Load Calc, Backfeed Calc and Panel Upgrade Options

[sarge_in]

I am currently in the process of getting solar from a local company (did not work out with Tesla due to type of roof, and I wanted microinverters anyway). I had placed the order for 2 Powerwalls as well, but that will now be considered next year or later. I am in SCE territory, and wanted to review my load calcs, backfeed calcs and panel options here.

Current Panel = 200A main breaker, no idea about bus as there is no label anywhere.
Objectives:
a) 60A for solar (34 x 360W LG panels, so 12.24kW) now
b) 14-50 outlet or hard-wired charger for EV (planned for near future)
b) 2 Powerwalls at sometime in future

What I have understood from the local company is that total backfeed cannot be more than 120% of the bus amp rating. So assuming 200A bus, max backfeed = 240A.
And 60A for solar + 30A*2 for 2 powerwalls = 120A backfeed. --> Is this correct?

Load Calcs and Backfeed Calculations
=====================================
Tesla did load calc with Optional way using breaker values, and came up with 123A (without EV outlet). Proposed (I think) derate to 125A, 60A for solar, 60A for 2 powerwalls = 245A backfeed. This is >120% of 200A panel, so not sure if that was going to be feasible.

My load calcs - Did load calcs both ways using appliance nameplate ratings, Standard and Optional. After including 48A for the 60A breaker for hardwired Tesla EV charger, I came up with these numbers:
1) Standard = 171A
2) Optional = 147A

So I see the following options for my electric panel:
1) Keep current 200A panel (max 240A backfeed):
a) Derate to 150A, 60A for solar, 30A for only 1 powerwall = 240A backfeed
2) Upgrade main panel to 200A/225A (max 270A backfeed):
a) Derate to 150A, 60A for solar, 60A for 2 pwerwalls = 270A backfeed
b) Derate to 175A, 60A for solar, 30A for 1 powerwall = 265A backfeed
c) No derate at this time so use full 200A, 60A for solar = 260A. When get solar choose either of 150A or 175A derate options at that time.
3) Upgrade main panel to 400A. Waiting for meter spot from SCE to find if it is even feasible.
4) Go the GMA route, upgrade panel later when get powerwalls.


Does the above reasoning and calculations sound correct (assuming load calc is ok)? I find it hard to believe that all homes with similar-sized solar and 3 or more Powerwalls have 400A service? What am I doing wrong here?
If it matters, I have 40A breaker for a sub-panel for spa. I don't need to backup this or the EV outlets if/when I get the Powerwalls.


Which option would you recommend for the electric panel?

 

[sarge_in]

Hello...any thoughts or recommendations?

 

[Get Real]

If I were you I would upgrade your service panel to a solar ready 200 amp/225amp bus bar since you are going all out anyways. They also typically have lots more slots for breakers Compared to Most home service panels which is nice if you want to add Additional circuits in the future

it will A bit cost for the labor but these panels can be had from Home Depot or Lowe’s for under $200:
Square D Homeline 200 Amp 30-Space 42-Circuit Outdoor Ring-Type Semi-Flush Mount Solar-Ready Main Breaker Plug-On Neutral CSED-SC3042M200PF - The Home Depot

 

[CrazyRabbit]

I believe the 120% rule allows 120% for bus load (load and generation added together) when bottom feeding generation.
I would just put in a 200 amp generation panel or use the optional gateway bus.
You don’t state which enphases inverter. Do use their compatibility tool.

 

[SoundDaTrumpet]

Greetings @sarge_in,

Here some recommendations/insights:
1. Contact your next door neighbor and get a photo of the panel label. I assume your home was part of a tract development with an identical panel next door. Without such information, it will be difficult to advise.
2. Your backfeed sum of 120A is correct.
3. Did Tesla truly “derate” you to 125A? They may have simply put the backed up loads along with the Powerwalls on a branch breaker, which typically are available at the maximum 125A. This is how it is done for my home. Partial backup... call it what you will... that excludes the EV chargers and A/C units.
4. Your load calculations are very typical at 120-170A.
5. Option c. Let your solar folks get in and get out. The Powerwall folks will scramble up what the solar folks did and set you up properly. I find it laughable how small solar backfeed is in comparison to solar + Powerwall backfeed. Derating with smaller main breakers is very straight forward, but there no reason why an installer will derate it for you without good reason. Main breakers cost a $100-200, and requires effort and safety prep to pull the meter for a main breaker swap, which is extra work scope.
6. 400A upgrade could be pricey if you have underground service and the test mandrel fails to pass thru the conduit due to tree roots. Cost major reason for me to abandon 400A. My inspector did not make it an issue even though my NEC load calcs were over 200A when I told him I have two EV chargers installed. (I even bought “standby power interlock breaker kit” for the occasion only one EV charger can be used.) Overhead service removes much of the ambiguity.
7. The reason why you see folks w/o 400A service isn’t about backfeed. Nearly all of the 400A folks on this forum have 50/50 dual 200A bus bars. I researched an found exactly 1 compliant true 400A panel. The 400A is for getting more service to charge EVs and really large homes. The backfeed is question you have is solved by adding more subpanel downstream towards the load. The Tesla Energy Gateways (TEG v1 / v2) have 200A busses and the backup panels have 200A busses. One simply protects the service panel using a circuit breaker to limit loads to the the service panel (sum of breakers <200A) and put a “do not add loads sticker.” Excuse the brevity.

