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Help me understand my Powerwall 2 & Solar install (want to add more solar)

Discussion in 'Tesla Energy' started by Sivart321, Jul 29, 2018.

  1. Sivart321

    Sivart321 Member

    Joined:
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    Location:
    San Jose, CA
    Tesla just finished installing a single Powerwall at my house which has an existing 4.3 kW solar system. I intended to be able to backup all loads, but Tesla managed to screw up my SGIP application and overall order and I ended up with only one Powerwall and two load centers and a split system (only the <30 A circuits are backed up on a different load center). A new main panel was installed where the meter and all of the > 30A circuits live, which feeds to a sub-panel with all of my backed-up loads, the powerwall, gateway, relays, etc. (Partial home backup).

    My questions revolve around a desire to increase my solar production in the hopes of further decreasing my usage from PG&E. I also plan to get 1 or more EVs in the near future which would increase my consumption even further and give me more reason to upgrade the solar.

    Questions:
    How do I add solar capability to this system? Where does it feed? I understand there is some limit to the size of the solar system feeding the powerwall? I plan to install another 7 kW of panels or so.

    What visibility does the powerwall/gateway have to loads on the main panel? Is the app showing me power consumption of the whole house? How does it know what is being consumed outside of its isolated sub-panel, etc?

    I know a bit about this install, but some of it is still a black box to me. Anything to help me better understand what I am working with would help my sanity and education, but also help me when working with the new solar installer (who is not Tesla/Solar city due to fast quoting, pricing, etc).

    Install photos are below.

    Main Meter Panel

    https://www.dropbox.com/s/7du0xajr56u0e1c/Powerwall%20Install%201.jpg?raw=1

    Backup Load Center, Gateway, Relay, etc.

    https://www.dropbox.com/s/f2nd4v3pbv7v2tm/Powerwall%20Install%202.jpg?raw=1
     
  2. Sylvia Else

    Sylvia Else Member

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    The system will include current transfomers clamped on to the conductors coming from the grid. Given that the system knows the voltage, it can use the current transformers to calculate the power.
     
  3. arnolddeleon

    arnolddeleon Supporting Member

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    I would add the additional solar in the generation panel (where you current solar and Powerwall are).

    The problem with excess solar occurs when the net PV (after any loads that are running) exceed what the battery can absorb. The worst case scenario is your batteries are not full but there is too much solar and they can't charge because the system shutdowns down. There is a simple work around if that occurs while you are home, turn off the additional solar.

    If you are lucky the additional (and maybe existing) solar can throttle down instead of shutting down when the battery needs to curtail PV production. The Powerwall will start moving the frequency to signal the PV down slow down. If the PV doesn't slow the frequency will move out of spec and the PV will shutdown.
     
  4. Sivart321

    Sivart321 Member

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    Got it. That is what I assumed was the case. Thanks for confirming!

    I want to feed the grid the excess power and get paid for that. I am on a Net Energy Metering plan with PG&E. My goal is to generate as much as I consume. I currently have my PowerWall setup as Time Based Control for cost savings, plugging in my off-peak and peak Time of Use schedule to get optimal cost savings. I never want my PV system to shut down as that sounds like wasted money? I don't care about being off-grid.

    What are the alternatives?
     
  5. miimura

    miimura Active Member

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    In California, Tesla Powerwall systems always include a Backup Gateway. This is what you described in the first post. This means that the system can continue to power loads on the "Generation Panel" when the grid is down. A single Powerwall can only charge and discharge 5kW continuously. Normally, Tesla does not want to put more than 6 or 7 kW of solar per Powerwall because you can get into that weird situation Arnold described when the grid is down and the batteries need charging but they can't because the solar is overpowering the Powerwall. The best solution is to add another Powerwall with the new solar. However, if you don't care that much about the backup situation, then you can have the solar installer move the existing solar outside the Backup Gateway so that it will just shut down whenever the grid is down and the new and larger solar can keep you running during a grid outage. When the grid is up, everything will work together and it doesn't matter whether loads and solar are inside or outside the Gateway.

    One more thing. You don't need to generate as many kWh as you use to zero out your utility bill - especially with a Powerwall. So, I would carefully consider how many miles you plan to drive electric in the future and ask the solar installer to estimate how much solar you need to zero out your bill on the EV rate. Adding more solar than that will not have much if any financial benefit. It will only give you a warm green feeling that you've generated as much as you've used. You know, carbon footprint and all.... If you really care about carbon footprint, then you should go for the big solar and convert all your gas appliances to electric the next time they need to be replaced. Clothes dryers and hybrid electric water heaters are particularly good candidates. Central heating, not so much.
     
  6. Sivart321

    Sivart321 Member

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    Thanks. That helps a lot.

    I understand the calculations about when I am using power and the prices I am getting when I am feeding power back to the grid with NEM. I am only sizing the system to match those dollar amounts and not necessarily the kWh's consumed vs generated. My simplified explanation wasn't helpful there. Sorry for that. I am also making some estimated guesses about my commute given a Model 3's watt-hour per mile usage, etc.

    I also know I don't want a surplus of power as the PG&E surplus compensation rate is something crazy low like $.03-.04 per kWh. I am currently getting $0.37 per kWh credited from PG&E with the Powerwall during peak Tier 1 usage and $0.45 per kWh when I am dipping in to Tier 2. Ideally that Tier 2 usage pro/con is gone with the extra solar.

    It sounds like my best option is as you suggested and to ask them to move the old system to the main panel and the new system to the backup load center/generation panel. I can't imagine I could get another Powerwall 2 in any reasonable time frame, and it sounds like the SGIP rebates are all consumed as well? I am looking to add the extra solar in the next 4-6 weeks based on the scheduling of this solar installer.
     
  7. arnolddeleon

    arnolddeleon Supporting Member

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    The shutdown conditions I was talking about only happen when there is a power outage. When the grid is up you effectively have an infinite load available to you, so the solar always has a place to go. So don't worry about your ability to sell excess production being limited.

    If you are not concerned about the backup configuration then it doesn't much matter where you put the additional solar. I still recommend just installing it on the generation panel. If it's installed in the current generation panel it would make the CT wiring probably easier. Done properly you won't even need Tesla to add more CTs. The installer just need to loop the existing CTs. This also assumes the generation panel is sized big enough.

    I think I missed what tariff you are on.

    @miimura was touching on it, make sure you know what your motivations are, saving money, generating as much power to offset your usage (green), you just like technology. It's going to usually be a mix but the weight of each one will affect the choices you make.
     
  8. Sivart321

    Sivart321 Member

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    Got it. All of your help is appreciated. It's starting to make considerably more sense now.

    I am not super concerned with the backup configuration. My critical loads are limited to the backup load center and the stuff on there isn't excessive and will likely last through a typical (and rare) power outage in Campbell where I live. Anything longer than that I don't care too much about.

    How can I tell if the generation panel is sized big enough? I think my photo shows that in the top right? It has a 100 amp "main" breaker.
     
  9. Sivart321

    Sivart321 Member

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    I gave the direction to the solar installer to simply add the new system to the existing generation panel. What should I keep an eye out for when doing this? I will be adding another 7.2 kW to an existing 4.3 kW system.
     
  10. mongo

    mongo Well-Known Member

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    There are likely current clamps on your existing solar feeds. To allow the system to measure total solar, the installer will need to route the new feeds though the appropriate sense transformers (same phase same direction).
     

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