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 deerhunter 09-03-2010 07:54 PM

How does electricity meter calculate kwh?

Recently I learned about the concept of power factor and did some research into it. I read some old thread on power factor correction and it seems that the prevailing belief is that it's useless for residential users. However, I've been also researching about solar power recently, and was informed that the new digital meter won't turn back if a solar panel system feed current back into the grid. Some even say that you are charge for pushing current into the grid. What ever electricity you give the grid, it will be considered as kwh used.

This seems to contradict with the claim that utility company charge the "true power" used by residential user. Because what happens with device that has PF less than 1 is that it first store more energy than it uses, and then, it pushes the extra energy back into the grid. So, at first, the meter will count the extra energy as kwh used. If a meter doesn't "turn back", wouldn't it mean that it doesn't deduct the electricity "returned" to the grid from the kwh used? This would result in the meter charging more than the "true power" used.

Would someone who is familiar with how the digital meter calculate kwh please explain this to me? I can't think of how a meter can avoid over charging like I described without being about to reduce it's kwh number when current is pushed back into the grid.

 nap 09-03-2010 08:43 PM

in the systems I have seen where you are allowed to sell your excess power, you must have a second meter to read the power distributed back to the grid. It would then be calculated as an offset to your bill.

Your power factor concern has nothing to do with selling your own power to the POCO. Power factor is a comparison of real power compared to apparent power. There is no power factor involved with supply. It is only a concern of consumed power.

Quote:
 Because what happens with device that has PF less than 1 is that it first store more energy than it uses, and then, it pushes the extra energy back into the grid.
a device that has a poor power factor does not store power nor does it return energy to the grid.

 deerhunter 09-03-2010 10:20 PM

I have a rather good understanding of power factor. What happens with induction or capacitor is that it causes the current and voltage to be out of phase. This means that for a certain period of time during a AC cycle, the device works as a power source and push the current back to the grid -- only for a brief period. The energy is stored either in a capacitor in the form of potential energy, or in the form of magnetic field in the case of induction, and then part of this energy is returned to the grid.

All I want to know is how meters calculate Kwh. I mentioned solar panel system because what I heard about the way digital meters doesn't spin back makes me think that it may not charge the true power for a device with power factor less than 1, but more than that.

Quote:
 Originally Posted by nap (Post 495640) in the systems I have seen where you are allowed to sell your excess power, you must have a second meter to read the power distributed back to the grid. It would then be calculated as an offset to your bill. Your power factor concern has nothing to do with selling your own power to the POCO. Power factor is a comparison of real power compared to apparent power. There is no power factor involved with supply. It is only a concern of consumed power. a device that has a poor power factor does not store power nor does it return energy to the grid.

 deerhunter 09-03-2010 11:46 PM

To be more specific about my question. My understanding of the analog meter is that it effectively takes the integral of the product of instantaneous current and voltage. This means that if the current is pushed to the grid, then the integral will be negative and thus the meter spins backward. The kwh measured this way will be the true power used. Since many people say that the new digital meter doesn't "spin" backward, that means the kwh isn't measure in this way. That is why I want to know exactly how the digital meter measures kwh.

I have tried to communicate with the utility. But the people I talked to don't know what I'm talking about. They redirect me to someone who didn't answer the phone and haven't called me back so far. I think I may have to buy a watt meter and make the measurement myself.

 bobelectric 09-04-2010 04:29 AM

This should go to a Professional site such as ElectricianTalk.com. This site is for DIY.

 a7ecorsair 09-04-2010 08:39 AM

Quote:
 Originally Posted by deerhunter (Post 495679) I have a rather good understanding of power factor. What happens with induction or capacitor is that it causes the current and voltage to be out of phase. This means that for a certain period of time during a AC cycle, the device works as a power source and push the current back to the grid -- only for a brief period. The energy is stored either in a capacitor in the form of potential energy, or in the form of magnetic field in the case of induction, and then part of this energy is returned to the grid. All I want to know is how meters calculate Kwh. I mentioned solar panel system because what I heard about the way digital meters doesn't spin back makes me think that it may not charge the true power for a device with power factor less than 1, but more than that.
Ah, so you've studied ELI the ICE man. Voltage (E) leads current (I) in an inductive (L) circuit and current (I) leads voltage (E) in a capacitive (C) circuit.
Have you had a discussion with your power company? Your AC isn't going to get you free power. Note the first entry below.
• Power dissipated by a load is referred to as true power. True power is symbolized by the letter P and is measured in the unit of Watts (W).
• Power merely absorbed and returned in load due to its reactive properties is referred to as reactive power. Reactive power is symbolized by the letter Q and is measured in the unit of Volt-Amps-Reactive (VAR).
• Total power in an AC circuit, both dissipated and absorbed/returned is referred to as apparent power. Apparent power is symbolized by the letter S and is measured in the unit of Volt-Amps (VA).
• These three types of power are trigonometrically related to one another. In a right triangle, P = adjacent length, Q = opposite length, and S = hypotenuse length. The opposite angle is equal to the circuit's impedance (Z) phase angle.

 deerhunter 09-04-2010 11:23 AM

I know the relation between true power, reactive power and apparent power. The big question is how the utility calculate kwh, this is all that matters. I find that most people simply assume that the meter measures the true power. However, I won't automatically assume this.

There are a few points that I want to make about how meter works.

