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Deciding on PW. 1? 2? 3? and TOU!

Sep 24, 2015
833
696
San Diego (Oceanside)
I have been mulling and lightly researching the purchase of a Powerwall (or similar) for some time in anticipation for some forthcoming TOU changes that will negatively affect the value of my solar production. Fortunately, the change to 4-9pm peak is 2.5 years away for me, but the recent power outages due to red flag weather along with the new CAL FIRE codes has got my a bit more motivated to look into options.

Stats:
9.9 KW solar
Peak production ~70 kWh summer. Half that for winter
Average day consumption just under 50 kWh including daily Model 3 charge.

Current peak is 11-4 during summer which is great ROI for solar. System generates more than daily usage and peak NEM rates are about 2:1. This helps out in the winter months when the solar generation does not cover daily usage.

Power outages in my area are rare. The thought of whole home backup is desirable and neat, but just for that there is little ROI on the purchase.

Rate arbitrage would be useful but this is an area PW isn't designed for. My neighbor has a battery solution from another manufacturer and an EV, so with his cheap EV plan, he charges both up at night super off peak, uses solar to power the house and sell excess back to the utility at a mid tier rate during the day, power his house from battery during peak until super off peak, then charge again. This ability may be useful enough that it may drive me to another solution than PW unless there's some inherent PW features that are more desirable.

Before I speak to Tesla or a third party installer, I feel I need some more knowledge on how PW can be useful in my house.

Questions:

Being that it's currently a good ROI to sell back to the utility during summer peak, is a PW useful with this rate plan?

PW must charge from the solar. It seems advantageous to charge from sunrise to 11a during the summer then sell back to the utility during peak. I cannot seem to determine via the SolarEdge web portal how long it would take to generate enough power to charge a PW. It only seems to show the output at that moment in time in KW and the reports area is lacking. Any suggestions on how to get this data?

During winter months, peak and semi peak are largely the same rate (differs by a fraction of a cent). Solar could charge PW but there may not be enough excess to sell back to the utility. While the utility acts as a big battery, I do get charged for the costs associated with transmitting the electricity used during times no solar generation. Using PW will reduce those non bypassable charges, but unsure if it's really worth it.

How does one size the PW install? Using the Tesla website it recommends 2 for my size house, but if I add EV it suggests 3. On a good solid solar day in winter, solar produces 30 kWh, house uses 10 of that, leaving 20 back to the utility. That would charge 1 PW fully and another partially. If I had 3, would one PW effectively be useless in the winter?

Forgive me if some of these questions seem elementary. When I learned that PW can't be charged from the grid if one has solar, it opened up a lot of uncertainty on how useful it may be. There isn't a lot of documentation on what level of control one may have via the software, but outside of adjusting for TOU, it seems like it's largely on autopilot (forgive me!)

Thanks for reading and offering suggestions and clarifications.
 

power.saver

Supporting Member
Mar 4, 2018
572
609
Arcadia, CA
Based on the information you provided, I think 2 PWs would be ideal. You won't charge the EV from the PW, so your house load during the 11-4 peak will not fully discharge them. And you will likely produce enough from sunrise to 11 to replenish what was taken out the day before. If you have west facing panels, you'll even start the recharge after 4 most of the year.

The TOU controls are good for offsetting your peak usage, I do that and don't draw any peak power.
 

chrisbailey13

Member
Sep 17, 2019
96
75
Central Florida
I have a 19KW Solar system and 3 PW2's. I can charge all three of them in <4hrs from 20% to full in Sept-Oct time of year. I know the sun is different in FL than in San Diego, but the fact that they recharge relatively quickly Is a good deal...My morning house usage is around 2-4K, leaving 15-17KW to charge the PW's.
 

GenSao

Member
Aug 3, 2017
550
938
Pleasant Hill, CA
Being that it's currently a good ROI to sell back to the utility during summer peak, is a PW useful with this rate plan?

In general with a regular TOU rate schedule, batteries do not have a good overall ROI (if any). With an EV TOU rate schedule, there is a better chance for an actual ROI. This is all dependent on how much rate arbitrage is done (saving during peak period), how much you value backup power, and/or how much you value on being green.

PW must charge from the solar. It seems advantageous to charge from sunrise to 11a during the summer then sell back to the utility during peak. I cannot seem to determine via the SolarEdge web portal how long it would take to generate enough power to charge a PW. It only seems to show the output at that moment in time in KW and the reports area is lacking. Any suggestions on how to get this data?

