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Expanding My 6.0kW System - 4.08kW + 2x PW

jsurpless

Member
Feb 20, 2020
99
9
Massachusetts
Hi everyone

Looking to get some thoughts on this.

I currently have a 6.0 kW DC solar system (24x 250W panels, installed in early 2016 by Sungevity) and am considering adding 2x PowerWalls.

It occurred to me the other day that while this system generally should be able to charge the batteries on a sunny day (except in the winter), as my peak generation is around 38 kWh in the summer, I won't be guaranteed to do so by the 2PM potential start of Connected Solutions.

As National Grid's Connected Solutions program in Massachusetts is what makes the PW truly affordable, I need to make sure that it'll work for me.

So it got me thinking that I might want to consider adding Tesla solar alongside the PWs. I wouldn't need much as the current system pretty much meets our needs (although, I do have an M3 that will impact our usage) and so the 4.08 kW 'small' system would be good. Tesla Experience cautioned me that I might not be approved by National Grid because my system would be just over the 20kW limit for the transformer w/o upgrade.

Thoughts on this? What would you all do?

Thanks in advance!
 

wwhitney

Member
Nov 2, 2017
965
1,310
Berkeley, CA
I would think any kW limit would be based on the AC continuous rating of the inverters involved. That's 5 kW for each Powerwall, so if there's a POCO 20 kW limit, just make sure your two inverter ratings add up to no more than 10 kW.

Cheers, Wayne
 

jsurpless

Member
Feb 20, 2020
99
9
Massachusetts
I would think any kW limit would be based on the AC continuous rating of the inverters involved. That's 5 kW for each Powerwall, so if there's a POCO 20 kW limit, just make sure your two inverter ratings add up to no more than 10 kW.

Cheers, Wayne
Thanks for the feedback, Wayne - any idea how I might find out what/if there is a limit?

Also, it seems unlikely to me that I'd be able to find a 4kW AC inverter (my current one is SE-6000A-US)
 

Er1c*

Member
Jul 4, 2021
36
33
RI
I have national grid in RI. I have 10kw of solar and three powerwalls. On my net metering application they list my system as 25kw. I had no problem. Now my house is new construction and they did install a new pole and transformer when they built my house so it might be that you are on older equipment.
 

jsurpless

Member
Feb 20, 2020
99
9
Massachusetts
I have national grid in RI. I have 10kw of solar and three powerwalls. On my net metering application they list my system as 25kw. I had no problem. Now my house is new construction and they did install a new pole and transformer when they built my house so it might be that you are on older equipment.
Thanks for the info, much appreciated. My house is relatively new (built 2013) so maybe I'll be OK.

I sent an email to [email protected] so maybe they'll be able to clarify
 

daveh85

Member
Aug 6, 2020
34
54
Cohasset, MA
In my case I got Tesla to cover the cost of the transformer. They (NG) bizarrely take the sum of the PW and Inverters. In my case 6 PWs and 3 7.6 Inverters. Sum of 52.8 which is over the 50kv transformer. Depending on who your case manager is at NG they will be flexible and work with you, I’ve heard horror stories though.
 

jsurpless

Member
Feb 20, 2020
99
9
Massachusetts
In my case I got Tesla to cover the cost of the transformer. They (NG) bizarrely take the sum of the PW and Inverters. In my case 6 PWs and 3 7.6 Inverters. Sum of 52.8 which is over the 50kv transformer. Depending on who your case manager is at NG they will be flexible and work with you, I’ve heard horror stories though.
I think that the reason that they use the combined sum is because, in theory, if the PV system was producing maximum output AND your PWs were discharging in a ConnnectedSolutions event, that's what the output would be.

I heard back from NG - the gentleman tried to explain things but ultimately, there are too many variables to be able to specify a hard-fast limit. So we'll see what happens.
 

daveh85

Member
Aug 6, 2020
34
54
Cohasset, MA
Agree. I did come close the other day if the app is to be trusted.
8683F23E-6654-4472-B98C-6B47F84E1D3E.png
 

jsurpless

Member
Feb 20, 2020
99
9
Massachusetts
So I'm still in the design phase with Tesla and am growing increasingly frustrated with them.

I'd like to get thoughts on my experiences, am I making a big deal out of things?

