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  • powerwall2 behavior when paired with solar

    i've been told on another forum that if your powerwall2 is configured to interoperate with solar, then it is impossible for the PW2 to charge from the grid. not sure if this is a software restriction or somehow physically impossible.

    for any PW2+solar owners, is this true? the tesla website implies that it is not, but websites do lie...

  • #2
    Originally posted by astroboy View Post
    i've been told on another forum that if your powerwall2 is configured to interoperate with solar, then it is impossible for the PW2 to charge from the grid. not sure if this is a software restriction or somehow physically impossible.

    for any PW2+solar owners, is this true? the tesla website implies that it is not, but websites do lie...
    I believe there is an OPTION to only have powerwall 2 charge from grid, but this was just an option. The reason this option is needed is that IF your battery charges 100% from solar, then you can get the 30% federal tax credit. If it charges from the grid, then no federal solar tax credit.
    Last edited by blueman2; 10-14-2019, 05:35 PM.

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    • #3
      understood - what i was told is that if an installer is pairing the powerwall with solar they will set this option at install time and the user cannot undo it. that's what i'm trying to suss out.

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      • #4
        Originally posted by astroboy View Post
        understood - what i was told is that if an installer is pairing the powerwall with solar they will set this option at install time and the user cannot undo it. that's what i'm trying to suss out.
        My understanding is that the option to charge from the grid is not user configurable. The installer sets it based on whether the installation has solar, because with no solar the only option would be to charge from the grid. All my information comes from the Tesla Energy sub topic on the teslamotorsclub.com forum. You may find that a useful resource for information about Tesla Powerwalls since it is a user forum like this but tends to have mostly Powerwall and grid tie solar users. It is completely independent of Tesla Motors and some of the members who I trust I have known from some other forums. No doubt there are a lot of Tesla Fanboys on that site.

        The reason for this limited option is to insure that users of a PowerWall will get the Federal Tax Credit. If the user gets audited I am sure they can point to that configuration to prove that the Powerwall could only be charged from the solar. In my case, with a hybrid inverter I took the credit but I have kept logs to prove that the batteries are charged from the grid. I had a reservation on a Powerwall but let it go because I thought I could get more flexible load shifting. I also wanted the ability to incrementally add more battery capacity at less cost than buying another Powerwall in the future.
        Last edited by Ampster; 10-14-2019, 07:37 PM.
        9 kW solar, 42kWh LFP storage. EV owner since 2012

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        • #5
          Originally posted by astroboy View Post
          understood - what i was told is that if an installer is pairing the powerwall with solar they will set this option at install time and the user cannot undo it. that's what i'm trying to suss out.
          I remember reading somewhere that it could be changed after installation but you need to call Tesla to do it.

          Another more serious limitation to me is that when Powerwall is configured to charge from solar it also can't be charged from a generator.
          Last edited by solardreamer; 10-14-2019, 09:49 PM.

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          • #6
            Originally posted by solardreamer View Post

            I remember reading somewhere that it could be changed after installation but you need to call Tesla to do it.

            Another more serious limitation to me is that when Powerwall is configured to charge from solar it also can't be charged from a generator.

            right - i was looking at SMA's "island" inverters and at least according to their marketing material those inverters can handle a variety of generation types.

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            • #7
              Originally posted by solardreamer View Post

              I remember reading somewhere that it could be changed after installation but you need to call Tesla to do it.
              I have spent four years on the Tesla Forum and to date I have not heard of anyone who has been successful in having their PowerWall charge from the grid if they have solar. There is an exception, totally in the control of Tesla called Storm Watch. If they detect a future condition they can do an over the air exception for those areas that are affected and let them charge from the grid. That actually happened in California last week when PG&E and SCE announced they were going to turn of power to certain areas due to high winds that threatened to cause fires. That allowed those people to charge from the grid shortly before the announced power outages so they could enter the blackout with fully charged batteries.
              Another more serious limitation to me is that when Powerwall is configured to charge from solar it also can't be charged from a generator.
              Are there any circumstances that a Powerwall can be charged from a generator? To date I have not heard of any.
              Last edited by Ampster; 10-15-2019, 10:46 AM.
              9 kW solar, 42kWh LFP storage. EV owner since 2012

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              • #8
                yeah those shutoffs are why i've been thinking about this again. really, for me, the purpose of powerwalls would be to have backup power, as only during the summer months do i overproduce power. but i realized that since i'll be on EV-A for another year, load shifting would offset the cost of the powerwalls quite a bit. however, it's sounding like this is impossible if there is solar on the premises.

