CT placement for Net Metering

[peppersass]

This question applies to both the GEM and DashBox. It's come up as I've begun planning configuration changes to my power monitoring configuration for a solar tracker, installed but not commissioned yet, and a pair of Tesla Powerwall 2 batteries to be installed for a special pilot program offered by our electric utility.

Reading through many posts here about Net Metering, I found that sometimes the solar company runs the PV system through a separate cutoff/fuse/breaker box and wires it directly to the Mains before they enter the main panel, and sometimes they connect the PV system through a breaker in the main panel. Our PV system runs through a separate cutoff/fuse box, then connects to the Mains through a pre-existing junction box upstream of (before) the main panel. I have a pair of CT-200s coming that I had planned to put on the output lines in the PV cutoff/fuse box. But then I read the installation manual for the PowerWall 2 (click here to download), and found that the batteries will connect through a 30A breaker in the main panel. The diagram on page 41 shows this, and also shows a PV system connected through a main panel breaker. As our main panel is full, my guess is that Tesla won't change how our PV system connects to the Mains, and they might install a separate cutoff/fuse/breaker box for the batteries and connect them where the PV system connects to the Mains before they enter the main panel.

This got me to thinking about how the CT positions might change how GEM and DashBox compute power generation and net metering. If the PV and battery CTs are downstream (after) the Mains CTs, as they are in the Tesla diagram, then the Mains CTs see any power taken from the mains and, if generation exceeds consumption, negative power exported by the PV or batteries. I assume the Net designation causes the GEM and DashBox to use the positive and negative CT readings to compute total power consumed, generated and exported. But if the PV and battery CTs are upstream of the Mains CTs, then the Mains CTs see total power consumed but can't distinguish between mains power, PV power and battery power. Also, the Mains CTs don't see the power generated or consumed by the PV and batteries. I think with the data provided by the PV and battery CTs, the GEM or DashBox can still compute the total power consumed, generated and exported, and can add the power consumed by the PV and batteries to total consumption, but the algorithm would be different, wouldn't it? I haven't seen a configuration option that tells the GEM or DashBox how they CTs are arranged. Is there one, or must the PV and battery CTs always be downstream of the Mains CTs?

If so, then I'll have to move my Mains CTs from the main panel to the junction box upstream of where the PV connects, and there's not a lot of room for that.

A related question is the relationship between Mains and Total Consumption with and without Net Metering. Without Net Metering, they're equal. With Net Metering, does Mains only measure power drawn from the mains, which may be less than total consumption?

It would be great if we could get a more detailed (and illustrated) description of exactly how the GEM and DashBox do Net Metering, and the effects of different placement options for the CTs, if any.

[ben]

Yeah, you wouldn't set your Mains to Net as you won't have any generation on that end. You want your Solar to be set to Net.

I think this is the setup you'll want:

- Mains - Main setting
- Solar - Net Setting
- Net Solar - Inverter
- Create a group with Mains + Net Solar

If you want the header, etc. to show NET consumption instead also set Net Solar to Main.

I think you also may want to same settings for the battery as solar. I would try that out.

[peppersass]

Thanks, Ben.

It's still not clear to me. It would be very helpful to get a detailed description of how you keep track of and use the wattage values for channels designated as Main (with and without the Net designation) as well as for channels designated as Net, Net Inverter, etc.

I read in a post for one of the ECM-12xx products that you have two counters, one for absolute watt-seconds and one for polarized watt-seconds. Is that or something similar true for the GEM? And if so, does "absolute" mean total power flowing in either direction, while "polarized" means the net of positive and negative flow? How are the counters used to compute Total Consumption? Is that always computed from the CTs designated as Mains, and if so which counter is used when a channel is set to Net and which counter if not set to Net? How are the counters used to report power generated, power exported, etc.?

Beyond a better understanding of the algorithms and effects of relative CT placement, here's what I want to know from the system:

- Total power consumed by all devices in the house, regardless of source.
- Power provided by power company
- Power generated by solar
- Power supplied by batteries
- Power exported by solar
- Power exported by batteries
- Power used by the solar inverter/tracker (ideally included in the first item of this list.)
- Power used to charge the batteries (ideally included in the first item of this list.)