1. The analog meter measures true power, there is no doubt about that. So the power correction device won't save money if such meters are used.

2. It seems that there are two types of meters, bi-directional and uni-directional. Analog meter should be bi-directional. There are meters that only measures energy that flows in one direction.

3. This seems to related to the term "net metering". A meter capable of net metering should be bi-directional. Since meters are not automatically net metering, there are those in residential use that only measures in one direction. For such meters, I have a big question on how true power is measured. The simplest way to measure true power is as I stated in my previous post -- simply take the integral over time of instantaneous current times voltage, and allow the value to be negative when they are not in the same direction. If a meter is uni-directional, there will be no negative values, so this method cannot be used.

4. As stated by wiki, there are 4 states that doesn't require net metering. My state, Mississippi happens to be one of them.

 TimPa 09-04-2010 04:56 PM

my understanding's and assumptions are:

- incoming power travels through the meter
- the current creates a magnetic field (magneto-motive force) which in turn is used to turn the meter rotor shaft
- the rotor shaft is mechanically linked to the count-wheel system
- the greater the current, the greater the mmf, the faster the rotor spins
- some meters may not be physically capable of running/registering backwards, by default design
- here in pa, if you intend to incorporate a power source, you must pre-establish with the power company so the proper bi-directional equipment can be installed
my \$.02

 a7ecorsair 09-04-2010 08:58 PM

Quote:
 Originally Posted by deerhunter (Post 495827) 1. The analog meter measures true power, there is no doubt about that. So the power correction device won't save money if such meters are used. 2. It seems that there are two types of meters, bi-directional and uni-directional. Analog meter should be bi-directional. There are meters that only measures energy that flows in one direction. 3. This seems to related to the term "net metering". A meter capable of net metering should be bi-directional. Since meters are not automatically net metering, there are those in residential use that only measures in one direction. For such meters, I have a big question on how true power is measured. The simplest way to measure true power is as I stated in my previous post -- simply take the integral over time of instantaneous current times voltage, and allow the value to be negative when they are not in the same direction. If a meter is uni-directional, there will be no negative values, so this method cannot be used. 4. As stated by wiki, there are 4 states that doesn't require net metering. My state, Mississippi happens to be one of them.
1. Why won't it? The unit would run more efficiently and use less power. How do you think they have improved the SEER of different appliances?
2. Are you expecting to return power to the grid by improving the efficiency of your house? If the meter ran backwards that would mean you are using less electricity then what you are producing. That won't happen just by correcting the power factor.

 deerhunter 09-04-2010 09:58 PM

"Returning energy to grid" basically means that the instantaneous current and voltage have different direction. This makes their product negative, which means the instantaneous power usage is negative. This will happen if the current and voltage have don't have the same phase. And this is the reason why the apparent power is greater than the true power. Without the negative power usage accounted for, I can't think of how the true power can be measured. That is why I suspect a uni-directional meter doesn't really measure true power.

Quote:
 Originally Posted by a7ecorsair (Post 496043) 1. Why won't it? The unit would run more efficiently and use less power. How do you think they have improved the SEER of different appliances? 2. Are you expecting to return power to the grid by improving the efficiency of your house? If the meter ran backwards that would mean you are using less electricity then what you are producing. That won't happen just by correcting the power factor.

 deerhunter 09-04-2010 11:14 PM

Why this is considered "off topic".

 Scuba_Dave 09-05-2010 08:19 AM

Because it has nothing to do with DIY

 deerhunter 09-05-2010 11:38 AM

I ask this information with the intention to install a power factor correction capacitor if it's confirmed that it will help. If I don't get an answer to my question, I will have to find the answer DIY way. I don't understand the rigidness in determining if it's relevant to DIY.

I know you can argue that intention doesn't count. But the point is, information collection is part of DIY process. Do I have to spell it all out in order to be not "off topic"? When people start to ask question about how a system works on a DIY forum, can't it be assumed that the person is contemplating about doing something with the system?

Quote:
 Originally Posted by Scuba_Dave (Post 496146) Because it has nothing to do with DIY

 nap 09-05-2010 04:25 PM

Quote:
 deerhunter;496236]I ask this information with the intention to install a power factor correction capacitor if it's confirmed that it will help.
trying to determine a properly sized PF correction cap for a residential use is nearly impossible unless you utilize a variable level system. First, a residence has such a low usage of power that the correction would generally not be cost effective. Then, it is so varied that any correction you would install would be counterproductive if the loads used to calculate the Var correction were not drawing power all the time. You would actually cause the exact opposite problem you have with the inductive load you are trying to correct for.

 deerhunter 09-05-2010 08:07 PM

Actually, there is one appliance that use the most electricity: a 5 ton heat pump. My plan is to correct it individually. I'm thinking about turning on the capacitor the moment the heat pump turns on. May be I can also correct the refrigerator. Other loads are mostly resistance or nonlinear and don't amount to much anyway.

Quote:
 Originally Posted by nap (Post 496358) trying to determine a properly sized PF correction cap for a residential use is nearly impossible unless you utilize a variable level system. First, a residence has such a low usage of power that the correction would generally not be cost effective. Then, it is so varied that any correction you would install would be counterproductive if the loads used to calculate the Var correction were not drawing power all the time. You would actually cause the exact opposite problem you have with the inductive load you are trying to correct for.

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