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Single Powerwall Backup Entire 200amp panel?

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I'm in Florida, I ordered a 7.2kw system with a single Powerwall. I've been informed by Tesla that they won't be installing a sub panel. The single Powerwall will backup the entire house. This sounds great unless it's (worst case) the middle of summer, it's dark and I'm using the oven and charging the car (50amp circuit) when the lights go out. I'm expecting it to trip almost right away and drain fast if the central AC is running with nothing else running. My home is new(er) built within the last few years and my central AC is efficient yet I'm highly skeptical that a single Powerwall will run the main AC longer than a hour or two if the sun isn't out.

While I'm sure they know what they are doing I almost feel like they (the designers) are doing everything possible to cut costs and corners. I wonder what the impact of this system design will have on the Powerwall long term.
 
On the AC side, it’s possible to wire a relay to the gateway so that when the power is out the AC won’t run. Now if that’s an ok limitation for you, it’s a path forward without having to put in a new panel. Not so sure on the EV side (especially if you don’t have a Tesla EV).
 
I'm in Florida, I ordered a 7.2kw system with a single Powerwall. I've been informed by Tesla that they won't be installing a sub panel. The single Powerwall will backup the entire house. This sounds great unless it's (worst case) the middle of summer, it's dark and I'm using the oven and charging the car (50amp circuit) when the lights go out. I'm expecting it to trip almost right away and drain fast if the central AC is running with nothing else running. My home is new(er) built within the last few years and my central AC is efficient yet I'm highly skeptical that a single Powerwall will run the main AC longer than a hour or two if the sun isn't out.

While I'm sure they know what they are doing I almost feel like they (the designers) are doing everything possible to cut costs and corners. I wonder what the impact of this system design will have on the Powerwall long term.
1 Powerwall can backup your whole house if you manage your loads, but this is not very much energy. I would not run an AC unit or charge the car, unless I had excess PV production and was looking for a place to use it during the day.

FYI, the Tesla cars will not pull power off-grid, unless you let them. They have a setting in the powerwall app that is the reserve percentage, below which the car will not charge. I have mine set at 90% so only a very full Powerwall battery will discharge and only the top 10% will discharge.
 
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I have a single PW, gas heat, electric dryer, and an EV. I can run the AC when the sun is shining. If we're in a power fail mode, I'd not do laundry or run the AC outside of the 2-4 hours of peak solar generation if the grid was down. Power has bee remarkably stable since installing the system last fall, so never tested in real world conditions.
 
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I think I'll be ordering another Powerwall in a year or so. Management seems to be key with the limitation I have, I was expecting a subpanel to deal with this but now I think about it I kind of like this. When the system went online I was using solar plus the battery to run the AC, charge the car with minimal being pulled from the grid. I think I may keep a window AC on standby for any long term outages as without solar my main AC will drain the single Powerwall fairly quickly.
 
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I think I'll be ordering another Powerwall in a year or so. Management seems to be key with the limitation I have, I was expecting a subpanel to deal with this but now I think about it I kind of like this. When the system went online I was using solar plus the battery to run the AC, charge the car with minimal being pulled from the grid. I think I may keep a window AC on standby for any long term outages as without solar my main AC will drain the single Powerwall fairly quickly.
you can def heavily manage a single pw as you said ... but if you want a 2nd i would HiGHLY recommend you get it at same time if at all financially possible ... always read ppl on here regreting not getting enough pw's due to cost or avail of adding them later ....never read anyone say i bought too many 😝
 
you can def heavily manage a single pw as you said ... but if you want a 2nd i would HiGHLY recommend you get it at same time if at all financially possible ... always read ppl on here regreting not getting enough pw's due to cost or avail of adding them later ....never read anyone say i bought too many 😝

In general its cheaper to get it now than try to get it later.

