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How is feed in and usage calculated by a digital electricity meter?

Semiconductor

To ask this question as clearly as possible, I do it by example: Three phase connection (Nos. 1, 2 and 3), loads at the same point of time: Phase 1: Usage of 10kW. Phase 2: Feed in of 25kW. Phase 3: Feed in of 2kW. How will this be read by a digital meter from AGL? By this i mean: If these are the stable loads for one hour, how many kWh will we be shown on the meter for usage and how many for feed in for that period?

5 REPLIES 5

Re: How is feed in and usage calculated by a digital electricity meter?

Super Charger

I am not familiar with your setup (beyond what you provided in this post), but normally 3 phase power systems take into account the sum of all electricity being used on all the phases and then subtract that from the amount of solar energy being generated to calculate the import or export amount for billing. In your example, assuming these are not values from your bill, but simply a hypothetical question: The calculation should be Total FiT - Usage. Hence 25 kWh + 2 kWh - 10 kWh = 17 kWh (FiT) x your FiT Rate. I would expect the inverter (assuming 3 phase) you have installed would sort out the consumption first (Against the Solar generated) and the excess (Electricity generated by solar which was not consumed) on the other phases would be exported to the grid (creating a FiT). The meter read by the energy retailer should not have incremented in value because the inverter provided exported energy and none was drawn from the grid. This, in theory, is how it is supposed to work. Not sure if I answered your question.

Re: How is feed in and usage calculated by a digital electricity meter?

Semiconductor

Schander, many thanks for the extensive reply to my original post. It may sound funny, but I am not sure either if your explanation does answer my question.

From the research that I did about three phase inverters it is clear that up to this date there is no inverter on the market that can shift the power fed-in from solar flexibly to each of the three phases out. So if there is, at one point of time, a production from the panels of say 3 kW, each phase of any three phase inverter on the market these days does supply a load of 1 kW. To put it differently, there is no way that a current three phase inverter can, e.g., distribute this 3 kW production to the outlets as follows: 2 kW onto Phase 1, 0.5 kW onto Phase 2, 0.5 kW onto Phase 3.

I found further that, for technical reasons, you cannot combine the three phases when wiring single phase outlets in your property. That means, some single phase outlets in the property have to be connected to Phase 1, some to Phase 2, some to Phase 3. Assuming that you have a single phase consumer on Phase 1 taking a load of 2 kW, and no consumption on the two other phases, I wonder where the „missing“ 1 kW for Phase 1 is taken from, if not from the grid. As said, I am positive that a current three phase inverter cannot provide this from solar production if the total production is 3 kW. It would have to be 6 kW instead, because then each of the three phases out could supply a load of 2 kW.

This however does not contradict your statement that „the inverter (assuming 3 phase) you have installed would sort out the consumption first (Against the Solar generated)“. It does so on all three phases, but individually per phase.

Practically this leads back to my original question. The „sort out“ would be: Phase 1: 2 kW of consumption against 1 kW of production makes 1 kW taken from the grid. (I therefore doubt your statement: „none was drawn from the grid“ is correct.) Phase 2: 0 kW of consumption and 1 kW of production makes 1 kW fed into the grid. Phase 3: 0 kW of consumption and 1 kW of production makes 1 kW fed into the grid.

In my opinion, there are two ways that this could be counted for billing purposes, if the process is stable and goes on for one hour:

a) 1 kWh consumed from the grid and 2 kWh fed-in (net count of each phase individually), or

b) 1 kW fed-in (net count across all three phases).

If it’s a) or b) that is counted for billing purposes by a net meter of AGL is the essence of my question.

Many thanks again.

Re: How is feed in and usage calculated by a digital electricity meter?

Super Charger

Hi Jakob,

 

My initial response was in reference to your question about how feed-in and usage being calculated by a DIGITAL ELECTRICITY METER.  As far as I am aware, the meter records energy and if it is a digital one, it may have the ability to record the energy on a frequent basis and communicate that information to the energy retailer for monitoring/billing purposes.  Since the meter's sole purpose is to record the net usage and the direction of the energy flow which is either "delivered" or "received", I believe there is a value for FIT (excess unused solar electricity fed to the grid) and GRID (drawn from the electrical supplier).  The digital meter simply is reconfigured by the energy retailer when someone installs a solar panel system so that it can recognise the energy flow and record the same.

 

If there is some decision being made by a device as to how much to draw from the grid versus how much should be fed to the grid, I'd think that a device would have to be present before the FIT or GRID direction is presented to the digital meter so that the draw or feed would be presented to the digital meter (based on consumption across the phase). 

