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Configure Powerwalls To Ignore Solar?

Discussion in 'Tesla Energy' started by wwhitney, Jun 13, 2018.

  1. wwhitney

    wwhitney Member

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    Hello,

    I'm curious whether anyone with Powerwalls installed and a solar system has asked Tesla to install their system as if they had no solar, and if so whether Tesla agreed. This obviously wouldn't work for someone taking the ITC, as that requires charging the Powerwalls only from solar generation. But for anyone not taking the ITC, then depending on one's rate plan, it could be economically beneficial to be able to charge from the grid.

    One issue is whether during island operation (power outage) the Powerwall needs to be able to monitor the solar generation via CTs in order to properly charge from it. I don't think it does, but I'm not sure.

    A longer commentary on my situation is below, for anyone interested.

    Cheers, Wayne


    As my two Powerwalls are scheduled to be installed next month, I started to take a look at the available modes of operation and how to maximize the economic benefit from the Powerwalls. A little background:

    - I'm on PG&E's EV-A schedule, which has Peak, Shoulder (Part Peak in PG&E speak), and Off-Peak rates on weekdays; and just Peak and Off-Peak on the weekend.
    - I have a small solar system (2.5 kw-AC).
    - I'm not currently planning to take the ITC [a close decision; as I'm an SGIP self-developer, it's 30% of the post-SGIP cost versus the extra SGIP hassle (monitoring plans) and the restriction on use (no grid charging).]
    - With solar and storage, the PG&E tariff allows instantaneous discharge from the storage directly to the grid, but it restricts total annual export to the total annual production of the solar system.

    So if it were possible, the most effective way for me to use the Powerwalls would be to time-shift all my consumption to Off-Peak times while simultaneously time-shifting all my solar production to Peak. On the weekend that is a pretty simple algorithm: during Off-Peak, charge to avoid exporting energy and to reach 100% at the beginning of Peak; during Peak, discharge to cover usage plus that day's full Off-Peak solar generation. Weekdays would be more complicated due to the Shoulder period, but a similar algorithm would work by forecasting the net generation expected during the Shoulder periods, and not fully charging during Off-Peak to allow absorbing the Shoulder period net generation.

    Unfortunately Tesla doesn't offer this operational mode or any time-shifting of solar production. So it seems the best I can do is to time-shift all of my usage to Off-Peak. When grid charging is not prohibited the Powerwall operational mode that does this best is "No Solar TBC". That would allow all of my Peak and Shoulder generation to go directly to the grid.
     
  2. NuShrike

    NuShrike Member

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    I think you're one of the very few that is installing standalone storage, instead of paired, even when there's solar.

    I think you would have to run "custom" TBC software (as somebody in the TBC thread has), in order to possibly get an operational mode that matches your use-case.

    I'll be approaching this too, whenever my installer ever gets any stock from Tesla.
     
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  3. ewoodrick

    ewoodrick Member

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    There's a lot to be talked about here.

    "Tesla doesn't allow time shifting of solar production" - No matter how powerful people believe Elon may be, he doesn't completely control mother nature. Solar production is controlled by the sun and blocked by the clouds. So the only choice that you really have is that you want to do everything possible to not sale back electricity.

    So while it makes sense that the conclusion is that you want the batteries to drain at night and fill during the day, there are some realities that exist. What are the loads during the day (and these change during seasons)? With a small solar system, are you actually able to maintain your loads during the day? What about the A/C, refrigerator, everything else, what are they using during the day. You can generally only shed so much power during the day.

    Let's say that you can control the load and charging, what would you want to do?
    1. You want to maintain the battery for good life. That suggest you are maybe only charging between 20% and 80%.
    2. You want all solar production to go to charging the battery. In reality this mean all solar production in excess of daytime load
    So it sounds as if you need to figure out how much solar energy you are creating above your daily load. Once you know that, then you will want to drain the battery this far each evening. Just to make things fun, this number tends to vary weekday vs weekend and per season.

    I guess then you would want to make sure that the battery drains this much on the associated day. That's sounding a little complex.

    But all of the makes the assumption that your solar can produce more than your daytime load. If it can't then the solution is a lot easier.