During winter months, peak and semi peak are largely the same rate (differs by a fraction of a cent). Solar could charge PW but there may not be enough excess to sell back to the utility. While the utility acts as a big battery, I do get charged for the costs associated with transmitting the electricity used during times no solar generation. Using PW will reduce those non bypassable charges, but unsure if it's really worth it.

I don't have Solar Edge to help you there.

Regarding non-bypassable charges, consider it a wash. There is approximately a 90% round-trip conversion loss (AC to DC then DC to AC) for the Powerwalls. So 10 KWH in would yield 9 KWH out. Furthermore, there is a vampire drain of roughly 1KW/day for each battery to maintain the batteries (varies by location/temperature dependent).

How does one size the PW install? Using the Tesla website it recommends 2 for my size house, but if I add EV it suggests 3. On a good solid solar day in winter, solar produces 30 kWh, house uses 10 of that, leaving 20 back to the utility. That would charge 1 PW fully and another partially. If I had 3, would one PW effectively be useless in the winter?

I agree with Power.saver as two is optimal:
Based on the information you provided, I think 2 PWs would be ideal. You won't charge the EV from the PW, so your house load during the 11-4 peak will not fully discharge them. And you will likely produce enough from sunrise to 11 to replenish what was taken out the day before. If you have west facing panels, you'll even start the recharge after 4 most of the year.

The TOU controls are good for offsetting your peak usage, I do that and don't draw any peak power.

Also, two Powerwalls are required for 9.9 KW of solar. Per the Tesla Powerwall FAQ:
To ensure reliable operation during power outages, at least one Powerwall is required for each 7.6 kW AC of solar included in the backup circuit.
 
Jun 22, 2017
527
337
Bay Area, California
To get your solar data, if you are good with large spreadsheets, you can NREL's PVWatts calculator to export a hourly production. The model is based on real data, so it is quite accurate.

I would avoid calculating your ROI based on current because 2.5 years remaining on your EV TOU is small in comparison to a ROI ranging from 10+ years (entirely depends on the net cost). I have 1.5 years remaining on my EV TOU equivalent rate schedule. Powerwalls used as "damage control"/protection from changing rate schedules is secondary. ROI for solar itself for me was 7 years under EV TOU, and 9 years under tiered rates. 9 years is the commonly accepted ROI for solar in California. The *new* TOU rate would push solar ROI to 12 years (my estimate). Adding Powerwall may or may not help depending on net price paid.

If you are running A/C, 3 PW can improve your chances of being able to start the A/C. Do you own A/C? Scroll type? How many LRA on the largest compressor? <--this is for those who can't live without A/C in the summer.

The CAL FIRE alerts impact vary hugely depending on where you live. I was not affected, so the perceived value of my Powerwalls did not go up. During this time of year, with no rain clouds and no A/C loads, I could in theory go off-grid, and be able to charge my EV for my commute even with 2 PW (with home loads of ~10kWh per day this time of year).
 
Last edited:
Sep 24, 2015
833
696
San Diego (Oceanside)
Thank you everyone for your replies.

Based on the information you provided, I think 2 PWs would be ideal. You won't charge the EV from the PW, so your house load during the 11-4 peak will not fully discharge them. And you will likely produce enough from sunrise to 11 to replenish what was taken out the day before. If you have west facing panels, you'll even start the recharge after 4 most of the year.

The TOU controls are good for offsetting your peak usage, I do that and don't draw any peak power.

I'm a net exporter during the 11a-4p peak during summer. For the next 2.5 years, the more I send out during that time the better. Is it possible to draw from the PW during that time and export 100% solar? i.e. charge up sunrise to 11a and 6p to sunset? I have 12 east facing panels, 12 west facing panels and 6 south facing panels. My generation curve isn't sharp but a more rounded, balanced curve.

To get your solar data, if you are good with large spreadsheets, you can NREL's PVWatts calculator to export a hourly production. The model is based on real data, so it is quite accurate.

I would avoid calculating your ROI based on current because 2.5 years remaining on your EV TOU is small in comparison to a ROI ranging from 10+ years (entirely depends on the net cost). I have 1.5 years remaining on my EV TOU equivalent rate schedule. Powerwalls used as "damage control"/protection from changing rate schedules is secondary. ROI for solar itself for me was 7 years under EV TOU, and 9 years under tiered rates. 9 years is the commonly accepted ROI for solar in California. The *new* TOU rate would push solar ROI to 12 years (my estimate). Adding Powerwall may or may not help depending on net price paid.

