AC electrical power usage

[Muse]

Just got my new Sharp R-21LCFS microwave oven. Was concerned because the sister model, Sharp R-21LCF was measured by a user in 2013 (Amazon review) to have standby power usage of 9 watts.

I plugged it into my Kill-a-Watt meter and set it to watts and it reads 3 watts. However, set to amps the Kill-a Watt says 0.09 amps. An online calculator indicates that at 120 volts 0.09 amps would equate to 10.8 watts. Using my multimeters and an extension cord I adapted with banana plugs I'm getting around 0.08 amps, which the same online calculator equates to 9.6 watts.

So, what is the actual power usage here? 9 or 10 watts or 3 watts?

Do I want to set my Kill-a-Watt device to Khours and see what it says after a day or two to get a figure I can have some confidence in???

 

[nakedfrog]

Over 9000!!

 

[Charlie22911]

Aren't there some other weird things like power factor that come into play?

 

[Muse]

Aren't there some other weird things like power factor that come into play?

One online amps--->watts calculator wanted me to enter the power factor. It started at .1, then .2, etc. I have no idea what the power factor is or what it represents. What is the power factor? How do I determine what it might be? I don't remember that in school.

I was also asked if it was 3 phase or 1 phase. What's that???

 

[us3rnotfound]

https://en.wikipedia.org/wiki/Power_factor#Power_triangle

I think what your Killawatt is reporting (3W) is the Real Power in the Power Triangle. Your microwave is drawing 0.09 amps but it isn't utilizing all those amps perfectly. A big portion of the current is being wasted but still flowing from the utility company and you're paying for it. It's just the Microwave for sure is consuming 3 W. But there's phenomena going on in that there Sharp that has to do with capacitive loads. Do me a favor, what's the PF output on the Killawatt? Theta seems to be about 74 degrees. Well, well well, is power factor about 0.3?

 

[NL5]

No way power factor would cause a discrepancy that large. Plus, residential meters only measure true power and ignore reactive power as it is usually pretty small in a home. If you are a large industrial customer, they will measure reactive power with a separate meter and charge for it. I'm going to say that with measurements so small, it's inaccuracies in the testing tools.

 

[Red Squirrel]

How are you measuring the current, if it's with a clamp meter, those tend to be inaccurate at lower currents as there is too much noise while the signal (the electromagnetic field it's measuring to determine current) is very small.

 

[Muse]

How are you measuring the current, if it's with a clamp meter, those tend to be inaccurate at lower currents as there is too much noise while the signal (the electromagnetic field it's measuring to determine current) is very small.

Well, I've used two methods:

1. Plugging my Kill-a-Watt into a power strip that's plugged into the wall, then plugging the Sharp into the Kill-a-Watt. Shows 3 watts when I hit the watts button. After 11.5 hours of this, hitting KWH button shows 0.03. If that's accurate I think that suggests that the 3 watts figure is pretty right.

2. I have an extension cord that I cut one conductor in the middle (it's a two conductor cord, no ground). I ran two conductor wire from the cut ends to banana plugs, which I plug into a multimeter to test amperage. The Sharp I plug into the female end of the extension cord and the male end I plug into the wall or power strip. I have two good multimeters and they show around 0.08 amps.

I don't know what you mean by clamp meter.

AFAIK, electromagnetic fields should have nothing to do with either of my methods. The current is going right through either the Kill-a-Watt or a multimeter.

 

[Muse]

https://en.wikipedia.org/wiki/Power_factor#Power_triangle

I think what your Killawatt is reporting (3W) is the Real Power in the Power Triangle. Your microwave is drawing 0.09 amps but it isn't utilizing all those amps perfectly. A big portion of the current is being wasted but still flowing from the utility company and you're paying for it. It's just the Microwave for sure is consuming 3 W. But there's phenomena going on in that there Sharp that has to do with capacitive loads. Do me a favor, what's the PF output on the Killawatt? Theta seems to be about 74 degrees. Well, well well, is power factor about 0.3?

I don't understand this. The KillaWatt is showing 0.05 kwh usage after 18 hours. Would that be dependable or is it possible that it's actually closer to 0.10?

 

[us3rnotfound]

I don't understand this. The KillaWatt is showing 0.05 kwh usage after 18 hours. Would that be dependable or is it possible that it's actually closer to 0.10?

What's the VA option in the KillaWatt show? Is it a number closer to (0.09 * 120 = 10.8)? I really think that the Watts (real power) is what the unit under test actually absorbs, while the VA (or apparent power) is the power being delivered to the unit. I'd appreciate for someone much smarter than me to chime in and explain. Is the power circuitry driving the LED clock in the Sharp really bad at being efficient with its AC/DC and DC/DC conversions?

To add to this, I still stand by my theory of your new microwave having a very low power factor. The power actually being used by the microwave in this standby state is less than power delivered. The Sharp is "sending" power back to the source because of its internal A/C rectifier being not optimal or something similar.

Edit2: Basically, the sine waves of the voltage and current aren't lined up perfectly. So when you go to multiply the instantaneous current and voltage, you get a fraction of the answer you'd get in a purely resistive network.

 

[nakedfrog]

The surveillance modules take some power.

 

[Red Squirrel]

Clamp meter is the one that uses electromagnetic field of conductor to determine current draw. They tend to not be as accurate for lower current. You sometimes have to ensure it's reading 0 before you plug in the load as well, because sometimes it's catching other sources of magnetic fields and reading it as current. You can usually hit a button to zero it out first. Not sure how accurate killawatt is either. It's probably more accurate for higher resistive loads. The microwave on standby is probably a highly capacitive load, but depends on what kind of converter it uses for the electronics.

Power factor is an odd thing though, I won't pretend to fully understand it, but in simple terms it's the ratio of the current wave form vs the voltage wave form offset. In a perfect situation they should both be in sync, but a combination of inductive and capacitive loads can desync them either way. So high inductance will cause the wave form to lag behind and capacitive loads will push it further the other way. That's why those "power factor saver" devices usually only hvae a single capacitor. Those don't do crap though, you would need a much larger capacitor bank, and some kind of system that measures power factor and puts capacitors in/out of the circuit as required.

But long story short, that's probably why your reading is off. If you get a true RMS meter it will give you a proper reading. They are more expensive though.

 

[us3rnotfound]

Power factor is an odd thing though, I won't pretend to fully understand it, but in simple terms it's the ratio of the current wave form vs the voltage wave form offset. In a perfect situation they should both be in sync, but a combination of inductive and capacitive loads can desync them either way. So high inductance will cause the wave form to lag behind and capacitive loads will push it further the other way. That's why those "power factor saver" devices usually only hvae a single capacitor. Those don't do crap though, you would need a much larger capacitor bank, and some kind of system that measures power factor and puts capacitors in/out of the circuit as required.

Yeah I've been looking back into PF, and those power saver devices ("KVAR" savings devices). And what I've found to be the conclusion from NIST physicists is that they're a waste of money. Because even though the current draw is higher without the corrective capacitor installed, the power factor is lower, and when you put the corrective capacitor in the circuit, current goes down and PF goes up, nullifying any savings you would've had. The only good you'd be doing by installing these around the house is, on the aggregate of residential houses, putting a little less current draw on all the power lines, saving on some CO2 emissions. But that'd have to be on a huge scale.

Check this out: https://www.nist.gov/news-events/ne...ifies-utility-power-factor-correction-devices

So actually in my first response I was wrong, he's not paying for that extra current draw. The Sharp is essentially putting some current back on the line to the utility company, so the KWh reading of .05 is probably right in terms of what you'll be paying your utility company. End of story.