    And then when you look at your electricity cost, it gets more confusing. The answer tends to be shed load during peak periods. Luckily, the solar is probably producing during this period, but the worse is that's when you are wanting and needing to use the most energy.

    So I guess the answer is simple, discharge the battery during the evening, but make sure that it is charged by peak period. If it is cloudy, use the night before to charge. If it is sunny, use solar to charge. The amount that I want to drain is dependent on the temperature that it is going to be and whether anyone is going to be home. I want to make sure that I never charge the battery during peak, so if it hits minimum, switch to the really expensive power. But don't start charging until the morning, unless the morning is cloudy, in which case I need to charge earlier.

    By the way, the power companies have mainframes and years of data and large staffs of people that do this.
     
  4. wwhitney

    wwhitney Member

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    A battery with the proper controls can absolutely be used to time shift solar production from the point of view of the grid, just as it can be used to time shift consumption. Simply charge the battery "from solar" during one period of solar generation, and then force the battery to discharge during another time period.

    However, that is not an operational mode that Tesla supports, I'm not aware of any way to get the Powerwall to discharge more than the current usage at any point in time. (Other than tricking the system about your current usage by, e.g., routing a conductor through a CT twice so that it reads as double the current.)

    Cheers, Wayne
     
  5. wwhitney

    wwhitney Member

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    Quick update: I had my walk through this week, for a July 9 install. It absolutely is possible to configure the Powerwall to ignore the solar. The installers pointed out that to get the total consumption right, it will still be necessary to put a CT on the solar production, and to configure it as an additional (negative) load in the Wizard.

    I didn't get an answer as to the behavior of the Powerwall during a grid outage while unmetered solar is producing, but my impression is that it will just work. I'm also waiting to find out if there is going to be some bureaucratic obstacle to doing the install this way. But for anyone not taking the ITC and with NEM, it is definitely an economic win to be able to charge the Powerwall from the grid.

    Cheers, Wayne
     
  6. power.saver

    power.saver Member

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    Wayne,

    Are you going to connect your solar behind the gateway so that it continues to operate during outages, but configure the PW so it doesn't charge from solar normally? During an outage though, the PW would charge from your solar if your house load is less than the solar production, correct?
     
  7. wwhitney

    wwhitney Member

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    That is the goal, if Tesla cooperates. Since I'm not taking the ITC, there's no requirement to only charge from solar, so it is more economically efficient to charge from off-peak grid.

    That is a remaining question. I think so, but I'm not sure.

    Cheers, Wayne
     
  8. miimura

    miimura Active Member

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    If this is what you're going to do, you need to make sure your PG&E Interconnect agreement is filled out accordingly. I would think by default, Tesla will indicate the batteries are charged from solar if you have it. I believe there is not only export criteria, but also charging criteria in the interconnect application. However, I've not seen the application since mine was done for me by my installer.
     
  9. wwhitney

    wwhitney Member

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    I did my own interconnect application, so I got that covered. : - ) Doing the interconnect application was part of convincing the SGIP to allow me to be my own developer.

    Cheers, Wayne
     
  10. miimura

    miimura Active Member

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    So, was I right that you need to check a different box for grid charging than solar charging?
     
  11. power.saver

    power.saver Member

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    I would think you want that, so an extended outage replenishes the PW.

    I think the challenge will be to configure the PW to know they should charge during grid failure from solar, but only charge off-peak during normal operation (assuming that's your plan).
     
  12. wwhitney

    wwhitney Member

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    I had to answer some question about how the Energy Storage System was going to be charged, and at what rate. So it was a bit more than checking a box.

    Cheers, Wayne
     
  13. wwhitney

    wwhitney Member

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    So here's my understanding:

    When operating in grid-tie mode, the Powerwalls make an independent decision about whether to charge or discharge. So the brains (in the Gateway?) need to have various inputs for this decision, namely the various CTs. If the Powerwall is set up to only charge from solar, then it just sets its charge rate to be limited to what the solar CT is seeing. If the Powerwall is trying to handle Peak period consumption, then it sets its discharge rate to what the consumption CTs are seeing.

    Now I think, but I'm not sure, that none of this applies when the Powerwalls are operating in island mode, and the CT inputs are not required. I think the island mode inverter topology just automatically responds to changing load/supply and either sources or sinks the requisite current to maintain a stable reference voltage.