If you are running A/C, 3 PW can improve your chances of being able to start the A/C. Do you own A/C? Scroll type? How many LRA on the largest compressor? <--this is for those who can't live without A/C in the summer.

The CAL FIRE alerts impact vary hugely depending on where you live. I was not affected, so the perceived value of my Powerwalls did not go up. During this time of year, with no rain clouds and no A/C loads, I could in theory go off-grid, and be able to charge my EV for my commute even with 2 PW (with home loads of ~10kWh per day this time of year).

I tried to set up PVWatts when I first got solar, but my array faces three directions - W, E, and S. I can't recall the issue now but I think it was limited to 2 directions?

I live in a coastal city (the inland part of it) and really don't run the A/C much. Maybe a month a year. Growing up inland seems to have allowed me to tolerate moderate heat in the house during summer. When that humidity hits though, the A/C is on. House has 2 A/C's - 3.5 ton and 5 ton. Unsure how to tell if it is scroll type. LRA 105/144. I'd imagine running the A/C would drain the PW's fairly fast. I guess the lack of understanding how much something like that draws is adding to my uncertainty.

Ultimately, I don't want the food in main fridge, garage fridge, and garage chest freezer to spoil or thaw in an outage. :)
 

power.saver

Supporting Member
Mar 4, 2018
572
609
Arcadia, CA
You can export all solar during your 11-4 peak, and charge up the rest of the time the sun is out.

Forget about the 5 ton A/C. Maybe the 3.5 ton. But even if it could start with PW+Solar, it would drain your battery quickly.
 
Sep 24, 2015
833
696
San Diego (Oceanside)
I stopped into Tesla to get more info about PW. No one there had any idea short of trying to find it on the website for me and suggested I put the $99 down to get someone to call me. Maybe I have to as I've submitted info for someone to call me a few times but nothing so far.

I spoke to a third party installer and got some additional info. I was hoping to do a "simple" whole home backup and avoid a sub panel, but my panel is physically full and unknown if there's room on the bus energy wise. Apparently each PW requires 30a. Sub panel will need to be done either way.

They didn't have much info about SGIP and recommended I contact the CSE for more info. They stated they fill out the forms and send them in but no idea if/when there will be funding and if there is, how much so don't count on it. No problem, but SGIP seems to be confusing and there are specific requirements I don't understand quite yet, so more research.

Sizing - 1, 2, or 3? They said I may not need two, but that if I go to 3, there's additional requirements in my locality. Specifically, it is no longer considered residential but commercial due to the size (>10,000 watts) which requires a separate meter for SDG&E to monitor the battery use behind the main meter. This is new to me. I have started looking for more info online but have not found any. Anyone in San Diego familiar with this?
 

jjrandorin

Moderator, Model 3, Tesla Energy Forums
Nov 28, 2018
9,635
10,873
Riverside Co. CA
I stopped into Tesla to get more info about PW. No one there had any idea short of trying to find it on the website for me and suggested I put the $99 down to get someone to call me. Maybe I have to as I've submitted info for someone to call me a few times but nothing so far.

I spoke to a third party installer and got some additional info. I was hoping to do a "simple" whole home backup and avoid a sub panel, but my panel is physically full and unknown if there's room on the bus energy wise. Apparently each PW requires 30a. Sub panel will need to be done either way.

They didn't have much info about SGIP and recommended I contact the CSE for more info. They stated they fill out the forms and send them in but no idea if/when there will be funding and if there is, how much so don't count on it. No problem, but SGIP seems to be confusing and there are specific requirements I don't understand quite yet, so more research.

Sizing - 1, 2, or 3? They said I may not need two, but that if I go to 3, there's additional requirements in my locality. Specifically, it is no longer considered residential but commercial due to the size (>10,000 watts) which requires a separate meter for SDG&E to monitor the battery use behind the main meter. This is new to me. I have started looking for more info online but have not found any. Anyone in San Diego familiar with this?

Just that on the SGIP SDGE is fully subscribed at this point, and there is likely a waitlist (and a lottery) so you should not plan on getting it at all, and hope that, maybe, in a couple years or something you could get it.

I also am fairly sure that 3 Powerwalls ends up being "commercial" instead of residential as you mentioned, but I am not sure what happens with the service based on that.
 