I hope this helps.

 

[miimura]

Let the solar people do their thing and likely derate your main breaker to handle the 120% rule. Powerwalls have many more considerations and more ways to handle the issue. Don't worry about it now.

My advice would be different if you were on 100A service and were going to get a main panel upgrade and service upgrade just for the solar. In that case, I would want to make sure the new panel would be workable for Powerwalls too.

 

[sarge_in]

Thanks you all, this gives me a lot to think about!

The obvious reason I am considering the panel upgrade right now is to get the 26% off as part of solar install, especially if I decide to hold off on Powerwall till after next year. Other part is that I will be pretty much out of spaces for any additional circuits after this project, so might as well make some space and save some money while doing that.

Will try to get label pic from a neighbor's home - good idea!
Valid points about 400A ending up being quite expensive. SCE did approve 400A at same location, but also mentioned "No new load" - any idea what that implies? I guess that would still be dual 200A bus bars...any recommended panels for this application?

Seems it may come down to cost. The current difference between 200/225A and 400A was ~$1,200. Is that reasonable? And would this affect any of the recommendations? Am I able to change mind at time of install if it is indeed discovered that tree root swill cause it to be way more expensive?

Btw, I am getting the Enphase IQ7A Microinverters, and here is the diagram Tesla provided:

 

[BrettS]

Btw, I am getting the Enphase IQ7A Microinverters, and here is the diagram Tesla provided:

So in that diagram from Tesla, they are planning to keep your existing main service panel, so it looks like you would not need to upgrade that panel at all for your powerwall/solar install. The way solar connects to your electrical system without powerwalls is different than how it connects to your system with powerwalls, especially when you have a main panel that also contains your meter, like you do. Because the solar and your loads wind up behind the gateway then you don’t need to worry about the 120% rule for your meter main panel in a system with powerwalls.

Based on that, then I would suggest that you try to do what you can to make your existing panel work with solar for the few months that you have until you install the powerwalls, even if that means derating the panel.

Otherwise you’re going to wind up spending $3,000 on a new panel that you don’t need to save $1500 by installing solar in time to take advantage of the 26% tax rebate. It just doesn’t make financial sense.

 

[SoundDaTrumpet]

@sarge_in The Tesla diagram shows use of a 125A branch breaker. That is exactly what Tesla did to my home. This is actually a “value” configuration in my opinion. In 2019, it was a $900 installation from a 2017 contract. The exception is you will be getting the new Tesla Energy Gateway (TEG2) which allows you to land Powerwall (PW) and Solar PV breakers instead of having an extra panel, which is a cleaner install, but you do lose breaker space. Legacy TEG1 owners usually included an Eaton 200A compact load center. It is smaller than the Murray 125A load center stocked at Home Depot that it replaced. SunWorks installed the Murray sub-panel to make more space in my service panel. My service is a main lug only (MLO) panel (i.e. no main breaker), so there’s a NEC requirement that limits the panel to maximum of 6 circuit breaker throws. The conversion kit to add a main breaker is obsolete/unavailable. That said, I find it very useful to add a garage subpanel to free up space in the service panel. The garage subpanel is a great place to land breakers for EV chargers, future heat pump water heaters (next SGIP incentive), etc. The solar installer or for your future projects could employ quad breaker / half width breaker to replace your full-size breakers to free space.

