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Powerwall - Partial Back Up/Gateway Question

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Have 2 AC units and looking at a mid-size 8.16 KWH system with 2 Powerwalls. I understand that 2 PW's will only support 1 AC + rest of the home loads and as such my system will only have a partial back up configuration. My question: when there is not a power outage do the Powerwalls support the entire home/system? For example, if power from PG&E is up and I turn on both my AC's on while in a Cost Savings mode will the PW's feed both AC's? Pardon if this is an elementary question.
 
Agree with @arnolddeleon 's short answer.

@acalbear no such thing as an "elementary question" - everyone has different experiences and knowledge. (I am impressed by this forum's willingness to share both experience and knowledge which makes me happy to do the same.)

I have 2 ACs with a 17.64kwh + 3 PW2s system (details in signature) and what I did was to determine all load demands in my household. (I skipped smaller items like lamps and such.) But, for the bigger appliances like the ACs I gathered data sheets.

Basically, looked into amperage and voltage demands at start-up and operational stages to get kilowatts used during operation. Using AC amperage required at startup (LRA; locked-rotor amperage) and with a plan to install soft start devices I determined that for whole house backup I needed a minimum of three power walls.

Without pulling up my sheets I think one of my ACs was 80+A and the other was mid-70sA without the sure starts. Adding them reduced them to an assumed mid-30sA each. That demand is very very short in duration (milliseconds?), but can cause problems since the Powerwall2s are limited to ~7kw of peak power each. (Power = Voltage * Amperage [P=VxI] 16.8kW = 240V * (35A+35A); three powerwalls will handle peak of 21kw demand for a few seconds.)

What I ended up designing for was some amount of existing power draw and both ACs starting up with sure starts at the same moment and that put me at three powerwalls. In hindsight I do wish I opted for four powerwalls in the initial install. That desire is primarily driven by the other design consideration, how much capacity do I want in the batteries. I purchased for need and not want. In the future, as the batteries' retention declines I'll likely need to get a fourth battery to fill in the gap.

Tesla had one sure start on the truck that they installed. Not sure if that was by design or not on their part. When I asked about them installing a second sure start on the second AC they gave me basically a blank stare and not really a good answer. I am still not sure if the installer just did me a solid, didn't have the second sure start in the truck and didn't want to come back, or what. But, having the first sure start installed by them really helped with knowing what the wiring should look like for the second. I ordered a second unit and installed it a few weeks later. The install should be done by an electrician, but handy homeowners somewhat familiar with electricity, schematics, and wiring can get it done with some caution and time.

Back to your specific ask, if the grid is available it is a giant pool from which your house can make demands. During normal, grid available operation the powerwalls will provide your home with capacity, but in the situation that both ACs or other house demands exceed the capabilities of the powerwalls you will concurrently get partial demand satisfaction from the utility. In the app you would see solar production, battery supply, and grid all going into the house if you happened to catch it at the right moment.

In the event of a planned or otherwise grid outage you likely would want to turn off one of your ACs to minimize potential of the powerwalls being overloaded and turning themselves off or under powering (brown out) the house.
 
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Thanks for the responses so far. The way that I conceptualize the Gateway is that when the grid goes down, then only those items that are downstream of the Gateway are powered by the PW's - the other non-backed up items are effectively disconnected and not powered. When the grid is up, the gateway connects/closes the connection such that the entire household (backed up and non-backed up) are available to take advantage of the PW's. If this is correct, then the non-backed up AC will never tax the PW since its not connected under a situation when the grid is down - I think...

For additional context, the 2 AC units have and LRA of 83.
 
Thanks for the responses so far. The way that I conceptualize the Gateway is that when the grid goes down, then only those items that are downstream of the Gateway are powered by the PW's - the other non-backed up items are effectively disconnected and not powered. When the grid is up, the gateway connects/closes the connection such that the entire household (backed up and non-backed up) are available to take advantage of the PW's. If this is correct, then the non-backed up AC will never tax the PW since its not connected under a situation when the grid is down - I think...

For additional context, the 2 AC units have and LRA of 83.


Yes, what you describe is correct assuming the installer can agree to engineer it that way. But the challenge is finding an installer that is willing to do this configuration.