Sep 24, 2015
833
696
San Diego (Oceanside)
Just that on the SGIP SDGE is fully subscribed at this point, and there is likely a waitlist (and a lottery) so you should not plan on getting it at all, and hope that, maybe, in a couple years or something you could get it.

I also am fairly sure that 3 Powerwalls ends up being "commercial" instead of residential as you mentioned, but I am not sure what happens with the service based on that.
To answer the 3 PW question, SDG&E requires another meter to ensure what you're sending back to the grid under NEM is solar generated, not battery.

I sent Tesla my $99 a week ago Friday. How long did they take to contact you?
 

jjrandorin

Moderator, Model 3, Tesla Energy Forums
Nov 28, 2018
9,635
10,873
Riverside Co. CA
To answer the 3 PW question, SDG&E requires another meter to ensure what you're sending back to the grid under NEM is solar generated, not battery.

I sent Tesla my $99 a week ago Friday. How long did they take to contact you?

It only took a couple of days for someone to call me back. I am not sure how they assign people there, but I have been pretty pleased with the person they assigned to me. With that being said, the person assigned to me, right out the gate, told me that there was no guarantee that I would be able to get my powerwalls "this year", because of the backlog and number of installs etc. I live in temecula but work down in Oceanside, so dont know if tesla is installing directly in Oceanside etc.

I am not sure how they work internally but I would be happy to give you the contact I have there now who is working with me if you want. Not sure that would help, but if you want that contact info just PM me.
 

sroh

Supporting Member
Sep 10, 2017
653
2,246
San Jose, CA
Watching this thread... @Nathaniel, it sounds like I am in a similar situation as you.

We're in San Jose and use PG&E. We had a 10.56 kW system installed 2 years ago. We have two Model 3s charging at home. With that, net metering will end up being a significant credit for us. So any benefit of adding a PW or two will be for power outage protection. In the past, this wasn't a compelling reason. This year has changed our minds.

I put down the $99 deposit today and called Tesla Energy. The rep was not helpful. She couldn't give me an estimate of how long it would be for an advisor to call me, what kind of backlog they had, and was adamant that I needed two PWs. I'm not sure whether she was suggesting it was required or she was recommending it for the size of our system.

Timeshifting has little benefit for us. This would be mainly for power outage backup. When power goes out, we would minimize power usage to two refrigerator/freezers, a few power outlets (cable modem, wireless router, a few LED lights, may be an LED TV). No A/C or other power hungry appliances. So I think we can get by with just one PW.

Questions:
1. Can we get just one PW? Someone above mentioned Tesla requires two if PV system is greater than 7.6 kW.
2. For our use case, are there reasons we might consider two? Futureproofing, lower per unit cost, etc.
3. Do you think it's possible to have it installed in time to claim 2018 tax credit?
4. How important is it to install inside? Our main panel is on west facing exterior wall, on opposite side of house as our garage and basement, the two best inside locations.
 

GenSao

Member
Aug 3, 2017
550
938
Pleasant Hill, CA
Watching this thread... @NathanielQuestions:
1. Can we get just one PW? Someone above mentioned Tesla requires two if PV system is greater than 7.6 kW.
2. For our use case, are there reasons we might consider two? Futureproofing, lower per unit cost, etc.
3. Do you think it's possible to have it installed in time to claim 2018 tax credit?
4. How important is it to install inside? Our main panel is on west facing exterior wall, on opposite side of house as our garage and basement, the two best inside locations.

1. It is possible. Depends if your solar inverter and the Tesla Gateway can communicate to limit power when needed.

2. Lower cost per unit is a good. See below estimate. Roughly 32% additional cost for double the capacity (27 kWH) and power output (10 kW).

Capture.JPG


More capacity allows you to store more of your solar when the grid is down and mitigate for battery degradation (10 year warranty for 70% degradation). As you'll be forced onto the EV2-A rate schedule, the added capacity can cover more of your household use during peak periods.

More power allows you you back up most (if not all) loads. Depending on the size of your home, the AC may be included. EV's may be charged off the grid with reasonable efficiency on a 40 A circuit/plug.

3. It is possible. If not installed in time, there is the 5% "safe harbor" allowance you/Tesla can consider.

4. Ideally inside as batteries prefer to be in a nominal temperature range. Outside is fine as well. I have mine installed outside and noticed they use a bit more power than average to maintain the battery.
 