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                • #9
                  Originally posted by astroboy View Post
                  ........... but i realized that since i'll be on EV-A for another year, load shifting would offset the cost of the powerwalls quite a bit. however, it's sounding like this is impossible if there is solar on the premises.
                  PowerWalls have a mode called Time Based Control which is primarily a load shifting mode. They charge from the sun in the morning. Then they power the critical loads with solar duing the early peak and then use the battery to power the critical loads after the sun goes down. Is that what you would like to do?

                  With Self Generation Incentive Program incentives the payback in California can be fairly attractive. That would explain why reportedly over 50,000 have been sold. Others are clearly eyeing that market. Even Generac has a battery product to do that. That way they can offer a solution that can leverage existing grid tied solar and offer extended capability during a blackout.

                  I am also on EV-A and NEM 2. Are you on NEM 2?
                  9 kW solar, 42kWh LFP storage. EV owner since 2012

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                  • #10
                    Originally posted by Ampster View Post
                    PowerWalls have a mode called Time Based Control which is primarily a load shifting mode. They charge from the sun in the morning. Then they power the critical loads with solar duing the early peak and then use the battery to power the critical loads after the sun goes down. Is that what you would like to do?

                    With Self Generation Incentive Program incentives the payback in California can be fairly attractive. That would explain why reportedly over 50,000 have been sold. Others are clearly eyeing that market. Even Generac has a battery product to do that. That way they can offer a solution that can leverage existing grid tied solar and offer extended capability during a blackout.

                    I am also on EV-A and NEM 2. Are you on NEM 2?

                    i am on NEM-1 - PTO in november 2015. which is why it looks like i only have until november 2020 to stay on EV-A. EV2-A, at least according to my scripts, will take my yearly bill from +/- $200 to +$1200 per year. E-TOU-B would bring me to like $800 per year and E-TOU-A like $900 per year. as far as i can tell NEM-2 is pretty much the same as NEM-1 except that they get to charge you a one-time $150 interconnection fee... does that sound right to you?

                    also from reading around it sounds like installing powerwalls would let them force me onto NEM-2.

                    my idea was actually more radical than just the peak/partial-peak load shifting. because i didn't do enough reading at the tesla forum, i thought that i would be able to do off-peak to peak load shifting with the powerwall, at least for this last year while there's a 30 cent spread between the two on EV-A. however, 1) PGE explicitly forbids this and only lets you export as much energy as your solar plant would make, 2) tesla won't let you charge the PW2 from the grid, and 3) you can't claim the federal ITC (and certainly not the SGIP rebate) if you are charging from the grid. with some creative accounting i thought maybe i could claim the 26% ITC in 2020 but that might have been a stretch.

                    but it turns out what you describe here is the only option - to load shift between peak and partial peak. i guess i'd have to model how this would really work money-wise. i suppose as long as the powerwall lets you keep exporting energy at peak it could still pay off (especially if you are running your house from the partial-peak power stored in the battery and exporting all the solar during peak) but the deltas would seem to be pretty incremental. even if the PW2 cost is reduced by the state and federal incentives it will still take a very long time to pay for itself thru load-shifting.

                    since i'm interested in the PW2 primarily for backup power, and the incentives are targeted at self-generation and grid supplanting, my goals and the state's are kind of at odds. but it doesn't matter since tesla essentially forces you to operate the powerwalls in a mode that's aligned with what the state wants. so i might as well try to get the credits, although it does seem like the SGIP is getting harder and harder to claim. based on the SGIP spreadsheets it seems like the installers in my area might already be on step 3.