Tesla thought my house was too difficult to install solar so they wouldn't help me. But I spent months talking to local installers (East Bay) who insisted up and down that the scenarios you describe (when the utility was up and when the utility is down) were not possible. Instead they were willing to create a micro-sub panel to backup a few loads. Or, they'd require like 4x Powerwall to wrap the whole house (they said 4x was necessary for 2x 80 LRA ACs without soft starts).

But, the most frustrating thing I learned was that the installers refused to buy into the concept of putting CTs on the circuit feeding the ACs. I spoke with 5 different installers who insisted that with a partial backup, even with the utility up, there was no way Solar or Powerwall energy would find its way to ACs unless they were "behind" the Gateway. Basically they said if I did a partial home backup, my ACs would ALWAYS use utility energy only.

I can't pinpoint the exact reason why the installers made things so difficult, but I think it boils down to the concept that a partial home backup is rather new compared to what was possible under LG Chem, Generac, and Brightbox. And the behavior of the Tesla CT sensors was new... many installers weren't comfortable back-feeding clean energy from the solar and battery to the ACs.

@Vines was a godsend in my thread about this (wanted to partial backup with 2 ACs outside of the gateway)
Advice on whether this installation is possible

Anyway, moral of the story... try to go with @Vines if you're shopping for a partial home backup in the SF Bay area haha.
 
But, the most frustrating thing I learned was that the installers refused to buy into the concept of putting CTs on the circuit feeding the ACs. I spoke with 5 different installers who insisted that with a partial backup, even with the utility up, there was no way Solar or Powerwall energy would find its way to ACs unless they were "behind" the Gateway. Basically they said if I did a partial home backup, my ACs would ALWAYS use utility energy only.

I can't pinpoint the exact reason why the installers made things so difficult, but I think it boils down to the concept that a partial home backup is rather new compared to what was possible under LG Chem, Generac, and Brightbox. And the behavior of the Tesla CT sensors was new... many installers weren't comfortable back-feeding clean energy from the solar and battery to the ACs.

My home was built in 2005 and has the original 2 AC units (one in the 80s LRA and one over 100 LRA). Was originally told that 3 PW would be fine for all the loads in my home with AC if soft starts were installed, but then my installer could not find a HVAC sub that would warranty the work.

We ended up with a slightly different solution: installing Lumin SmartHome Home - Lumin

In technical design, I have a whole house backup, but the circuits I selected to go through the Lumin are then configurable both on time schedule, manual disconnect via the App or either protected / disconnected if the Grid goes out. I'm personally only using it in the case that the Grid goes out; it disables my Large AC double pole circuit. For me the huge upside of this is that as I upgrade one of my loads to be more efficient (ex. variable speed pool pump, high efficiency HVAC) I don't have to have any redesign of the panel / PV / PW setup.

It was a long road but I am very satisfied with the outcome. In my case, you only see the main panel on the outside of my home.
The 3 PWs and all the other panel work was done inside the garage (wall opposite the main panel).
 
My home was built in 2005 and has the original 2 AC units (one in the 80s LRA and one over 100 LRA). Was originally told that 3 PW would be fine for all the loads in my home with AC if soft starts were installed, but then my installer could not find a HVAC sub that would warranty the work.

We ended up with a slightly different solution: installing Lumin SmartHome Home - Lumin

In technical design, I have a whole house backup, but the circuits I selected to go through the Lumin are then configurable both on time schedule, manual disconnect via the App or either protected / disconnected if the Grid goes out. I'm personally only using it in the case that the Grid goes out; it disables my Large AC double pole circuit. For me the huge upside of this is that as I upgrade one of my loads to be more efficient (ex. variable speed pool pump, high efficiency HVAC) I don't have to have any redesign of the panel / PV / PW setup.

It was a long road but I am very satisfied with the outcome. In my case, you only see the main panel on the outside of my home.
The 3 PWs and all the other panel work was done inside the garage (wall opposite the main panel).


I like what Lumin is capable of, but I feel like it's not really necessary anymore with the Tesla Gateway 2. The amount of time it would take for someone to put your ACs on the "non backup loads" side of the Gateway 2 seems shorter than the time it takes to get Lumin set up and configured. Also, Lumin seems to add a level of complexity that I feel would be a tough sell to the experts doing the work.

In my situation...

Pre-Vines... all the partial-home backup installers just wanted to put backup loads behind the Gateway and left the ACs and other large loads on the outside. They wouldn't even try to backfeed the large loads ACs with the clean energy even if the utility were up. So Energy from the PV and Powerwall would never make it to the ACs and there was absolutely no reason to try something like Lumin.

