Some questions about how Powerwalls, Gateway, Solar and CTs work together

[njmurvin]

I have a pending installation of 2 PWs, gateway and critical loads panel with existing 10kw solar that is stalled due to some vendor conflicts. Without getting into details (in another thread), I have some questions about how Powerwalls, gateway, solar and CTs all work together. A better understanding of this might help me move forward with my project.

1) I live in SCE territory. My primary objective is to, on a daily basis, charge the PWs during non-peak hours when the sun is shining and use the batteries during peak hours when the rates are highest. I know this is a common objective. Are there any caveats? Will I be able to charge in the non-peak hours even on cloudy/rainy days (i.e. from the grid)?

2) My solar currently lands in a sub panel far away from the main panel, gateway and other equipment. Due to vendor issues, I am considering leaving the solar where it is and making the sub panel a critical load from the gateway as a means to get solar output to the gateway. If I do this, is it imperative that I have a CT measuring just the solar output and separate CTs on the loads or can have a single (at the gateway) CT which I'm assuming would measure the net of the solar against the loads in the sub panel? I don't care about seeing the actual solar generation data in the Tesla app. I have that in my Enphase app.

3) The main panel has a single 400amp input. If I execute the plan in (2) above, the main panel will have only the non-backed-up loads and a breaker feeding the gateway. Do I need CTs on all of the non-backed-up loads or do they matter?

4) Does the Gateway enforce the charging policies (rules for when to charge, how much, etc.)? Or is that done at the PWs themselves? Or, something else? What parameters determine when batteries get charged or not and how are those affected by the CT readings?

Sorry if these questions seem too basic. I'm trying to resolve an installation issue and need to understand the possible ramifications of any proposals I make.

 

[aesculus]

I have a pending installation of 2 PWs, gateway and critical loads panel with existing 10kw solar that is stalled due to some vendor conflicts. Without getting into details (in another thread), I have some questions about how Powerwalls, gateway, solar and CTs all work together. A better understanding of this might help me move forward with my project.

1) I live in SCE territory. My primary objective is to, on a daily basis, charge the PWs during non-peak hours when the sun is shining and use the batteries during peak hours when the rates are highest. I know this is a common objective. Are there any caveats? Will I be able to charge in the non-peak hours even on cloudy/rainy days (i.e. from the grid)?

I am not in SCE territory (PG&E) but this is the common goal for the Cost Based setting in the App. Works pretty well but significant weather changes can screw it up. Probably not common in SCE land.

2) My solar currently lands in a sub panel far away from the main panel, gateway and other equipment. Due to vendor issues, I am considering leaving the solar where it is and making the sub panel a critical load from the gateway as a means to get solar output to the gateway. If I do this, is it imperative that I have a CT measuring just the solar output and separate CTs on the loads or can have a single (at the gateway) CT which I'm assuming would measure the net of the solar against the loads in the sub panel? I don't care about seeing the actual solar generation data in the Tesla app. I have that in my Enphase app.

I have the same. But I have very little constant load on that panel, except at night. Tesla put two sets of CTs on that circuit. One feeds the backed up loads input and the other the solar input (this is GW1). So in the solar producing time when I have next to 0 load, it very accurately measures the solar production. At night, when there is no solar production it deals with the load (mostly lighting) well. The algorithm in the GW (or ??? see below) seems to be smart enough to through away negative solar etc. I was worried that was going to be an issue and it was, before Tesla doubled up on the CTs for that panel.

3) The main panel has a single 400amp input. If I execute the plan in (2) above, the main panel will have only the non-backed-up loads and a breaker feeding the gateway. Do I need CTs on all of the non-backed-up loads or do they matter?

I only have one non backed up breaker in my main panel so they just put my CTs there. It may be a challenge to find CTs that can measure the entire panel accurately. There is another thread on this where a user got the wrong scale of CTs so their grid loads were inaccurate.

4) Does the Gateway enforce the charging policies (rules for when to charge, how much, etc.)? Or is that done at the PWs themselves? Or, something else? What parameters determine when batteries get charged or not and how are those affected by the CT readings?

I think its a combination of the GW and back end SW at Tesla. You tweak some of this with your backup reserve and your pricing model. Many of us have different Sell/Buy values to offset NBCs and/or to force certain behaviors. Tesla monitors your usage/production and also maybe? looks at projected weather, to figure out the best (most cost effective in Cost Mode) operations. Not perfect but it's pretty good as long as everything is stable (production, usage, weather etc.).

 

[jjrandorin]

Are there any caveats? Will I be able to charge in the non-peak hours even on cloudy/rainy days (i.e. from the grid)?