Vines

Active Member
Jul 20, 2018
1,949
2,289
Silicon Valley, CA
Watching this thread... @Nathaniel, it sounds like I am in a similar situation as you.

We're in San Jose and use PG&E. We had a 10.56 kW system installed 2 years ago. We have two Model 3s charging at home. With that, net metering will end up being a significant credit for us. So any benefit of adding a PW or two will be for power outage protection. In the past, this wasn't a compelling reason. This year has changed our minds.

I put down the $99 deposit today and called Tesla Energy. The rep was not helpful. She couldn't give me an estimate of how long it would be for an advisor to call me, what kind of backlog they had, and was adamant that I needed two PWs. I'm not sure whether she was suggesting it was required or she was recommending it for the size of our system.

Timeshifting has little benefit for us. This would be mainly for power outage backup. When power goes out, we would minimize power usage to two refrigerator/freezers, a few power outlets (cable modem, wireless router, a few LED lights, may be an LED TV). No A/C or other power hungry appliances. So I think we can get by with just one PW.

Questions:
1. Can we get just one PW? Someone above mentioned Tesla requires two if PV system is greater than 7.6 kW.
2. For our use case, are there reasons we might consider two? Futureproofing, lower per unit cost, etc.
3. Do you think it's possible to have it installed in time to claim 2018 tax credit?
4. How important is it to install inside? Our main panel is on west facing exterior wall, on opposite side of house as our garage and basement, the two best inside locations.

1. 2 PW are required for a 10.5 kW system installed 2 years ago almost certainly. Most (all?) older inverters aren't "Rule 21" complaint. Therefore your PV is so large it would fight the single Powerwall and not charge correctly. The PW would keep frequency shifting the PV out of range and shutting it down. If you really wanted 1 PW only you could either leave the second one off of the backup system, or manually disconnect one inverter each outage.

2. Think of how little power you get from low angle winter sun, and how much more you will want power if its deep winter. If it gets cold do you have gas for space and water heat? Plus if you have 10 kw of solar you wont be able to use half of it in an outage, further reducing your winter harvest and storage. Plus in 10 years there will likely be some capacity loss, plan for it.

4. In our climate not terribly important to go inside, but I would if I could. In colder climates or in deep winter it can be a significant drain to keep themselves heated during winter outages. The Powerwall can expend approx. 1500W to run its heater and itself in very cold climates.
 

jjrandorin

Moderator, Model 3, Tesla Energy Forums
Nov 28, 2018
9,635
10,873
Riverside Co. CA
Watching this thread... @Nathaniel, it sounds like I am in a similar situation as you.

We're in San Jose and use PG&E. We had a 10.56 kW system installed 2 years ago. We have two Model 3s charging at home. With that, net metering will end up being a significant credit for us. So any benefit of adding a PW or two will be for power outage protection. In the past, this wasn't a compelling reason. This year has changed our minds.

I put down the $99 deposit today and called Tesla Energy. The rep was not helpful. She couldn't give me an estimate of how long it would be for an advisor to call me, what kind of backlog they had, and was adamant that I needed two PWs. I'm not sure whether she was suggesting it was required or she was recommending it for the size of our system.

Timeshifting has little benefit for us. This would be mainly for power outage backup. When power goes out, we would minimize power usage to two refrigerator/freezers, a few power outlets (cable modem, wireless router, a few LED lights, may be an LED TV). No A/C or other power hungry appliances. So I think we can get by with just one PW.

Questions:
1. Can we get just one PW? Someone above mentioned Tesla requires two if PV system is greater than 7.6 kW.
2. For our use case, are there reasons we might consider two? Futureproofing, lower per unit cost, etc.
3. Do you think it's possible to have it installed in time to claim 2018 tax credit?
4. How important is it to install inside? Our main panel is on west facing exterior wall, on opposite side of house as our garage and basement, the two best inside locations.

Just based on what they (tesla) said to me when I spoke to them, they were "slammed" with orders from people in northern california. They warned me it could be "up to 6 months" before I got my powerwall installed, and there was absolutely zero guarantee they would be even able to start work before the end of the year. Mine is moving much faster, as I think because I have a solar city leased system, and have made no changes other than to get a permitted installed HPWC since that install, they have all my information etc.

