Undersized Transformer? Tesla wants to downgrade my powerwall?

[amagangan]

My system has been running for a few months, awaiting PTO from Xcel energy. Telsa has come back and said that Xcel has requested a transformer upgrade and that will be at my own cost. They have suggested that I replace one of my Powerwall+ with a Powerwall 2.1 and that my production will not be affected at all. However I am confused about this - how does this setup reduce the amount of energy sent to the grid but not affect my home system?

This is from Tesla:

"We have two options:

1. Keep the installed design and go through the transformer upgrade process. This could take 6-12 months and would be at your expense. The cost is unknown at this point. Usually, Xcel requires $1000 to complete the transformer upgrade.

2. Redesign to (1) Powerwall+ and (1) 3.8 + AC Powerwall to avoid upgrading the transformer. This would not affect the production of the system.

It would be a matter of lowering the backfeed into the utility grid. Not the in-feed to the home. This is all tied to the inverter and the AC rating of the system. If we use two Powerwall+ the AC inverter rating becomes higher than what the current transformer could support in terms of the backfeed whereas if we redesign the system to reduce the DC size to 11.6KW and AC rating to below the requisite threshold per the utility company we can proceed with the install. The reduction is in the backfeed to the utility company; not the feed into the home"

So my question: is there any downside to me going from 2 x powerwall+ to 1 x powerwall+ and 1 x powerwall 2.1?

 

[arnolddeleon]

My advice is take offer #2.

It looks to me they are relying the different specs of the different versions of the Powerwall to determine maximum back feed into the grid. That only thing that you are probably giving up is the ability to export all your solar production and all your Powerwall capacity at the same time.

I suspect that in the future Tesla could do this with just software instead of of having to replace the equipment. They probably have the capability to do that already and Xcel just doesn't accept it.

 

[Vines]

They are offering to reduce your inverter size. The powerwall + has a 7.6 kW inverter, and so when paired with the offered 3.8 stand alone inverter the utility will be satisfied.

Depending on your PV system size, this may have various affects to your production, I cannot imagine a case where this difference would be zero unless the initial design had much more inverter than needed.

 

[cwied]

My system has been running for a few months, awaiting PTO from Xcel energy. Telsa has come back and said that Xcel has requested a transformer upgrade and that will be at my own cost. They have suggested that I replace one of my Powerwall+ with a Powerwall 2.1 and that my production will not be affected at all. However I am confused about this - how does this setup reduce the amount of energy sent to the grid but not affect my home system?

This is from Tesla:



So my question: is there any downside to me going from 2 x powerwall+ to 1 x powerwall+ and 1 x powerwall 2.1?

Without knowing the size of your PV system, it’s hard to guess what the production impact would be. It sounds like they’re proposing reducing your total inverter capacity to meet the limitations of the transformer.

 

[arnolddeleon]

@Vines and @cwied have bring up the essential point about the PV capacity of both the inverter and panels.

If we take that Tesla's claim of no production difference as true then they likely installed more PV inverter capacity than needed. They likely used two PW+ for ease of installation.

 

[BGbreeder]

I may be wrong, but the way I read the note from Tesla is that option #2 does reduce the power of your array.

I don't see many people these reducing their electrical power usage. I think that with time, you are likely to want the extra power from the larger inverter. I would lean toward keeping the larger system and upgrading the transformer.

All the best,

BG

 

[MP3Mike]

It sounds like you have 11.6kW of panels, and you have two Powerwall+ that can handle 7.6kW each for a total of 15.2 kW. So you have more inverter capacity than you need.

The new system would be 11.6kW of panels, one Powerwall+, one Powerwall, and one 3.8kW inverter. So enough inverters to handle 11.4kW of panels, though inverters can usually support a little more than rated capacity. (And you would rarely be producing at panel rated capacity anyhow.)

So I would probably go with option #2, not because I didn't want to pay for the transformer upgrade but because I wouldn't want to wait for the transformer to be upgraded.

 

[slazinger_7]

Our PTO was also denied on the basis that the transformer serving the house is apparently too small.

Tesla wants to add some sort of "conductor limits" to the Gateway to limit the output of the Powerwalls through use of the Panel Control Settings. Tesla is claiming this would only affect the amount of power that could be exported to the grid, and not what the system generates or what we are able to consume.

Moreover, Tesla is also telling us that, since our PV system is smaller than the size of the transformer, the conductor limits won't affect the amount of solar we can export to the grid and it won't affect the system performance in any way since the Powerwalls don't export to the grid anyway.

Does anyone have experience with this problem and/or understand the actual consequence of the "solution" being offered by Tesla?

