Kill a Watt(tm) power meters

[Bernie Hunt]

Here on Long Island that's $37 per year.

Bernie

[Dan Musicant]

On Sun, 01 Nov 2009 15:28:15 -0600, cjt wrote:

:Dan Musicant wrote: :> On Sat, 24 Oct 2009 23:16:31 -0500, zxcvbob wrote: :> :> :Anybody familiar with them? Is the "EZ" (model P4460) worth an :> :extra $15 over model P4400? :> : :> :I want to see how much electricity my freezers, TV sets, computers, :> :etc are *really* using, plus check the frequency stability of my :> :portable generator and that little inverter than I mounted in my truck. :> : :> :Thanks, :> :Bob :> :> Two things: :> :> I. :> :> I bought the P4400 in February of 2008. I left it plugged into a power :> strip and plugged my desktop computer's power strip (and etc.) plugged :> into the Kill-a-Watt meter, measuring power draw (watts), for an :> indefinite time. It stopped working. It came back to life, don't :> remember the details, but leaving it off and unplugged for a while, it :> started working somehow. Someone said they thought that the meters :> aren't designed to be used in this way, so I stopped doing it. :> :> II. :> :> I've found that the P4400 (don't know about the other(s)) doesn't :> measure low power draw at all accurately. IOW, if your device is drawing :> 3-4 watts, say, the P4400 doesn't give you anything like an accurate :> reading. I only know this because I have a far more accurate way of :> measuring power draw, which I have used for years. The P4400 is a lot :> easier to use, but my system is much more accurate and I feel I can :> depend on it more. It's a simple thing I put together for next to :> nothing: :> :> 1. I already had a decent digital multimeter that measures AC amps, :> easily converted into watts (amps x 120 volts = watts). It cost me :> around $100 in the early 1990's. :> :> 2. I bought a couple of banana plugs that will fit in the multimeter's :> input jacks and attached them to a length of AC two conductor cord, :> around 6 feet long. The other end of this ~6 foot length of cord is :> soldered (both leads) to either end of a cut wire in a short extension :> cord. IOW, I cut one wire in the middle of that extension cord and :> connected the cut ends to the cord, the other end of which has the :> banana plugs. Plugging the banana plugs into the multimeter first :> (ALWAYS do it this way or you will likely cause a dangerous short !), I :> THEN plug the extension cord into a power outlet and then a device into :> the female end of the extension cord. The multimeter gives me the :> current draw and simple arithmetic gives me the watts. Amps x 120 = :> watts. :> :> Using this system I determined that the P4400 is useless for low draw :> measurement. I don't remember the cutoff where the P4400 starts being :> useful. I assume that this holds true for all of them and that I don't :> have a lemon here. :> :> Dan :> :> :> Email: dmusicant at pacbell dot net : :Your "accurate" system measures VA, not Watts. Read up on power factor, :the effect of an out-of-phase relationship between volts and amps.

Could you explain further? I'm measuring amps on 120v AC. My understanding is AC amps x AC volts = Watts. Not so? Why?

Dan

PS What's VA?

Email: dmusicant at pacbell dot net

[gfretwell]

VA is amps times volts. Watts is amps times volts after to take into account power factor.

[cjt]

Amps times volts equal watts when the current and voltage are in phase (i.e. a purely resistive load). When they are out of phase, there's an additional "power factor" applied to convert what is then volt-amps (VA) to watts (W). It all goes back to the true definition of AC watts, which is an integral (as in calculus) over time of instantaneous voltage times instantaneous current. If you actually do the calculus, you should be able to determine power factor as a function of phase angle. I bet the folks in Wikipedia (or your favorite source on the Internet, whatever it might be) have an even clearer explanation. :-)