 

As to your question regarding what is counted for billing purposes, I think AGL would have to answer (wasn't sure if this was a hypothetical question or an actual scenario you have configured and therefore wanting to know and remains unanswered from my original reply). 

 

From a theoretical point of view, using the two values you supplied in your second example, you state the following:

 

1 kWh consumed from the grid

2 kWh fed-in (net count of each phase individually)

 

The meter, I assume, is expecting either a draw in power or a feed-in of power at any one time. On the bill, I would expect to see 0 FIT during the 30 minute interval in which the house was drawing power from the Grid (1 kWh). If Solar was generating 1 kWh and the household required 2 kWh, then 1 kWh is offset by the solar energy and the other 1 kWh is drawn from the grid (net effect).  The assumption being, there is solar energy being produced at the time and that the household consumption exceeds the demand of the solar energy and therefore the inverter had to redirect to the grid to draw power to service the consumption. 

 

If a person presented to me an electric bill where the FIT had a positive value and the GRID had a positive value for the same date/time, I'd be asking what is going on here.  The reason being is that theoretically, there is no way you should have fed anything to the grid at the same time you were drawing from the grid as that would defeat the purpose of getting a properly configured solar panel system (which is to offset the household consumption first before feeding excess energy to the grid).  

 

Incidentally, this actually happened in my scenario because I had a defective Inverter. Once the inverter was replaced, I saw zero values for FIT and a lower GRID consumption.

 

I should probably mention that I have a three-phase electricity system at my property. (i.e. Lights, Large air conditioner, etc.)

 

There's a good article which does explain a bit about three-phase solar inverters here:

https://www.solarquotes.com.au/blog/three-phase-solar-inverters/

 

It says that if a person has a three-phase supply, they can opt for different inverters, but it will not affect how the customer is billed for exports or how much solar energy is self-consumed overall. It should still follow a netting effect as far as a digital meter reading goes.

 

I should probably offer up that I'm not an electrician, engineer, or solar expert nor do I work for any energy retailer.  So it is possible I didn't offer up an answer to your question in the way it should have been answered (apologies if this is the case).

 

 

 

 

 

 

Re: How is feed in and usage calculated by a digital electricity meter?

AGL Community Manager

@Schander , great response, thanks very much for taking the time to go into so much detail. 

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Re: How is feed in and usage calculated by a digital electricity meter?

Semiconductor

Hi Schander, many thanks again for the effort you have taken. I think my question is answered by your last reply. Just to clarify, both examples I gave were theoretical in preparation for getting a solar system for my property. This system would not work out financially when the count for billing purposes was as I summed it up in my reply of this morning under a). But it will pay off when it is counted as I summed it up under b). As my solar system is going to be quite large and the investment therefore will be, too, I wanted to make sure in advance what the way of counting feed-in and consumption for billing purposes will be.

The reason for starting this blog was an article by Finn Peacock: https://www.solarquotes.com.au/blog/3-phase-solar-wars-menace-phantom-imports/

I am not an electrician either and that is why I got concerned when reading the article in regard to the financial benefit from my future system. My reaction to the article was the one you described with: „If a person presented to me an electric bill where the FIT had a positive value and the GRID had a positive value for the same date/time, I'd be asking what is going on here.“

According to the article, this seems to have been the case with a configuration of a three phase supply and one single phase inverter feeding solar into one of the phases (termed „solar phase“ in the article). The key statement in the article is:

„According to my contact, he’s tested some 3 phase meters and has discovered that some are now being configured the latter way: i.e. to calculate exports only on the solar phase, to benefit the electricity company at the expense of the unknowing solar owner.“

Going with the article’s six paragraphs above the quote, this lead to a positive FIT value and a positive GRID value for the same point of time. What I did in my (theoretical) scenarios is that I translated this scenario to a system with a three phase inverter.

So I think the answer to my scenario of this morning depends on how the meter is configured: If it is, in your words, „expecting either a draw in power or a feed-in of power at any one time“ for each phase individually, it is a). If it is „expecting either a draw in power or a feed-in of power at any one time“ across all three phases, it is b).

So it is actually AGL to answer, depending on the configuration of their (individual) meters.

The fair configuration would be, in the words of Finn Peacock: „1) Look at the total amount of energy being consumed on all 3 phases. 2) Look at the solar being generated on the single ‘solar’ phase. Subtract (1) from (2) to calculate the exported energy.“

If that is the configuration with AGL’s meters generally, I would consider it also fair in the scenarios I described. Translated to the scenario of a three phase inverter system it would mean that the meter would count net „across all three phases“ (at any point of time). That would lead to a result for billing as I mentioned it this morning under b).