Kill-a-watt meter -- What does it measure?

[sophiecentaur]

TL;DR

How useful is a kit-a-watt meter for non-resistive loads?

I've been reading this thread about power consumption of a domestic fan and @SentinelAeon says he's using a kill-a-watt meter. The accuracy for power measured to an induction motor will depend very much on the way the meter works. A Google search only turned up sales and 'user' pages. Does anyone have actual experience of these things?

 

[berkeman]

My experience was that they were not very accurate, especially at light loads. I don't remember how power factor affected their accuracy, though.

 

[russ_watters]

Mine works pretty well (readings in watts):

...and this was with the TED whole house readings not being completely stable.

Vs an older version of this: https://www.theenergydetective.com/content/the-energy-monitoring-system-you-have-been-waiting-for

 

[SentinelAeon]

I could compare the readings from killawatt to current and voltage reading from multimeter. Though if i understand this correctly, the problem is similar to using multimeter to measure current when driving high frequency speakers - multimeter is simply to slow with its 60Hz and you would need something like Osciloskop?

 

[Hankelec]

TL;DR Summary: How useful is a kit-a-watt meter for non-resistive loads?

I've been reading this thread about power consumption of a domestic fan and @SentinelAeon says he's using a kill-a-watt meter. The accuracy for power measured to an induction motor will depend very much on the way the meter works. A Google search only turned up sales and 'user' pages. Does anyone have actual experience of these things?

I have been using a brand name "Kill-a-Watt" meter for several years. Quite happy with its performance. My guess as to its accuracy would be 95%. I base that guess upon a device's printed power rating and the meter's readout. A killawatt meter measures and records a device's power consumption. Power = volts x amps. P=EI. Power is expressed in watts. Example: If you have your Kill-a-Watt meter inline with 10 - 100 watt lightbulbs, at the 60 minute mark, the meter will read 1.00 kilowatt hour. kWh. 10 - 100 W lightbulbs, when energized, will draw 1000 W. If there is a constant load of 1000 W (1 kW) for 1 hour, your power consumption has been 1 kWh.

 

[sophiecentaur]

Power = volts x amps. P=EI. Power is expressed in watts

That's ok for DC Power but the Power of a perfectly sinusoidal AC supply is actually P = V I Cos(phase angle). Cos(phase angle) is referred to as the Power Factor. A device that's not pure resistance (for example an induction motor) can have a very low PF.
You need to read these two links (and more) I think.

 

[OmCheeto]

I suspect my killawatt meter is functioning adequately, though I've never gone to the trouble of checking its accuracy.

The readings it gives for my 35 year old refrigerator are as follows:
408 volt amps
210 watts
3.39 amps
121.4 volts
0.50 power factor
60 hertz

I found a posting where someone compared the readings of a Killawatt meter($20) with a Fluke 1735($2000). The Fluke was generally more accurate. Much more so at very low wattages.

 

[sophiecentaur]

my killawatt meter is functioning adequately,

Absolutely fine for high power heating appliances and that's what counts for your electricity bill.

 

[Hankelec]

That's ok for DC Power but the Power of a perfectly sinusoidal AC supply is actually P = V I Cos(phase angle). Cos(phase angle) is referred to as the Power Factor. A device that's not pure resistance (for example an induction motor) can have a very low PF.
You need to read these two links (and more) I think.

You are correct on those issues. I responded to this thread because Kill-a-Watt meter is the brand name ,of the meter I use to track my power usage. I rarely use any other functions other than kWh and clock. I did scroll thru its other functions just now. It does display current watts, VA, and PF. It gave me a power factor of 0.93, a VA of 893, a wattage of 830. Amps reading was around 7.68 A, voltage reading around 117 VAC. 7.68 x 117 =~ 898 VA. So it looks like all things are quite on the eastern front.