Pre-Vines... all the whole-home backup installers said they didn't trust load shedding. It was 4x or more Powerwalls or else they'd switch to a partial backup solution using the above concepts.

Post-Vines... for the partial-home backup... installers finally agreed that what @acalbear described was possible.
 
Thanks for the responses so far. The way that I conceptualize the Gateway is that when the grid goes down, then only those items that are downstream of the Gateway are powered by the PW's - the other non-backed up items are effectively disconnected and not powered. When the grid is up, the gateway connects/closes the connection such that the entire household (backed up and non-backed up) are available to take advantage of the PW's. If this is correct, then the non-backed up AC will never tax the PW since its not connected under a situation when the grid is down - I think...

For additional context, the 2 AC units have and LRA of 83.
When the grid is up, the Powerwalls interact with the grid and and are given commands from the Gateway Computer how much to charge or discharge. In the most simple case, Self Powered Mode, the goal is to drive your electric meter to a zero reading. In order to do this, the system has to measure the power flow as close to the meter as possible. If you don't have any loads outside the Gateway Switch, ie. true Whole Home Backup, then nothing special needs to be done. In that case, the grid measurement at the grid input of the switch will be the same as the meter. If you have loads like an A/C compressor or EV charging circuit in the main panel outside the Gateway, then they need to install CT clamps (AC current measurement sensors) in the main panel to measure the real total grid power flow. My system is like this. I have the original Gateway 1 (gray box) and they simply removed the CTs from the factory installed location on the Gateway switch and extended the wires into my main panel. With the CTs placed this way, the Powerwall system can offset any usage whether the loads are backed up or not. The Gateway 2 is a more advanced device since it can have non-backup loads that are also measured by the factory installed CTs.

You are correct that if a load is not backed up, it will simply lose power when the grid goes down and the Powerwalls will continue to power only the loads that are on the Backup side of the Gateway.
 
The Gateway 2 is nice because now you can basically put the "generation panel" inside the gateway 2 and use the internal busbar... the internal breaker/busbar is also on the backed up side... So ideally the simplest thing would be to manually load shed yourself. you could use Smartthings with the code for the gateway and powerwall integration and just have a scene setup that if power goes out, then the smart outlet or switch tied into say a hot water heater would turn off, the thermostat would adjust a higher temp or turn off, etc... this would be a much cheaper method... you can put smart switches, outlets on pretty much anything. they even has large amp smart outlets...
 
Was originally told that 3 PW would be fine for all the loads in my home with AC if soft starts were installed, but then my installer could not find a HVAC sub that would warranty the work
I have 2 Powerwalls with partial backup but it was installed without AC. We had an outage for 5 days a few weeks ago, and it got really hot. The house was above 80F in one evening. Since then, I have looked at the two AC systems I had, and one of them had a scroll compressor which works best with a soft starter and LRA can be reduced by 60-70%. Unfortunately the only way to prove that LRA was reduced is to measure with an oscilloscope. The multimeters with inrush measurements have a high resolution of 100ms that cannot measure true LRA.

My other AC compressor was a reciprocating type and had a TXV and a long lineset. When I spoke with 2 soft starter manufacturers, one said it was not compatible, and the other said that they had a custom soft starter for reciprocating, but could only reduce LRA by 30-50% depending on how pressure is equalized on the lineset. I was more concerned about loss of torque at start, so I left it with a hard starter.
 
we will be doing a whole home as well and have a 3ton ac with an LRA of about 80. a soft start should bring it down into the 30's which will be more then sufficient for two power walls. i'm also changing out our hot water heater in the next couple of years with a hybrid... other then just using a toaster oven during outages instead of our normal oven. most of our large loads should be good
 
@holeydonut - isn't that what the non-backup lugs are for on the gateway? To allow those circuits that are not backed up, to receive clean energy when the grid is up?


Yes, for most applications you are correct. The non-backup lugs are there and would work great if you had a lot of non-backup loads on a panel. My house is weird though... it's more represented by the Gateway 2 installation manual page 55 (in PDF where its says 55 in the corner).
https://www.tesla.com/sites/default...erwall_2_AC_GW2_NA_EN_Installation_Manual.pdf

My ACs are forked off of the main as opposed to being on the same breaker panel as the house. So in my case, I need the CTs as shown on the pic to backfeed clean energy (when the utility is up) all the way upstream to the ACs.