You may or may not have grid charging enabled. This was previously blocked by Tesla in the US (grid charging) and has been enabled for many, but not all. I personally wouldnt "plan" on grid charging being enabled, as I dont know what circumstances dictate whether they turn it on for someone or not.

FWIW, I have grid charging enabled on my system, which is 2 powerwalls installed in Jan 2020 by Tesla, in SCE territory. Whether it would be enabled for you or not I dont know, and its not something I would plan on.

I dont have any other feedback for you on anything else myself though.

 

[spsheridan]

I’m in a similar situation with 8 kW of Solar (not Tesla solar roof but Panasonic panels with Enphase inverters), two Powerwalls, a Gateway, CTs supplied by Sense, and I have SCE as my provider. I also had existing solar and later had the PWs installed. However, our main panel only has a 200A input and I don’t have a sub-panel for our solar. I had similar questions as you just before I had our PWs installed.

1) My PW charging and grid usage is similar to your objective. Solar charges the PWs during the day and the PWs provide all home power at least during peak hours (4-9 pm for me) and usually for other times of the day. PW, grid, and solar power control are very well integrated into the Tesla app with lots of parameters to control.

In our home, we generally consume about 25 kWh a day, barring EV charging or heavy HVAC (heat pump) usage. We chose two PWs with 27 kWh of total storage to roughly align with one day of power draw from our home. Also, with our 8 kW solar system, get about 20-25 kWh of charge during a sunny mid-winter day and about 50-55 kWh of charge during a sunny mid-summer day. So we can usually be self-powered for a day unless we charge EVs or use a lot of HVAC. If it’s a completely overcast day, particularly in the winter, we will not generate enough to power our home for a day. But partially sunny/cloudy non-winter days generally give us sufficient power to fully charge the PWs to provide power for a day’s usage.

Previously in our location with SCE, grid charging of the PWs was disabled. However, I recently noticed the Tesla app now allows me to charge the PWs from the grid. You get a warning regarding tax implications about grid charging when it's enabled. I have not enabled grid charging. With the Storm Watch feature enabled in the Tesla app, the PWs will charge up to 100% in preparation for a predicted outage if they are not fully charged. If there is insufficient solar to charge the PWs, then they will charge from the grid which I have enabled.



2) My setup is a bit different so I can’t help you much here. I would consult with the PW installer to get the monitoring you desire. I also have the Enphase app but find the Tesla app more useful because it integrates solar, PW and grid power, and house load all in one location with good graphics. The Enphase app will only monitor your solar generation, although it gives you more detail about solar (e.g. individual panel performance).



3) If you don’t have CTs on the non-backed-up circuits, you won’t see power drawn through those circuits.

4) The Gateway controls the charge/discharge policies and all user control is through the Tesla app. You set all PW charging and usage parameters in the app. For example, you can set the system to be Self-Powered (minimizes reliance on the grid) or Time-Based Control (minimizes energy bill). In either condition, you define how low you allow the PWs to be drained before switching over to grid power. It is a very flexible system with sufficient user control to achieve most people’s charging policies.



Let me know if you have any questions.

 

[njmurvin]

Thanks for the great replies. Very helpful.

My sub panel where the solar lands is approximately 100 ft away from the gateway. So, I would either have to run a long cable or use Neurio to transmit the CT readings to the gateway - if that's even an option. The solar enters the panel on a 50amp breaker that looks like any other breaker (except for its size). In order to monitor the other circuits, don't I need to install a CT on each circuit? If multiple CTs like this, is there just one cable back to the gateway or do I need a cable for each of the CTs?

 

[spsheridan]

Thanks for the great replies. Very helpful.

My sub panel where the solar lands is approximately 100 ft away from the gateway. So, I would either have to run a long cable or use Neurio to transmit the CT readings to the gateway - if that's even an option. The solar enters the panel on a 50amp breaker that looks like any other breaker (except for its size). In order to monitor the other circuits, don't I need to install a CT on each circuit? If multiple CTs like this, is there just one cable back to the gateway or do I need a cable for each of the CTs?

I’m not sure how many CTs the gateway can handle. The app implies it can only measure power at four locations: grid, solar, battery, and home. I don’t know if the gateway hardware can handle multiple CTs to measure separate home circuits.

 

[aesculus]

I’m not sure how many CTs the gateway can handle. The app implies it can only measure power at four locations: grid, solar, battery, and home. I don’t know if the gateway hardware can handle multiple CTs to measure separate home circuits.

In my case Tesla added two splitter/joiner to combine two sets of CTs on the same inputs (solar and home load). I suspect they are just additive. They needed to do this because the circuits were in separate panels.