 

[h2ofun]

Our PTO was also denied on the basis that the transformer serving the house is apparently too small.

Tesla wants to add some sort of "conductor limits" to the Gateway to limit the output of the Powerwalls through use of the Panel Control Settings. Tesla is claiming this would only affect the amount of power that could be exported to the grid, and not what the system generates or what we are able to consume.

Moreover, Tesla is also telling us that, since our PV system is smaller than the size of the transformer, the conductor limits won't affect the amount of solar we can export to the grid and it won't affect the system performance in any way since the Powerwalls don't export to the grid anyway.

Does anyone have experience with this problem and/or understand the actual consequence of the "solution" being offered by Tesla?

And some said this could not happen. Another option are some or all panels have to be removed.

 

[BGbreeder]

Our PTO was also denied on the basis that the transformer serving the house is apparently too small.

Tesla wants to add some sort of "conductor limits" to the Gateway to limit the output of the Powerwalls through use of the Panel Control Settings. Tesla is claiming this would only affect the amount of power that could be exported to the grid, and not what the system generates or what we are able to consume.

Moreover, Tesla is also telling us that, since our PV system is smaller than the size of the transformer, the conductor limits won't affect the amount of solar we can export to the grid and it won't affect the system performance in any way since the Powerwalls don't export to the grid anyway.

Does anyone have experience with this problem and/or understand the actual consequence of the "solution" being offered by Tesla?

To me, the global question is why your power company is unwilling to upgrade the transformer, but moving on...

It sounds like Tesla is offering to set your export limit from the Gateway to be a level that your utility can live with and, furthermore, that the export limit level is more than your solar export, so you will get full credit for your solar power generation. If that works for utility, great!

The only potential fly in the ointment that I see is that the export limit has to be lower than solar and powerwall(s), (or the utility wouldn't be complaining), so in the event of a VPP, you might not (as in won't) be able to export everything. That isn't a big issue from where I sit, but you may feel differently. If you do, I would take the PTO as it stands, and explore what upgrading the transformer will cost you. I suspect that the upgrade won't be free, or they would be offering it now, so be prepared.

All the best,

BG

 

[STS-134]

Our PTO was also denied on the basis that the transformer serving the house is apparently too small.

Tesla wants to add some sort of "conductor limits" to the Gateway to limit the output of the Powerwalls through use of the Panel Control Settings. Tesla is claiming this would only affect the amount of power that could be exported to the grid, and not what the system generates or what we are able to consume.

Moreover, Tesla is also telling us that, since our PV system is smaller than the size of the transformer, the conductor limits won't affect the amount of solar we can export to the grid and it won't affect the system performance in any way since the Powerwalls don't export to the grid anyway.

Does anyone have experience with this problem and/or understand the actual consequence of the "solution" being offered by Tesla?

And some said this could not happen. Another option are some or all panels have to be removed.

Aren't transformers two way devices? If they can deliver a certain amount of power to you, they should be able to take the same amount of power from you. Adelman reported that PG&E was complaining about the transformer not being able to take 200A from him (despite the fact that it could deliver 200A to him) and he called BS on their claim, and smoked their transformer: Incoming utility cable capacity - A cautionary tale It sounds like PG&E is too cheap to upgrade transformers and is coming up with all kinds of excuses not to do it. Is the transformer undersized for your service? If so, then fry it and force them to replace it.

 

[jjrandorin]

Moreover, Tesla is also telling us that, since our PV system is smaller than the size of the transformer, the conductor limits won't affect the amount of solar we can export to the grid and it won't affect the system performance in any way since the Powerwalls don't export to the grid anyway.

I certainly can not speak to the technical aspects, but the "wont affect system performance since powerwalls dont export to the grid anyway" statement gives me pause. This is because, recently, many of us were, in fact, given the ability to export powerwall power to the grid (export solar only / export everything slider).

I dont know under what circumstances that is enabled for some and not others, etc. Not going down that rabbit hole, however, the statement that "it doesnt impact you because the powerwalls dont export" seems to "imply" that "if you did export powerwall power, this would impact you". This means you may not be able to export powerwall power.

if this doesnt matter to you, thats one thing. I dont currently intend to participate in any VPP program, and I currently neither import grid power, nor export powerwall power. with the above being said, I love choices, and I "could" if I so desired to. "could" means a lot to me, even if its something I dont think I particularly want right now. I have spent quite a bit of money on "could" (appliances, TVs, etc).

I only explain that to say that to me " could" has value, so it would bother me that I couldnt export powerwall power, even if I didnt want to do it right now. I would want to get that nailed down, as to why that statement was made.