 
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I think I'll be ordering another Powerwall in a year or so. Management seems to be key with the limitation I have, I was expecting a subpanel to deal with this but now I think about it I kind of like this. When the system went online I was using solar plus the battery to run the AC, charge the car with minimal being pulled from the grid. I think I may keep a window AC on standby for any long term outages as without solar my main AC will drain the single Powerwall fairly quickly.
I had put in an order for more solar + one Powerwall a few months ago, and while I decided to cancel before moving into detailed design phase, I preferred to be offered whole house backup with one Powerwall. I wasn't sure if they were going to offer it though. You have more options to manage your load, not less, without a subpanel, so I think it's better.

If you don't want your A/C to drain your Powerwall's, you can choose to turn it off, it's fully in your control. I think the Gen 3 wall chargers can also communicate with the Gateway to not drain your Powerwall. And also, your electric oven mostly draws a huge load the first few minutes, then cycles occasionally - so unless you're baking cookies all evening, it's highly unlikely it's cycling on right when an outage hits, giving you ample time to choose whether or not to turn the oven off.
 
Code says your backup source needs to be able to power all expected loads, to me that means all loads the automatically run or are always on. So that would be ac/heat, hot water, well pump, etc; and not microwave, oven, laundary, etc.
I do not believe this is correct for an optional backup system under 2017 NEC code. Section 702.4.B.2.b states:

In the 2020 code I understand it might not be as clear, but I haven't researched that myself.

From 2017 NEC:
(2) Automatic Transfer Equipment. Where automatic transfer
equipment is used, an optional standby system shall comply
with (2)(a) or (2)(b).
(a) Full Load. The standby source shall be capable of
supplying the full load that is transferred by the automatic
transfer equipment.
(b) Load Management. Where a system is employed that
will automatically manage the connected load, the standby
source shall have a capacity sufficient to supply the maximum
load that will be connected by the load management system.


Since the Powerwall has a load management system it will not enter an overload situation. The Gateway and Powerwall will sense an overload state and trip the Powerwall off.
 
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Since the Powerwall has a load management system it will not enter an overload situation. The Gateway and Powerwall will sense an overload state and trip the Powerwall off.
While I agree your NEC citation is on point, I wouldn't say "tripping off on overload" constitutes a load management system. The load management system is supposed to control the connected load to keep it below the inverter capacity.

I think this section is routinely violated by PW installs.

Cheers, Wayne
 
While I agree your NEC citation is on point, I wouldn't say "tripping off on overload" constitutes a load management system. The load management system is supposed to control the connected load to keep it below the inverter capacity.

I think this section is routinely violated by PW installs.

Cheers, Wayne
I think this point isn't super well defined, however this isn't a breaker that is tripping, I should have been more clear. It is an accurate measurement of the inverters ability to serve loads or not, based on accurate computer control. It is also not an overload as it relates to a motor circuit.

The internal power control system recognizes when it is safely loaded and when it is overloaded. When the connected load is beyond the inverter capacity it ceases to serve the load.

How is this different than other load management systems? Do you think the code requires discrete control over every circuit in the optional backup system? I do not see "load management system" as a defined term in the NEC, so we are left with whether the jurisdiction approves it.

I went to the mat about this in Palo Alto, and the head electrical inspector agreed with this interpretation.
 
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The internal power control system recognizes when it is safely loaded and when it is overloaded. When the connected load is beyond the inverter capacity it ceases to serve the load.
Right, but the text from the NEC says "Where a system is employed that will automatically manage the connected load, the standby source shall have a capacity sufficient to supply the maximum load that will be connected by the load management system."

That describes a system which connects and disconnects loads as required to keep the connected load smaller than the standby source capacity. E.g. less than 5 kW continuous for a single Powerwall. So load shedding.

I guess you could say that the PW has a system to do that, it just sheds all loads if the connected load gets too large, and 0 kW < 5 kW. So perhaps it complies with the letter of the rule.

But the spirit of the rule would be to disconnect specific loads to keep the load below 5 kW continuous. The "all or nothing" disconnection could easily lead to ping-ponging, where the system tries every 5 minutes (or whatever time period) to run the house but gives up immediately. Although I don't actually know how the PW will behave in that scenario.

Cheers, Wayne
 
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Right, but the text from the NEC says "Where a system is employed that will automatically manage the connected load, the standby source shall have a capacity sufficient to supply the maximum load that will be connected by the load management system."