    So if that's correct, then there's no special configuration required.

    Cheers, Wayne
     
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  14. miimura

    miimura Active Member

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    That begs the question: What rate will it charge at and will you have any control over the value?
    When the Powerwalls are in island mode (Backup) they must be regulating frequency and voltage for L1 and L2 simultaneously, so it cannot depend on CT values.

    Later this summer I will be testing injecting 120V power with a small grid-tie inverter. I will try to monitor power on each leg coming out of the Powerwalls so I can see the imbalance on each side of the neutral. It will be interesting to see how they handle load on one side and charge on the other. PW1 systems use an external autotransformer to handle this because the SolarEdge is a pure 240V inverter.
     
  15. wwhitney

    wwhitney Member

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    Hmm, I'm getting some pushback on configuring my Powerwalls to ignore the solar. As far as I can tell it's technically not had, all I'm hearing is that it is against Tesla's policy. I recall someone else said they were going to do it, but I believe they were using a third party installer, who might be more flexible.

    Well, we'll see how this plays. I obviously could reconfigure the system by running the Wizard after it has been installed. I don't want to get in a tug-of-war with Tesla, though, since they apparently have the ability to change the configuration remotely. I'd like to discuss the policy with a decision maker at Tesla, but I find Tesla is very reluctant to let you talk to anyone other than the front line people.

    Cheers, Wayne
     
  16. NuShrike

    NuShrike Member

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    You could also unplug the solar CTs afterwards, as long as you're not limited by any contract.

    As you've said, this is a configuration supported by SGIP, and not an alien configuration for TE for normal standalone installations.

    I do wonder in island-mode how does "standalone-mode" PW2s handle solar pushing into the micro-grid. Will the PWs push the AC waveform out of spec to get the "unknown generator" to back-off, or this is an untested/supported configuration after-all.
     
  17. wwhitney

    wwhitney Member

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    Update: yesterday I had my equipment installed by Tesla, yay!

    I also had a conference call with the installer's boss, a technical manager, and some people from Tesla's utility interconnection department. The basic answer from Tesla on my request was "this is not a supported configuration and we will not configure it as you suggest." The only external concern cited that rang true at all to me was whether PG&E would OK my proposed configuration. But we all agreed that if PG&E finalizes my interconnection request showing grid charging that would not be an issue.

    So it boils down to Tesla wanting to have a pretty tight control on how their Powerwalls are used and configured, and wanting to limit the supported set of configurations. As the product is clearly still under development, I can sympathize somewhat. Of course, as a knowledgeable user, it is a frustrating position, particularly since I view my proposed configuration as within the envelope of what they already support.

    The upshot is that I should perhaps reconsider taking the ITC, since the path of least resistance involves complying with its restrictions anyway. There are other workarounds available to me, but they are all probably sufficiently complicated that I won't be interested in pursuing them. [(i) reconfigure via the wizard and hope Tesla doesn't change the configuration back; if they do, disconnect the Powerwalls from the Internet; (ii) manipulate the behavior of the Powerwalls by manipulating the info gathered by the CTs.]

    Cheers, Wayne
     
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  18. wwhitney

    wwhitney Member

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    PS The technical manager agreed, with some hesitation, that in island mode the Powerwall inverter's basic ability to charge the battery does not depend on anything at the software level (the wizard configuration or the CT inputs). In the unlikely event I purse reconfiguring the Powerwalls, I would test this out first.

    Cheers, Wayne
     
  19. MorrisonHiker

    MorrisonHiker S 90D 2018.32.4

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    Thanks for the update. I believe they offer grid charging for solar customers with Powerwalls in Australia so we know it is possible. They just don't want to deal with the ITC and other credits here...even when some people don't want or can't get the ITC. <sigh>

    Maybe someday it will change. We won't be getting the ITC on our (referral) Powerwalls but would like to be able to charge from the grid before a predicted blizzard, etc.

    We'll probably be tweeting Elon once we get ours installed to ask when the US will have similar functionality as Australia.
     
    • Informative x 1
  20. miimura

    miimura Active Member

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    If you were desperate before an impending storm, you could move your solar CTs to your EVSE circuit and make it look like charging your car was generation and trick it into charging. However, you would have to make sure it didn't register on the Grid CTs.
     

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