Even with this diagram, folks just refused to install things the way Tesla indicated here. The installers were basically not running the CTs going to the non backup loads in the upper-right hand corner. And they wouldn't engineer the breakers/system to allow backfeeding.

That would effectively cause the Gateway to inefficiently balance energy use in my home since it'd lose sight of the activity happening with the ACs (even when the utility was online).

Basically the installers were telling me the only reason people get Powerwalls was for backup solution; they did not seem to believe me that the Primary thing I cared about was using my own energy as much as possible even after the sun set.

Anyway, I hope your installer is more adept than the ones I found. Because this experience with all these expert installers giving me weird designs was SUPER frustrating.
 
The information on this thread is quite educational. I now understand that the gateway non-backed up lugs serve two purposes: 1) to act as an internal CT and 2) to disconnect the non-backup circuits in the event the grid goes down. Also, per @holeydonut post and per Figure 24 in the PW install guide, it does look like the clean energy does "back flow" to circuits/loads upstream of the gateway but does so on a non-backed up basis.

I have a set up very similar to @holeydonut in that I have a main panel with several large loads (AC, furnace/blower, HPWC, Oven) and a sub-panel with the rest of the home loads/circuits. If I'm following so far, my entire home will benefit from PV and PW as long as the grid is up. So if I turn on the AC, it will utilize PW stored energy/PV and turn to the grid, if needed. If I wanted to back up any of the circuits on the main panel, I need to get enough PW capacity to support whole home which would put the Gateway in front of the Main panel and sub panel or (what follows is a question) - still have a Gateway downstream from Main Panel and then rewire those circuit/loads from Main Panel (e.g. - AC/HPWC) I want and place on a subpanel downstream of Gateway?

Is this a correct understanding?
 
Yes, for most applications you are correct. The non-backup lugs are there and would work great if you had a lot of non-backup loads on a panel. My house is weird though... it's more represented by the Gateway 2 installation manual page 55 (in PDF where its says 55 in the corner).
https://www.tesla.com/sites/default...erwall_2_AC_GW2_NA_EN_Installation_Manual.pdf

My ACs are forked off of the main as opposed to being on the same breaker panel as the house. So in my case, I need the CTs as shown on the pic to backfeed clean energy (when the utility is up) all the way upstream to the ACs.

Even with this diagram, folks just refused to install things the way Tesla indicated here. The installers were basically not running the CTs going to the non backup loads in the upper-right hand corner. And they wouldn't engineer the breakers/system to allow backfeeding.

That would effectively cause the Gateway to inefficiently balance energy use in my home since it'd lose sight of the activity happening with the ACs (even when the utility was online).

Basically the installers were telling me the only reason people get Powerwalls was for backup solution; they did not seem to believe me that the Primary thing I cared about was using my own energy as much as possible even after the sun set.

Anyway, I hope your installer is more adept than the ones I found. Because this experience with all these expert installers giving me weird designs was SUPER frustrating.

Preventing all these panel / wiring workarounds was my primary objective and why I pushed my installer so hard.
I was one of their first Solar + Battery installs, so they ate the cost of the Lumin. I didn't want any circuits left upstream of the gateway after I saw what a huge ordeal it was to move them. I didn't want to be caught in the future with a circuit that now COULD be backed up by the Powerwall because of change in LRA, specifically my AC units upon replacement was on my mind.

My main panel has just has the pass through to the Tesla Gateway.

Tesla Gateway is connected to the loads panel (contains breakers for Solar and Batteries) and the alternate "main" panel.

The alternative main then has one sub panel and some of the circuits in the main are then fed through the Lumin to provide the circuit control ability.
 
which would put the Gateway in front of the Main panel and sub panel or (what follows is a question) - still have a Gateway downstream from Main Panel and then rewire those circuit/loads from Main Panel (e.g. - AC/HPWC) I want and place on a subpanel downstream of Gateway?

Is this a correct understanding?

In my case, the original "main" panel (the one outside my house with the Meter / Utility feed in it) is essentially empty. It just passes through to the Tesla Gateway with very large wires. :)
Installed inside my garage is what I call the "alternate main" panel, which is just as big in circuit space as my now empty "main" panel on the exterior of the house. This means that every circuit had to be relocated from my original "main" panel to this new "alternative main" panel.