System Wiring Layout

Tesla is insisting that my local electrical code requires a total of (3) AC disconnect switches (2 new ones plus the existing 1)

1629741064977.png
1629741046530.png


I called the electrical inspector in Bellingham and based on my discussion with him, my understanding is that additional AC disconnects are not required. All that is required is method of isolating the individual inverters which could be accomplished like a friend's (also done by Tesla but at the same time, not expansion). That small box btw the disconnect and the inverters has individual breaker disconnects.

1629741199025.jpeg


I am just trying to minimize the amount of hardware on the side of my house and to simplify what is needed to quickly shut the system down. My reasoning being that a single AC disconnect is simpler than three of them - I'm sure that the local fire dept would appreciate the simplicity!

Panel Wiring & Potential Shading

The design shows all 12 panels on a single string from the Tesla inverter which has me concerned. The solar panels would be on the garage roof (NW corner of my house) and there is a cherry tree (for privacy, so cutting is not an option) that has some shading impact on the corner of the roof like so

1629741683742.jpeg


I am not worried about the minimal shading on an individual panel but that little bit adversely affecting the *whole* system where there is *no* shade at any time.

I have tried to explain this at length and they just keep coming back, saying that there's nothing that can be done as they don't use power optimizers with Tesla inverter. I have told them that I'm not saying that they need to use optimizers, just what I am trying to avoid. My understanding is that the 3.8 kW inverter supports a 2nd string that could be used for the "concerned" panels?

Many thanks in advance :)
 

h2ofun

Active Member
Aug 11, 2020
2,902
550
auburn, ca
So I'm still in the design phase with Tesla and am growing increasingly frustrated with them.

I'd like to get thoughts on my experiences, am I making a big deal out of things?

System Wiring Layout

Tesla is insisting that my local electrical code requires a total of (3) AC disconnect switches (2 new ones plus the existing 1)

View attachment 700449View attachment 700448

I called the electrical inspector in Bellingham and based on my discussion with him, my understanding is that additional AC disconnects are not required. All that is required is method of isolating the individual inverters which could be accomplished like a friend's (also done by Tesla but at the same time, not expansion). That small box btw the disconnect and the inverters has individual breaker disconnects.

View attachment 700450

I am just trying to minimize the amount of hardware on the side of my house and to simplify what is needed to quickly shut the system down. My reasoning being that a single AC disconnect is simpler than three of them - I'm sure that the local fire dept would appreciate the simplicity!

Panel Wiring & Potential Shading

The design shows all 12 panels on a single string from the Tesla inverter which has me concerned. The solar panels would be on the garage roof (NW corner of my house) and there is a cherry tree (for privacy, so cutting is not an option) that has some shading impact on the corner of the roof like so

View attachment 700451

I am not worried about the minimal shading on an individual panel but that little bit adversely affecting the *whole* system where there is *no* shade at any time.

I have tried to explain this at length and they just keep coming back, saying that there's nothing that can be done as they don't use power optimizers with Tesla inverter. I have told them that I'm not saying that they need to use optimizers, just what I am trying to avoid. My understanding is that the 3.8 kW inverter supports a 2nd string that could be used for the "concerned" panels?

Many thanks in advance :)
My building code had me put in 1 disconnect per battery, for 5, 1 for each inverter, for 2, and for the generator. Not worth fighting, they won.
 

h2ofun

Active Member
Aug 11, 2020
2,902
550
auburn, ca
You're probably right, any thoughts on the shading impact?
My attitude was for Calif, NEM3 might kill any new solar stuff, so I just decided to put on as many as my roof would fit, and hope PGE would approve. Some put teslas in their garage, and for me a waste of money. I put way too many panels on the roof, I like the 26% write off, a waste of money. But if I get EV's etc down the road, at least the solar never has to be touched again! If I had take this attitude with my batteries I would have had 7. Oh well, the more solar the better
 

wwhitney

Member
Nov 2, 2017
965
1,310
Berkeley, CA
My building department's crazy interpretation of the code had me put in 1 disconnect per battery, for 5, 1 for each inverter, for 2, and for the generator.
Fixed it for you.

[I haven't actually closely read the relevant portions of the California Electrical Code, but since most places aren't requiring individual bladed disconnects, it's fair to say that your building department's interpretation is an outlier.]

Cheers, Wayne
 

h2ofun

Active Member
Aug 11, 2020
2,902
550
auburn, ca
Fixed it for you.