Can I get all these with the solar and/or battery CTs upstream of the Mains CTs, or would it be better to move the Mains CTs upstream of the solar and battery CTs?

One complicating factor is that the battery CTs may be fed through a breaker in the main panel. If that's the case I don't believe I can use the settings you recommended because the solar CTs would be upstream of Mains CTs and the battery CTs would be downstream of the Mains. I think that means if I can't get Tesla to install a stand-alone battery breaker connected upstream of my Mains CTs, I'll have no choice but to move my Mains CTs upstream of both the solar and battery CTs, right?

Finally, how is billing affected by all this? I think I read somewhere that you don't deal with Net Metering rates yet and I don't see it in the rate cards. Are you able to keep track of power supplied by the power company and use that to compute total dollar cost for the period? With our battery program we'll be going on Time-Of-Use rates, including for Net Metering, so accurate billing calculations will be even more complicated. Seems to me that device costs would be meaningless without knowing the source of power, right?

[ben]

Thanks, Ben.

It's still not clear to me. It would be very helpful to get a detailed description of how you keep track of and use the wattage values for channels designated as Main (with and without the Net designation) as well as for channels designated as Net, Net Inverter, etc.

I read in a post for one of the ECM-12xx products that you have two counters, one for absolute watt-seconds and one for polarized watt-seconds. Is that or something similar true for the GEM? And if so, does "absolute" mean total power flowing in either direction, while "polarized" means the net of positive and negative flow? How are the counters used to compute Total Consumption? Is that always computed from the CTs designated as Mains, and if so which counter is used when a channel is set to Net and which counter if not set to Net? How are the counters used to report power generated, power exported, etc.?

Yeah, the channels operate the same as CH1/2 on the ECM-1240. Counters work as you guessed, you can get into the math here on how NET is calculated.

http://brultech.com/software/files/down ... at_2_1.pdf

On the DashBox, channels just track absolute power. The NET setting may be what's confusing you. When you enable NET it creates a new channel (at the bottom of the list) that does the NET conversion. That channel is named "<Channel Name> Gen".

Total Consumption is simply the summation of all channels marked as Main. If it's on a NET channel, it will use the NET calcuation. If it's on a normal channel, it won't take NET into affect.

Beyond a better understanding of the algorithms and effects of relative CT placement, here's what I want to know from the system:

- Total power consumed by all devices in the house, regardless of source.
- Power provided by power company
- Power generated by solar
- Power supplied by batteries
- Power exported by solar
- Power exported by batteries
- Power used by the solar inverter/tracker (ideally included in the first item of this list.)
- Power used to charge the batteries (ideally included in the first item of this list.)

Can I get all these with the solar and/or battery CTs upstream of the Mains CTs, or would it be better to move the Mains CTs upstream of the solar and battery CTs?

Leave the CTs at the panel, Total Consumption will be a lot easier to derive that way as it's just the sum of the Main Left/Right.

I'll have to think about this, my head is a bit off as I'm pretty sure I've contracted covid. With some grouping of your Main Left/Right + NET Solar and NET battery you should be able to get most these values.

One complicating factor is that the battery CTs may be fed through a breaker in the main panel. If that's the case I don't believe I can use the settings you recommended because the solar CTs would be upstream of Mains CTs and the battery CTs would be downstream of the Mains. I think that means if I can't get Tesla to install a stand-alone battery breaker connected upstream of my Mains CTs, I'll have no choice but to move my Mains CTs upstream of both the solar and battery CTs, right?

Finally, how is billing affected by all this? I think I read somewhere that you don't deal with Net Metering rates yet and I don't see it in the rate cards. Are you able to keep track of power supplied by the power company and use that to compute total dollar cost for the period? With our battery program we'll be going on Time-Of-Use rates, including for Net Metering, so accurate billing calculations will be even more complicated. Seems to me that device costs would be meaningless without knowing the source of power, right?

I don't think you'd have to move it. We'd just have to figure out the right way of grouping the battery channel together with your Main Left/Right to show the values you'd want.

