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
-- 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.
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:
The problem with that is that it gives Volt-Amps rather than Watts, ignoring power factor.
-- 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.
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?
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
You are incorrect.
If it does,
Yes. In fact, it will display power-factor as an option.
Does a watt-hour meter deal with
A watt-hour meter measures watt-hours.
Does power factor really come (significantly) into play
It can. For instance, I have a computer here that has a power factor of about 0.65.
-- 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.
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?
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
No, and the waveforms associated with switching power supplies are more complicated than that.
-- 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.
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.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.