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                    • #11
                      I think you need to separate Powerwall with solar and standalone.

                      My understanding is that in a Standalone configuration you can get the SGIP subsidy if you set it up for load shifting (charge at Off Peak and use in part and peak). Even in that mode you can still set a reserve so You have enough battery backup during and outage. What PG&E requires is that you use at least 52 x Full battery capacity in that mode. This is easily achieved even with a decent reserve.
                      Also the power wall has a Storm option that will automatically override the above mode and keep the full capacity for backup.

                      If interested, I made an Excel sheet that allows to simulate the charge and discharge of the battery in that mode based on your own hourly consumption, and computes the dollar saved based on your TOU rates
                      https://www.solarpaneltalk.com/forum...749#post405749

                      When used with solar, you cant charge from the grid if you want the Tax credit and SGIP, but the benefit to you are still the same, as the battery will be charging during solar hours, and be used during the peak hours (4 to 9 or so). So as a result, you end up not using peak hours electricity.
                      I have a similar version of the other Excel sheet to do NEW simulation that includes simulating the battery usage in that mode
                      https://www.solarpaneltalk.com/forum...393#post402393

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                      • #12
                        well the consensus on the tesla energy forum is that no installer would set up a PW2 as standalone if there was solar already on the premises. that seems to be the main problem. worse, what i wanted was standalone for a year and then solar past that point which is such a special case that i'm sure i could never get it to happen. heck, even the company that installed my panels on behalf of sunpower won't answer my calls/emails about installing batteries. if you call them when you want to spend money and they don't reply what are the odds they'll call you back when you want a favor?

                        i did hear about the storm mode and supposedly tesla did activate it before the PGE PSPS. what bothers me about that is that i'm relying on tesla to allow me to do this, rather than being able to just decide on my own.

                        IMO the easiest path is just to install the powerwalls and use them within the ITC/SGIP rules. however, i just read on the tesla energy forum that california has a new fire code for 2020 which effectively makes it impossible to mount more than one powerwall on your... wall. they want the batteries to be off by themselves on pedestals if > 20KWh of storage. that might kill the whole thing for me since i decided 2 powerwalls is the minimum to get me thru any prospective outages.

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                        • #13
                          Originally posted by astroboy View Post
                          well the consensus on the tesla energy forum is that no installer would set up a PW2 as standalone if there was solar already on the premises.
                          I don't see how that would even be possible, as they both connect to the same circuit. So I don't think this is a will thing, but a technical feasibility one


                          Originally posted by astroboy View Post
                          what i wanted was standalone for a year and then solar past that point .
                          I am not sure what would be the value of that even if it had been possible. By doing that you would only get SGIP subsidy on the battery instead of both SGIP and 30% tax credit by adding it to Solar and setting it up to be used for TOU arbitrage.


                          Originally posted by astroboy View Post
                          i did hear about the storm mode and supposedly tesla did activate it before the PGE PSPS. what bothers me about that is that i'm relying on tesla to allow me to do this, rather than being able to just decide on my own.
                          As I mentioned, I don't think you need to be concerned about that. Even when set to do TOU arbitrage (battery being used during peak hours), You can still set your battery reserve to a fairly large percentage and have plenty in case of an outage.
                          Remember that when connected to solar, you only need to have backup to last you one evening/night, as the next day you will be able to recharge the battery with solar. So with a 13.4 kWh battery, even if you only had it set with a 30% reserve, that would be 4kWh for one evening/night, which I think is plenty.

                          Using my Excel sheet, I checked how much battery is used in TOU arbitrage with various level of reserve (based on my hourly consumption and 8600kWh per year usage)
                          SGIP requires 52 full capacity cycle, or 697kWh used from the battery in a year.
                          If I set the battery usable capacity to only 15% (85% reserved for backup). I get a battery usage of 733 kWh for the year (more than required).
                          So that to me means you can really keep a very large backup capacity even in the TOU arbitrage mode required for SGIP
                          Last edited by scrambler; 10-15-2019, 02:57 PM.