We have a new beta F/W that allows for a separate net metering rate. I'm not sure where I left off with that (can't check as currently furloughed).

[peppersass]

I'll have to think about this, my head is a bit off as I'm pretty sure I've contracted covid.

Ben, please call your doctor ASAP! Cognitive issue are a reported symptom of COVID-19 and could indicate something that needs attention right away, like a neural infection or blood clots.

The description of absolute and polarized channels in the document was helpful. I didn't realize that polarized channels record power in only one direction, set by polarity, not the sum of polarized values for both directions.

But the document doesn't describe what you call the "NET conversion" or "NET calculation". The big problem here is that my solar isn't commissioned yet and I haven't configured the GEM and DashBox for it so I haven't been able to see the Gen channel(s?) or the numbers that result from the calculations. Once I see how the numbers are reported for different consumption and generation scenarios I'm sure it'll make a lot more sense.

That said, I tried to work out hypothetical results using some assumptions about the algorithms. I can see how not setting Mains to NET will work for the solar CTs being upstream of the Mains CTs, but I don't think Total Consumption is going to come out right with the battery CTs downstream of the Mains CTs. Seems to me that all generator CTs need to be either upstream or downstream of the Mains CTs. I can always create a group of all power consuming devices in the house, but I can't see how billing and budgeting would work correctly.

I won't move the Mains CTs yet. It may be a few weeks or months before the batteries are installed, so when solar is commissioned sometime in the next couple of weeks I'll set it up as NET with the two Mains channels not NET. Hopefully that will improve my understanding of the NET calculations. I'll try to get Tesla to put in an external breaker box so the batteries can be connected upstream of the Mains CTs, like solar, or I'll move the Mains CTs so solar and battery CTs are downstream of them.

[ben]

I'll have to think about this, my head is a bit off as I'm pretty sure I've contracted covid.

Ben, please call your doctor ASAP! Cognitive issue are a reported symptom of COVID-19 and could indicate something that needs attention right away, like a neural infection or blood clots.

I just had a bad headache for a couple days, it's gone now so I'm back to normal, just got a lingering cough now.

The description of absolute and polarized channels in the document was helpful. I didn't realize that polarized channels record power in only one direction, set by polarity, not the sum of polarized values for both directions.

But the document doesn't describe what you call the "NET conversion" or "NET calculation". The big problem here is that my solar isn't commissioned yet and I haven't configured the GEM and DashBox for it so I haven't been able to see the Gen channel(s?) or the numbers that result from the calculations. Once I see how the numbers are reported for different consumption and generation scenarios I'm sure it'll make a lot more sense.

That said, I tried to work out hypothetical results using some assumptions about the algorithms. I can see how not setting Mains to NET will work for the solar CTs being upstream of the Mains CTs, but I don't think Total Consumption is going to come out right with the battery CTs downstream of the Mains CTs. Seems to me that all generator CTs need to be either upstream or downstream of the Mains CTs. I can always create a group of all power consuming devices in the house, but I can't see how billing and budgeting would work correctly.

I won't move the Mains CTs yet. It may be a few weeks or months before the batteries are installed, so when solar is commissioned sometime in the next couple of weeks I'll set it up as NET with the two Mains channels not NET. Hopefully that will improve my understanding of the NET calculations. I'll try to get Tesla to put in an external breaker box so the batteries can be connected upstream of the Mains CTs, like solar, or I'll move the Mains CTs so solar and battery CTs are downstream of them.

The NET calculation only occurs on the channel that's created when NET is set. Go ahead and set a random channel to NET then remove it, you'll see the newly created channel, you can remove it by turning NET off. It won't have any affect on your setup.

The calculation itself is basically 2 * polarized diff - absolute diff.

Will the battery be serving for more then a power backup when power loss occurs (will you be doing load shifting)? Yeah it's probably easier if it's at the upstream. But if it has to be at the panel, it's something we'll have to look at once installed. Adding the power leaving the battery to the main consumption should give total consumption for example, would just need a grouping option like "ignore negatives (charging)" or something.