 

[njmurvin]

It's looking again like they might run the solar across the roof to land in the gateway after all. That way I can leave the sub panel running off the main panel (not backed up) and only have to be concerned about monitoring main panel loads with CTs. I assume (or hope) the gateway knows the backup loads and solar input if they connect directly to the gateway.,

 

[sperkin]

If you are approved for NEM2.0 then it doesn't matter when you pull from the gird. It's 1:1 so timing your PW is not needed. Think as if SCE is your giant battery. This is why they want to move away from 2.0 to 3.0.

 

[sunwarriors]

If you are approved for NEM2.0 then it doesn't matter when you pull from the gird. It's 1:1 so timing your PW is not needed. Think as if SCE is your giant battery. This is why they want to move away from 2.0 to 3.0.

That's only true for NEM1.0. NEM2.0 has TOU (Time of Use) so if you pull from say 4pm - 9pm, you could be paying the highest rates ($0.80 in SD) while it could be as low as $0.16 if you're on some of the EV plans.

One needs to be strategic when to pull from the grid on NEM2.0, usually, that means no grid use from 4-9pm.

 

[sperkin]

That's only true for NEM1.0. NEM2.0 has TOU (Time of Use) so if you pull from say 4pm - 9pm, you could be paying the highest rates ($0.80 in SD) while it could be as low as $0.16 if you're on some of the EV plans.

One needs to be strategic when to pull from the grid on NEM2.0, usually, that means no grid use from 4-9pm.

You are right.

I just looked at my friend's bill again. He is saving $100 a month on solar but they do charge him based on TOU for generating and delivering his kWhs. I was confused as his bill was only $9 with a true up of -$660. It was because due to legal issues on SCE, they had to credit him $2,500, so he had that to work with for many months. His true bill woukd be around $95/month if he didn't have those credits. If he didnt have solar it would be around $195/month.

Took awhile to understand his messy bill.

 

[jgleigh]

I’m not sure how many CTs the gateway can handle. The app implies it can only measure power at four locations: grid, solar, battery, and home. I don’t know if the gateway hardware can handle multiple CTs to measure separate home circuits.

There might be a limit, but you can definitely have multiple CTs. I have two site CTs due to having a partial backup setup. The main CTs in the Gateway monitor the backed up loads and I have remote wireless CTs for the non-backed up loads. There are also CTs on both inverters.

Solar is usually monitored by a single CT on each inverter. Grid is monitored either by the Gateway or at the meter with a pair of CTs (or both depending on panel configuration). Battery is reported directly from the Powerwall back to the Gateway. Home isn't measured directly. It's calculated based on all the other readings.

 

[spsheridan]

There might be a limit, but you can definitely have multiple CTs. I have two site CTs due to having a partial backup setup. The main CTs in the Gateway monitor the backed up loads and I have remote wireless CTs for the non-backed up loads. There are also CTs on both inverters.

With multiple CTs for house loads, does the app explicitly show the power draw as measured by each house-load CT separately?

 

[aesculus]

With multiple CTs for house loads, does the app explicitly show the power draw as measured by each house-load CT separately?

No. They are combined into the 4 categories as mentioned in post #6

 

[jgleigh]

With multiple CTs for house loads, does the app explicitly show the power draw as measured by each house-load CT separately?

You can see the separate readings in the API, but the app combines them into the four screens.

 

[spsheridan]

You can see the separate readings in the API, but the app combines them into the four screens.

So to see the separate readings, do you need a third-party app to read the multiple CT measurements through the API?

 

[jgleigh]

So to see the separate readings, do you need a third-party app to read the multiple CT measurements through the API?

Login to the gateway on your local network. Then you can access the api directly through a browser.

<local ip address>/api/meters/site

Will give you the various CTs in the system.

 

[njmurvin]

My install is complete (according to my battery installer) and I'm waiting for PTO from SCE. However, I still do not have CTs on any of my non-backed up circuits. 3 of my 4 locations (grid, battery, and solar) appear to be accurate. Home only includes the total of the backed up loads.

I can't assume I'll get much cooperation from my installer on this. They're adamant that the project is finished. What's the best way to get Home to include my non-backed up loads?

 

[miimura]

My install is complete (according to my battery installer) and I'm waiting for PTO from SCE. However, I still do not have CTs on any of my non-backed up circuits. 3 of my 4 locations (grid, battery, and solar) appear to be accurate. Home only includes the total of the backed up loads.

I can't assume I'll get much cooperation from my installer on this. They're adamant that the project is finished. What's the best way to get Home to include my non-backed up loads?

The best way is to get the installer to place the Grid CTs so they measure the total grid draw near the main breaker. My installer did this for me, but that was with Gateway 1 in 2018. They are simply taking the easy way out and using the Grid CT built into the the Gateway 2.