That describes a system which connects and disconnects loads as required to keep the connected load smaller than the standby source capacity. E.g. less than 5 kW continuous for a single Powerwall. So load shedding.

I guess you could say that the PW has a system to do that, it just sheds all loads if the connected load gets too large, and 0 kW < 5 kW. So perhaps it complies with the letter of the rule.

But the spirit of the rule would be to disconnect specific loads to keep the load below 5 kW continuous. The "all or nothing" disconnection could easily lead to ping-ponging, where the system tries every 5 minutes (or whatever time period) to run the house but gives up immediately. Although I don't actually know how the PW will behave in that scenario.

Cheers, Wayne
When a Powerwall is overloaded (assuming no on-site ICE generator) it requires the user to either flip the switch manually, or wait until the normal reset behavior. Every hour between 8am and 4 pm the TEG will wake up the powerwalls for 6 minutes. While this might lead to one "ping-pong" per hour it wouldn't be offensive, and wouldn't violate the code.

OTOH, If the system has a backup generator to backup the ESS, then the generator would take over and the Powerwalls would require a manual reset to reconnect. The PW would never wake themselves up without user intervention.

While what you say makes sense, some work is needed to NEC code language if the intention is that the load management system must have discrete control over every circuit that it is connected to, and try to run the optional system with as large a load as possible, but just under the limit. This description sounds like a legally required standby system or a critical ops power system.

These Powerwall systems are optional backup systems, so the definition assumes that loss of power has already happened, and any power you can give the home needs to be safe, but is not required to be constant or reliable. There is no requirement that the system must try to continue to run right at its 5 kW limit, when you load it to 10 kW.

It would not be allowed to install any single load that could NEVER start up (like an AC with 150 LRA and a 50A breaker) with 1 powerwall. This violates the manufacturers installation instructions.
 
While what you say makes sense, some work is needed to NEC code language if the intention is that the load management system must have discrete control over every circuit that it is connected to, and try to run the optional system with as large a load as possible, but just under the limit.
Thanks for the explanation of the PW behavior on overload. I wouldn't say the intention is for discrete control over every circuit--up to 5 kW (per load calculation) of circuits could be permanently powered, and any load beyond that would need to be sheddable.

But on reflection, while that may (or may not) have been the intention, I guess I wouldn't argue that the current wording precludes your interpretation.

Cheers, Wayne
 
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Thanks for the explanation of the PW behavior on overload. I wouldn't say the intention is for discrete control over every circuit--up to 5 kW (per load calculation) of circuits could be permanently powered, and any load beyond that would need to be sheddable.

But on reflection, while that may (or may not) have been the intention, I guess I wouldn't argue that the current wording precludes your interpretation.

Cheers, Wayne
Like many parts of the code, ESS have gotten popular well before the NEC codes really caught up fully with the technology.

To me, it seems practical that if your options are to have no power in an outage or to have limited power then limited power is better than nothing. As long as there are no safety concerns and the equipment isn't in a ping pong cycle endlessly where it is sure to break eventually, I am of the opinion that customers can manage the connected load themselves.

OTOH, what happens when the power goes out and you arent home, but you left your AC on? The NEC has opinions on these things for good reason.

The NEC is stronger than my opinion and I need to look forward and understand what the 2020 NEC code says about all this.

2020 NEC Code adds a qualifier to the otherwise very similar language, see new text added in Bold Underline

2020 NEC 702.4.B.2
(2) Automatic Transfer Equipment. Where automatic transfer
equipment is used, an optional standby system shall comply
with (2)(a) or (2)(b)
in accordance with Article 220 (load calcs) or another approved method.
(a) Full Load. The standby source shall be capable of
supplying the full load that is transferred by the automatic
transfer equipment.
(b) Load Management. Where a system is employed that
will automatically manage the connected load, the standby
source shall have a capacity sufficient to supply the maximum
load that will be connected by the load management system.


So it looks like the NEC is moving in this direction, recognizing that you cannot power a whole 6 bedroom house in Atherton with a single 5 kW inverter in a Powerwall 2 unless approved.