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                          • #14
                            Originally posted by scrambler View Post
                            I don't see how that would even be possible, as they both connect to the same circuit. So I don't think this is a will thing, but a technical feasibility one



                            I am not sure what would be the value of that even if it had been possible. By doing that you would only get SGIP subsidy on the battery instead of both SGIP and 30% tax credit by adding it to Solar and setting it up to be used for TOU arbitrage.
                            i guess i don't have any idea what the powerwall looks like in the schematic of the power setup at my house. i had assumed everything was all in parallel - powerwall, solar and grid, with cutoff switches to make sure the powerwall does not feed into the grid when it is down. clearly even if you have solar the PW2 has to be able to draw power from the solar inverter and push power to the grid. are you saying that it is designed to be installed in series between the inverter and the grid?

                            anyway part of my cunning plan was to operate the PW2 in such a standalone manner for a year to do peak vs. off peak arbitrage, and at that point switch to solar charging (one way or another be it hardware or software) and then claim the ITC at that time. i had already given up on the SGIP since tesla themselves are fully subscribed, but over the last day it does seem that there are installers in the area that can still make claims though as i mentioned they seem to be on step 3 already. but clearly this scheme is a non-starter for a bunch of reasons, so as you say the best thing to do is just get all the subsidies and forget about putting the screws to PGE.

                            Originally posted by scrambler View Post

                            As I mentioned, I don't think you need to be concerned about that. Even when set to do TOU arbitrage (battery being used during peak hours), You can still set your battery reserve to a fairly large percentage and have plenty in case of an outage.
                            Remember that when connected to solar, you only need to have backup to last you one evening/night, as the next day you will be able to recharge the battery with solar. So with a 13.4 kWh battery, even if you only had it set with a 30% reserve, that would be 4kWh for one evening/night, which I think is plenty.

                            Using my Excel sheet, I checked how much battery is used in TOU arbitrage with various level of reserve (based on my hourly consumption and 8600kWh per year usage)
                            SGIP requires 52 full capacity cycle, or 697kWh used from the battery in a year.
                            If I set the battery usable capacity to only 15% (85% reserved for backup). I get a battery usage of 733 kWh for the year (more than required).
                            So that to me means you can really keep a very large backup capacity even in the TOU arbitrage mode required for SGIP
                            being a belt and suspenders (and tape and rope) kind of guy i wanted to plan for scenarios where we can back up for a couple days even if it is cloudy or there is wildfire smoke. if we do nothing at all (like while on vacation) the house burns about 10kwh per day, so a single powerwall is only good for ~1 day in the absence of the sun. but i agree that the SGIP requirements are very modest; i'd just like to be able to have 100% when i know i'm going to need it... when we are occupying the house we use closer to 24kwh per day. obviously we'd have to just turn a bunch of stuff off in the interest of keeping the refrigerator running.

                            anyway if the new fire rules are real i think the whole thing is dead for me. i can't install these things anywhere else but on the side of the garage and i'm not sure just one powerwall gets me where i want to be. we'll see.


                            edit: i will check out your spreadsheets. i had written a perl program that takes all my PVOutput data and simulates the different TOU rates but that software has no notion of storage. it will probably be easier to try your spreadsheets than hack all of that stuff in at this point.
                            Last edited by astroboy; 10-15-2019, 03:51 PM.

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                            • #15
                              Well that seems completely asinine. One would think that it would charge from Solar preferentially but then be able to charge from the grid (or generator) when it cannot (or hadn't sufficiently) based on a schedule / heuristic for when power is cheap and the priority of the system -- e.g. load shifting vs. outage protection.

                              I guess if you live somewhere sunny where it's never cloudy, rainy, snowy, that might work,. But on the East coast, one would certainly get a lot more mileage from a nice 1800rpm, liquid cooled natural gas generator for the same price or less.

                              Realistically, even with 2 or 3 Powerwalls, you're unlikely to be able to run your HVAC for more than a few hours at 13.5kWh per unit. When Sandy hit, folks that were lucky enough to have natural gas generators ran for days/weeks on end. (while the rest of us tried to find gasoline).

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