[I haven't actually closely read the relevant portions of the California Electrical Code, but since most places aren't requiring individual bladed disconnects, it's fair to say that your building department's interpretation is an outlier.]

Cheers, Wayne
True. But at least they did not also force 3 feet between PW's that some locations are enforcing :)
 

BGbreeder

Member
Jun 19, 2020
357
218
Bay Area
So, is the cherry a dwarf, or a plain black cherry that will top out at 100'? Makes a difference for the future, doesn't it?

As shown, I wouldn't sweat the shading, but I would wonder about the effect of the second story as it would be south of that roof plane, if the cherry is in the NW corner.

All the best,

BG
 

jsurpless

Member
Feb 20, 2020
99
9
Massachusetts
So, is the cherry a dwarf, or a plain black cherry that will top out at 100'? Makes a difference for the future, doesn't it?

As shown, I wouldn't sweat the shading, but I would wonder about the effect of the second story as it would be south of that roof plane, if the cherry is in the NW corner.

All the best,

BG
It is a Kwanzan Cherry that's been there for about 6 years now, so it's probably at least 7 years old.

Regarding the effect of the 2nd story, this is how they have proposed the layout and there's how it shades around 12PM in mid-July. That shadow pretty goes away by 4-5 PM. I think that the panels wouldn't be in that shade?

1629764930322.jpeg
1629764972646.jpeg
 

Vines

Active Member
Jul 20, 2018
2,039
2,368
Silicon Valley, CA
Fixed it for you.

[I haven't actually closely read the relevant portions of the California Electrical Code, but since most places aren't requiring individual bladed disconnects, it's fair to say that your building department's interpretation is an outlier.]

Cheers, Wayne

Some local jurisdictions including CPA were until recently ruling that each Powerwall needs a bladed lockable disconnect and citing 2017 NEC 705.22 as the reason.

This interpretation is still being used in at least one other jurisdiction, for any ESS more than 5' from the breaker it is served by. I am working on writing a white paper to describe this question, and get various smart people's opinion on the matter while I have access to a bunch of smart inspectors at the IAEI meeting.
 
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Vines

Active Member
Jul 20, 2018
2,039
2,368
Silicon Valley, CA
So I'm still in the design phase with Tesla and am growing increasingly frustrated with them.

I'd like to get thoughts on my experiences, am I making a big deal out of things?

System Wiring Layout

Tesla is insisting that my local electrical code requires a total of (3) AC disconnect switches (2 new ones plus the existing 1)

View attachment 700449View attachment 700448

I called the electrical inspector in Bellingham and based on my discussion with him, my understanding is that additional AC disconnects are not required. All that is required is method of isolating the individual inverters which could be accomplished like a friend's (also done by Tesla but at the same time, not expansion). That small box btw the disconnect and the inverters has individual breaker disconnects.

View attachment 700450

I am just trying to minimize the amount of hardware on the side of my house and to simplify what is needed to quickly shut the system down. My reasoning being that a single AC disconnect is simpler than three of them - I'm sure that the local fire dept would appreciate the simplicity!

Panel Wiring & Potential Shading

The design shows all 12 panels on a single string from the Tesla inverter which has me concerned. The solar panels would be on the garage roof (NW corner of my house) and there is a cherry tree (for privacy, so cutting is not an option) that has some shading impact on the corner of the roof like so

View attachment 700451

I am not worried about the minimal shading on an individual panel but that little bit adversely affecting the *whole* system where there is *no* shade at any time.

I have tried to explain this at length and they just keep coming back, saying that there's nothing that can be done as they don't use power optimizers with Tesla inverter. I have told them that I'm not saying that they need to use optimizers, just what I am trying to avoid. My understanding is that the 3.8 kW inverter supports a 2nd string that could be used for the "concerned" panels?

Many thanks in advance :)
It is reasonable to put a single lockable AC disconnect in to do all ESS and PV disconnection at once. The issue with a large firm like Tesla is the designers want to make it easy across the country with various codes. The approach Tesla is taking is to just throw up a bunch of disconnects and it will never be wrong, but there are also more correct approaches, which use less equipment.

Make your request known to Tesla that you want as few disconnects as the code allows. If you aren't satisfied with the response, try a smaller installer. They will cater to your local market, and charge you more likely for the personal service.
 

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