, I may have to get one. Sounds
like a bargain....if it is indeed accurate. At one time I read that, for
a typical household, trying to compensate for power factor was just not
worth it.

Compensating for power factor is generally _not_ worthwhile, because
the power companies typically bear the costs associated with it.
Residential billing is normally based on real power, ignoring the
imaginary component. Rarely will a residential application require
a change of wire size because of a poor power factor -- in fact, I've
never encountered such a case. Of course, the power companies probably
appreciate any efforts by their residential customers to improve PF.

However, if you're trying to determine as accurately as possible where
your power is going, you should take it into account.

However, that was before switching power supplies became so

common. Does the kill-a-watt tell you if the power factor is due to
inductive vs capacitive loading?

No, and the waveforms associated with switching power supplies are
more complicated than that.


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[KarlB]

Just wanted to add that I did buy the Kill-A-Watt, and I am very impressed.
I've been somewhat surprised by some of the results. I am now shutting off
power to those many "standby" devices I have when they are not in use,
including two UPS units.


"KarlB" <none@none.com> wrote in message
news:5PZeh.159979$2t1.63156@newsfe18.lga...

Hi...

I saw a piece on CNN about how much power household electronics can
consume on "standby". In it, Alan Meier of the Lawrence Berkeley National
Lab was using a really neat little digital wattmeter that you could simply
plug into an A/C socket, then plug a device into it to read power
consumption.

Anyone know where I might find a meter like that, OR a good schematic?
Haven't found much with a Google search.