It is interesting that the code states that if you have manual transfer equipment instead of automatic then you can size the system
2020 NEC 702.4.B.1 "Where manual transfer equipment is used, an optional standbysystem shall have adequate capacity and rating for the supply of all the equipment intended to be operated at one time. The user of the optional standby system shall be permitted to select the load connected to the system"
 
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It is interesting that the code states that if you have manual transfer equipment instead of automatic then you can size the system
2020 NEC 702.4.B.1 "Where manual transfer equipment is used, an optional standbysystem shall have adequate capacity and rating for the supply of all the equipment intended to be operated at one time. The user of the optional standby system shall be permitted to select the load connected to the system"
Sure, because with a manual transfer system, the proper way to use it is to (a) shut off all the branch breakers (b) do the manual transfer (c) turn on the breakers that you want to power. So the user is the load management system that will keep the connected load below the power source rating. In theory.

Also, in the 2023 NEC cycle, there was a proposal (Public Comment No. 2120-NFPA 70-2021) to add a section to 702.4(B)(2) which would read:

"(c) Protected Equipment. Where the standby source(s) has demonstrated through its listing that in overload conditions it shuts down safely without causing breakers to operate or loads to be energized outside of suitable voltage and frequency limits, the standby source capacity shall be permitted to be less than the maximum connected load."

The panel rejected the change with the response "The panel understands the concept of the commenter but having an undersized standby source with manual load shedding creates problems for system reliability. Having manual load shedding in a requirement for automatic transfer does not correlate."

As the proposed section (c) describes behavior closely matching the PW overload behavior, and the proposal was rejected, it is clear that the intent is that the PW behavior alone is not sufficient to comply with 702.4(B)(2), and a load management system is required. A rule most installs break.

With a free account on nfpa.org, you can read all the details within their TerraView system by going to "Next Edition" on the page nfpa.org/70, and then to First Draft Report or Second Draft Report.

Cheers, Wayne
 
Sure, because with a manual transfer system, the proper way to use it is to (a) shut off all the branch breakers (b) do the manual transfer (c) turn on the breakers that you want to power. So the user is the load management system that will keep the connected load below the power source rating. In theory.

Also, in the 2023 NEC cycle, there was a proposal (Public Comment No. 2120-NFPA 70-2021) to add a section to 702.4(B)(2) which would read:

"(c) Protected Equipment. Where the standby source(s) has demonstrated through its listing that in overload conditions it shuts down safely without causing breakers to operate or loads to be energized outside of suitable voltage and frequency limits, the standby source capacity shall be permitted to be less than the maximum connected load."

The panel rejected the change with the response "The panel understands the concept of the commenter but having an undersized standby source with manual load shedding creates problems for system reliability. Having manual load shedding in a requirement for automatic transfer does not correlate."

As the proposed section (c) describes behavior closely matching the PW overload behavior, and the proposal was rejected, it is clear that the intent is that the PW behavior alone is not sufficient to comply with 702.4(B)(2), and a load management system is required. A rule most installs break.

With a free account on nfpa.org, you can read all the details within their TerraView system by going to "Next Edition" on the page nfpa.org/70.

Cheers, Wayne
So, in theory, if you can configure the Tesla Powerwall to require a manual transfer on power outage it would be compliant?

Seems simple enough, but honestly looks like going backward. I guess slow code development is safer, and certainly without our code community, we would have many more failures and industrial accidents. Practically there may be ESS systems that are harmed by overload.

That question of whether the product is tested to be safe under overload seems more appropriate for UL Standards Development to take on. Maybe the CMP didn't think that UL was addressing this potential issue to their satisfaction.

It will be interesting to see whether jurisdictions will start requiring load calcs to run your backup system as if the grid was up or some lower reasonable standard. I could see Tesla offering a software setting to just drop the loads on power outages and require manual transfer through the enable switch or even through the app.

Either way, we only recently started allowing installations of just 2 units. We will probably see how other contractors are dealing with this when they are installing just 1 unit before it's a show stopper for us. It will be interesting to see what the IAEI trainers will be saying about this, as the codes start changing over soon.