I was able to get past the design stage very quickly, as I have the perfect spot on the inside of my garage, which is on the same wall as my electrical panel (with the panel being on the outside of the house, and "the space" being on the inside of the garage basically opposite it.)

all that to say, I would not expect to get it installed by the end of the year (which is, in effect, roughly 8 weeks away... how time flies!). "might" happen, if you have a very cookie cutter setup, AND they get through the design stage, and you have very little questions etc... but I doubt it, personally.
 

sroh

Supporting Member
Sep 10, 2017
653
2,246
San Jose, CA
Thanks Vines! I think your answer to question #1 is what the Tesla rep may have been trying to communicate. Makes sense. So I would have to research the model of our Enphase inverters for 'Rule 21' compliance. Otherwise, get two PWs. I certainly don't want to deal with manually turning things on/off or disconnecting.

Wrt question #2, you're in Silicon Valley, so you understand the climate. We've yet to experience a major power outage during the winter. We have gas for home heating as well as water heating. So I've really just been thinking about this as emergency backup. But I take your point. You and GeoSao have me thinking two PWs is the way to go.

Question #4. Understood. Cold temps are not an issue. I am more concerned about hot temp during the summer. I know heat and Lithium batteries and heat don't play well together. I guess I'll have to wait for the site survey to determine the best location.

Thanks again!
 

Vines

Active Member
Jul 20, 2018
1,949
2,289
Silicon Valley, CA
No problem, happy to help. I have been slammed lately with work too, tons of people want PV plus Powerwalls and many many older customers are coming back and want backup now too. So I haven't been able to log in much lately.

We are serving our existing customers first, and are 4-6 months out with new installations depending how wet the winter looks.

If you are concerned about west sun, you can pretty easily shade the PW. with something after the installation. the PW have active cooling so they won't be damaged in all but the hottest climates. They could derate output and charge rate though in extreme heat which could be bad depending on severity.
 

woferry

Member
Mar 4, 2019
405
478
San Jose, CA
Questions:
1. Can we get just one PW? Someone above mentioned Tesla requires two if PV system is greater than 7.6 kW.

It's not so much the size of the system (and 7.6kW isn't the magic number), it depends on the inverters. I have a 13.32kWp system (though only 10.4kW of inverters) and a single Powerwall 2. But when they installed my system they installed two Delta Solivia 5.2 units, one is on the PW side of the Gateway and the other isn't. So if the grid goes down I lose half of my solar, but what I still have is within the 5kW charging limit of a single Powerwall 2. I intend to add a second PW sometime in the future (when I add AC), and when there is the extra charging capacity I expect they'll move the second inverter's breaker to the PW side of the Gateway.

4. How important is it to install inside? Our main panel is on west facing exterior wall, on opposite side of house as our garage and basement, the two best inside locations.

My PW2 (and the space for the second one) are both outside, as is my main panel. Inside installations simply aren't a requirement, and there's no way one would have fit in my garage anyway (and no basement). I have a few pictures in the album linked from my signature.

One thing that is probably worth mentioning on 1x PW2 vs. 2x is the upcoming CA fire code change (there's a thread about it on this forum, it's on the first page right now), that might make 2 more difficult next year. So there's a risk that 2 is "easy" this year (scheduling issues aside) and much harder next year w.r.t. getting permits / approval. I have to hope that the rules are being misunderstood or will be tweaked, but it could end up being a wrinkle for my plan to expand my system later.
 

Vines

Active Member
Jul 20, 2018
1,949
2,289
Silicon Valley, CA
It's not so much the size of the system (and 7.6kW isn't the magic number), it depends on the inverters. I have a 13.32kWp system (though only 10.4kW of inverters) and a single Powerwall 2. But when they installed my system they installed two Delta Solivia 5.2 units, one is on the PW side of the Gateway and the other isn't. So if the grid goes down I lose half of my solar, but what I still have is within the 5kW charging limit of a single Powerwall 2. I intend to add a second PW sometime in the future (when I add AC), and when there is the extra charging capacity I expect they'll move the second inverter's breaker to the PW side of the Gateway.

If you try to do this after the first of the year, they may require 2017 NEC version of rapid shutdown, which basically requires module level shutdown. They may let you off the hook since its an existing system, but if you are planning on relocating the AC feed for one of the 2 Powerwalls, I'd get the permit asap. Then at least you could get 6 months of grace, and possibly a year if you are granted a permit extension.
As far as the 1 vs 2 Powerwalls being a fire code issue with the upcoming code cycle change, I have no further clarity, though I don't expect this to be a problem with San Jose.
 

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