How does an electric (watt-hour) meter work?

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Bill in OK

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Location
Oklahoma City
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Retired electronics/software engineer
Greetings,

We have a guest house which draws its power from the feed coming into the main house, and electrical consumption for both buildings is registered on a single meter.

The guest house is currently used for my office and for my wife's work area where she homeschools our grandchildren.

I would like to try to find out how much of the electrical bill is being spent on the guest house by itself. To that end, I am trying to build a rudimentary electronic device for measuring and recording the electrical consumption of the guest house.

I understand generally how AC current can be measured, using toroidal coils around the feed lines for picking up the magnetic fields around the cables. (P=EI) What I would like to know is, how does a power meter measure the total current in both phases? Would it work properly to read the voltage generated by two toroidal coils wired in series (with the phases opposed)? Or do the separate phases need to be measured individually and added together, or some other kind of mathematic formula applied? Or is it something even more complicated than that? 🤔

I am a retired electronics and software engineer, so I should probably understand some of the technology involved, though I have never worked with high voltage - high power AC systems.

Thanks!
 
Bill in OK said:
Greetings,

We have a guest house which draws its power from the feed coming into the main house, and electrical consumption for both buildings is registered on a single meter.

The guest house is currently used for my office and for my wife's work area where she homeschools our grandchildren.

I would like to try to find out how much of the electrical bill is being spent on the guest house by itself. To that end, I am trying to build a rudimentary electronic device for measuring and recording the electrical consumption of the guest house.

I understand generally how AC current can be measured, using toroidal coils around the feed lines for picking up the magnetic fields around the cables. (P=EI) What I would like to know is, how does a power meter measure the total current in both phases? Would it work properly to read the voltage generated by two toroidal coils wired in series (with the phases opposed)? Or do the separate phases need to be measured individually and added together, or some other kind of mathematic formula applied? Or is it something even more complicated than that? 🤔

I am a retired electronics and software engineer, so I should probably understand some of the technology involved, though I have never worked with high voltage - high power AC systems.

Thanks!
Click to expand...
You can just get a kWhr meter cheap on eBay and run the guest house feeder through that. That's what I did on the branch circuit feeding our ceramics kiln to keep track of its energy use. I got a meter base at a good price at the local Habitat for Humanity store.
 
There are plug-in watthour meters.... of course, you'd probably need one for every outlet you're using...
If you pardon me saying so, you could turn off everything in the main house and get an average of the outbuilding...
 
BarryO said:
You can just get a kWhr meter cheap on eBay and run the guest house feeder through that. That's what I did on the branch circuit feeding our ceramics kiln to keep track of its energy use. I got a meter base at a good price at the local Habitat for Humanity store.
Click to expand...
Good idea! I'll check it out. Thanks!
 
Seen CT meters for 120/240 single phase where they only use one CT and run the two lines in opposite directions through the CT. Technically they are measuring 120 volts and the current. A 240 volt load will register twice the current in this arrangement and will still be accurate measurement.

The next trick that I don't know as much about is how to make it see watts and not VA when the power factor isn't 1.0.
 
kwired said:
Seen CT meters for 120/240 single phase where they only use one CT and run the two lines in opposite directions through the CT. Technically they are measuring 120 volts and the current. A 240 volt load will register twice the current in this arrangement and will still be accurate measurement.

The next trick that I don't know as much about is how to make it see watts and not VA when the power factor isn't 1.0.
Click to expand...
"Power Factor" adds an element that sounds complicated. I'll need to do some more studying on that. Thanks!
 
Never cease to be amazed at how many basic querries get asked by folks who identify as electronic engineers. Either 'exagerating' (nice workd for lying?) or schools today are woefully lacking in basics.

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If you don't need revenue grade metering, there are several 'home energy monitors ' available.

You can have an electrician install one of:

http://go.sense.com/sense/

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How the Gen 2 Vue Energy Monitor Works

The Emporia Gen 2 Vue Energy Monitor installs in your electrical panel to track energy use and solar generation of your home through a simple mobile app.
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Or others
 
Bill in OK said:
"Power Factor" adds an element that sounds complicated. I'll need to do some more studying on that. Thanks!
Click to expand...
True watthour meters like POCO's use do not measure VA, they register watts (actually watthours after factoring in time component), which like mentioned is a little more complex than just measuring based on VA. I understand the difference but don't really know how they actually go about measuring it. Such meters will count in reverse if the energy happens to be flowing opposite of what is conventional where one that only registers VA based figures won't know the difference in direction of energy transfer.
 
Old school mechanical watthour are essentially induction motors where the spinning disk is the rotor.

You have voltage coils (coils with large number of turns connected L-L or L-N) and current coils (coils with a very few turns connected input to output on a single line). Both coils create a magnetic field, and the torque on the disk is proportional to the instantaneous product of voltage and current.

An eddy current magnetic causes drag on the disk, so the speed is proportional to torque. This makes the disk speed proportional to power. The number of turns of the disk is proportional to energy.

Jon
 
230806-1541 EDT

Power is an instantaneous measurement. Average power is an average value of the instantaneous values evaluated over time. Many incremental values of power added up over some time period and divided by that time period is average power for that period.

If you do an average over a short time, then this should be done over an integral number of cycles. Over a long time period start and stop times do not contribute much error.

A wattmeter measures the instantaneous values of these two variables and instantaneously multiplies these together, and via the time constant of the meter provides the averaging time constant.

If instead you separately measured average voltage, and average current, and then multiplied these two average values together you might not get a correct average power value. Only if the load was pure resistance would the calculated value be correct.

.

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