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Powerwall questions (NEM, electrical internconnection limits, integration with existing solar)

TerranApart

Member
Apr 19, 2018
152
282
USA
I've been reading a lot of the powerwall documentation but having some trouble finding some answers to a few questions. Some of these are more esoteric and might need help from @miimura @wwhitney or other electrical engineering wizards. Thanks in advance to everyone for any insight you can provide.

Q1: Do powerwalls count toward the "120% rule" limit of the electric panel?
i.e. 200A rated electrical panel fed by 200A service can have up to 40A of solar connected, but are powerwalls also limited to the 40A envelope?
If yes, Does net metering have any effect on this? How does Tesla work around this and allow 6 powerwalls on a 200A service?

Q2: Do you know if Tesla can integrate an existing solar array into a powerwall install?
Example: I have a 3.4kW solar array now through an SMA inverter. Could I add a few powerwalls and 4kW of Tesla solar to the existing system, for a total of ~7.5kW solar?

Q3: Related to Q2...I have the existing 3.4kW solar array connected to a 100A (125A-rated) subpanel in the garage via a 20A breaker. Would new generation need to be connected to the same subpanel? Or could the new 4kW solar and powerwalls connect to a Tesla gateway with the 100A subpanel becoming a "partial home backup" panel while keeping its existing 20A solar?

This is still all hypothetical. The electrical engineering side of me wants to understand how this works for future home-improvement decisions. Thanks.
 

jjrandorin

Moderator, Model 3, Tesla Energy Forums
Nov 28, 2018
10,406
11,750
Riverside Co. CA
No one will add anything to the existing system. The 4kW system and your 3.4 kWexisting system will continue to be separate from each other, but can normally be combined to function as 1 "electrical" system and backed up by powerwall(s). Of course, every home is different, and what has to happen to do that may be different.

@arnolddeleon has 3 systems combined on one property.

Will let those more qualified take a stab at the other two questions.
 
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arnolddeleon

Supporting Member
Supporting Member
Jul 21, 2012
894
951
SF Bay Area
For question one you already invoked the names of people what would have a more definite answer but I will venture a guess that the short answer is yes, any generation will count towards the 120% rule. Dealing with that is I believe one of the features the Powerwall+ configuration. The straightforward answer on how to deal with the issue is via dedicated generation panel. My system with 3 Powerwalls is connected via a dedicated generation panel to the "home" side of the gateway. There are two sets of lugs on the gateway. The other set of lugs are connected to my main load panel. Each panel has breaker that limits the current flowing to or from it. I theoretically have 400A of supply but my load is limited to 200A so the 200A only gateway works.

For question two, the short answer is, it is technically possible, it's been done in the past. It is an open question if Tesla will do it now.

My understanding of question three is the solar can potentially stay where it is. Tesla might even prefer that. They would just need to put a CT on it so the production can be integrated properly. There are many possible configurations.
 
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miimura

Well-Known Member
Aug 21, 2013
6,494
6,255
Los Altos, CA
1: 120% rule is usually managed with a Generation Panel. This separates the grid supply and solar/PW supply so that they cannot overload a panel bus.

2: Existing solar will remain separate, but can be integrated in different ways. Depending on situation, it might be worthwhile to rewire it to land directly in a new generation panel instead of leaving it connected to the existing sub-panel. Alternatively, it can be measured with a remote Neurio meter.

3: This is pretty flexible. The only requirements are A) The solar should be connected to a backed up sub-panel. B) The solar has to be measured so that the PW system knows what to do with the generated energy. The placement of the Backup Switch or Gateway is dependent on other factors, not really dependent on the placement of your existing solar.
 
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Bitslizer

Member
Jun 16, 2021
224
79
IL
The Backup Gateway2 have an optional breaker circuit that can fit 6 double pole breakers, its very flexible what it can be used for.... as a backup/essential load panel, or as a generation panel. my system will be the latter, 1 PW+, 1 PW2, and a 3.8 Tesla inverters will be fed through that as the generation panel
 

Vines

Active Member
Jul 20, 2018
2,051
2,379
Silicon Valley, CA
1. While the NEC 2017 does not differentiate between generation sources, there is a difference between the PW and PV generation in that one can be controlled at any time by CT placement and Gateway 2 Power Control System settings. NEC 2020 does allow Power Control Systems (PCS) to act as a control over the backfeed to the busbar limits per 705.12. There are several ways around this but at the main service panel your options are usually limited.

However practically the Powerwalls will not backfeed to any loads on the grid side beyond any CT's, those loads they cannot see.
 

Bitslizer

Member
Jun 16, 2021
224
79
IL
1. While the NEC 2017 does not differentiate between generation sources, there is a difference between the PW and PV generation in that one can be controlled at any time by CT placement and Gateway 2 Power Control System settings. NEC 2020 does allow Power Control Systems (PCS) to act as a control over the backfeed to the busbar limits per 705.12. There are several ways around this but at the main service panel your options are usually limited.

However practically the Powerwalls will not backfeed to any loads on the grid side beyond any CT's, those loads they cannot see.
caveat being if the system is enrolled in a virtual power plant type program, where the utility can draw from your powerwall in certain peak load situation but they pay you for it. Still pretty rare in the USA though, I think Boston area maybe?
 
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Zinc_Saucier

Member
Jul 30, 2020
56
31
Massachusetts
caveat being if the system is enrolled in a virtual power plant type program, where the utility can draw from your powerwall in certain peak load situation but they pay you for it. Still pretty rare in the USA though, I think Boston area maybe?
I believe MA, NH, VT, RI, & CT all have utilities that run a program called “ConnectedSolutions” that allows for this. I know my power co. in MA does.
 
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