I have a "whole house backup" setup as far as Tesla is concerned, but that is where my Lumin comes in to disable certain circuits when the grid goes out.
 
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The information on this thread is quite educational. I now understand that the gateway non-backed up lugs serve two purposes: 1) to act as an internal CT and 2) to disconnect the non-backup circuits in the event the grid goes down. Also, per @holeydonut post and per Figure 24 in the PW install guide, it does look like the clean energy does "back flow" to circuits/loads upstream of the gateway but does so on a non-backed up basis.

I have a set up very similar to @holeydonut in that I have a main panel with several large loads (AC, furnace/blower, HPWC, Oven) and a sub-panel with the rest of the home loads/circuits. If I'm following so far, my entire home will benefit from PV and PW as long as the grid is up. So if I turn on the AC, it will utilize PW stored energy/PV and turn to the grid, if needed. If I wanted to back up any of the circuits on the main panel, I need to get enough PW capacity to support whole home which would put the Gateway in front of the Main panel and sub panel or (what follows is a question) - still have a Gateway downstream from Main Panel and then rewire those circuit/loads from Main Panel (e.g. - AC/HPWC) I want and place on a subpanel downstream of Gateway?

Is this a correct understanding?


To add more head-ache inducing complexity... as @Vines 's company was able to determine, a customer in California today actually gets access to SGIP incentives to get 3x or more Powerwalls. There is no economic benefit to pick 2x Powerwall at this time while the SGIP large-scale incentive program is funded. So, if your AC manufacturer allows soft starts, maybe you should just back up your whole house and be done with the mess of partial backup solution.

If you have many large loads on many different panels, maybe you can do what @MJ_CA_2019 says and incorporate Lumin. This would let you get the Manual's page 50 (Figure 20), but add that Lumin thing to load-shed heavy loads that couldn't be supported by the Powerwalls that you can afford.

Unfortunately even if I got 3x Powerwall, I would need to get soft-starts to include both AC condensers into the backup solution. And my AC manufacturer said my compressor warranty would be voided if I added soft starts. So, I have no choice but to leave my ACs on the non-backup side. Hopefully you can just use the 3x Powerwall to back up your whole house.
 
With new products there is always a bit of confusion as people figure it out.
Even the Gateway 1 had the ability to add CT to the main panel, or add CT just a few of the biggest loads so that they can be offset (for instance on a hard bused service without room for traditional Doughnut CT, or the expensive optional 600A rope CT for service cabinets on 600A and larger switchboards.

Setting up a partial home backup, with a 100A max non backed up loads, and 200A of backed up loads is the go-to as far as our company is concerned. We have engaged with several smart panel companies, but to this point we aren't looking for one more new addition. New customers who want these smart panels are usually asking for them specifically and so we do them as specials. The cost of an additional panel and wiring is pretty small, the difficulty of moving a breaker physically is also low.

Alternately leaving some small loads in the MSP can work as an alternate depending on the 100% rule calculation. THis works for homes with limited wall space.

Retraining our crews to install and program a new product, let alone supporting and programming it takes much more time. Once those smart panels mature a bit more we may change our decisions about this.

(what follows is a question) - still have a Gateway downstream from Main Panel and then rewire those circuit/loads from Main Panel (e.g. - AC/HPWC) I want and place on a subpanel downstream of Gateway?

Is this a correct understanding?

Yes your understanding is correct, but not the only way to design it. However its likely the best option from the sound of your project.
 
Have 2 AC units and looking at a mid-size 8.16 KWH system with 2 Powerwalls. I understand that 2 PW's will only support 1 AC + rest of the home loads and as such my system will only have a partial back up configuration. My question: when there is not a power outage do the Powerwalls support the entire home/system? For example, if power from PG&E is up and I turn on both my AC's on while in a Cost Savings mode will the PW's feed both AC's? Pardon if this is an elementary question.
Doesn’t seem many people answered exactly how to make sure non backed up loads show up but it’s important that both panels or an upstream panel have the ct clamps measure all usage. Said differently the Tesla gateway needs to see all usage of the house in both panels to offer battery use back to them. If you don’t see the usage the battery won’t discharge that amount back. Make sure to confirm with Tesla they install multiple neurio meters that monitor all usage and not just back up panel usage .