For 400A service upgrade, the utility (at least PG&E) requires at least $1000 engineering fee, and provides ~$1900 in offset credit when project is complete. There can be a service interruption for a few days if roots get in the way. If any trenching is involved, it’ll be an instant $1000+, an the electrician or utility (more costly) will have to immediately respond. The utility will set you up with temporary power, but I don’t know if that is an extra charge. The 3” conduit is required which I have, but a 400A panel I had in mind would not fit where the conduit came out of the ground to clear the gas meter by 18”. The bend radius is huge on the 90 degree elbow, so even with the best case of trenching not required, I still need to break concrete to get conduit to line up with the service entrance of the wider larger panel. If your service panel is on the right side of the house, you would be in better shape. My service panel would have grown towards the gas service. Service conduit not allowed to jog or S-bend. Too rich for me. 400A upgrade abandoned. I do not regret the decision for my home.

I am working on a service upgrade for another house (underground service, relocate panel, 400A, unoccupied residence, 2nd meter for ADU). It will be easily exceed $10k.

 

[sarge_in]

Thank you both, that was very helpful information and really helped me firm up my decision.
Also seemed like the SCE approval was just for the 400A panel upgrade, not service. Bottom line, I have abandoned the 400A upgrade but decided to still get the 200A/225A panel upgrade even though it is not critically required at this time - so option 2c in my first post. For ~$1700 after tax rebate, much more palatable financially.

A couple follow-up questions:
1) Any recommendations for the 200A/225A combo panel, that would also work with Powerwalls whenever I decide to get them? Perhaps something with high number of spaces and also high number of max breaker size for sub-panel if needed in future.
2) Install PV combiner box for the Microinverters outside or inside garage (just behind the main panel wall)? Will it be easier if it is inside for future Powerwall installation?
3) Should I think about getting Sense (mixed reviews on Amazon) and/or whole house surge protector?

 

[BrettS]

A couple follow-up questions:
1) Any recommendations for the 200A/225A combo panel, that would also work with Powerwalls whenever I decide to get them? Perhaps something with high number of spaces and also high number of max breaker size for sub-panel if needed in future.

As I mentioned above, powerwalls connect to your system in a completely different way than solar does. If you are sticking with a panel that has the meter base as part of it, then a high number of spaces won’t matter at all because all your loads will be moved out of that panel and put in another panel behind the gateway (assuming you’re going with whole home backup). What will matter is the ability to put a large breaker on the bus to feed the gateway. If you have 200A service you will need a panel that allows you to use a 200A breaker or 200A feed lugs. If you only have 125A service then you will need to make sure your panel will support a 125A breaker or 125A feed lugs.

Alternately (and this might make more sense) you could switch to a standalone meter base and separate panel. That way when they install your powerwalls they can connect the gateway directly to your meter and then leave your loads in the existing panel and just put it behind the gateway.

2) Install PV combiner box for the Microinverters outside or inside garage (just behind the main panel wall)? Will it be easier if it is inside for future Powerwall installation?

Not sure this matters too much as long as it’s reasonably easily accessible to run new cables to.

3) Should I think about getting Sense (mixed reviews on Amazon) and/or whole house surge protector?

There’s a whole different thread here on the Sense, but my opinion (as well as the general consensus) seems to be that it doesn’t provide a lot of benefit beyond monitoring your whole house power usage. And you’ll be able to monitor that with the Tesla app once your powerwalls are installed (and quite possible also with the enphase app once your solar is installed). I don’t think there’s a need for a Sense.

 

[BrettS]

Just realized I forgot to comment on the whole house surge protector. There’s also another thread on that. It’s certainly not required as part of a solar and/or powerwall install, but it may be a good idea. After my install I wound up adding two whole house surge suppressors. One on the panel where my power came in from the utility and one on the panel with the breakers for my inverters and powerwalls.

 

[sarge_in]

As I mentioned above, powerwalls connect to your system in a completely different way than solar does. If you are sticking with a panel that has the meter base as part of it, then a high number of spaces won’t matter at all because all your loads will be moved out of that panel and put in another panel behind the gateway (assuming you’re going with whole home backup). What will matter is the ability to put a large breaker on the bus to feed the gateway. If you have 200A service you will need a panel that allows you to use a 200A breaker or 200A feed lugs. If you only have 125A service then you will need to make sure your panel will support a 125A breaker or 125A feed lugs.

Alternately (and this might make more sense) you could switch to a standalone meter base and separate panel. That way when they install your powerwalls they can connect the gateway directly to your meter and then leave your loads in the existing panel and just put it behind the gateway.

Thanks @BrettS , that makes sense.
Question - Would the recommendation change if there are some loads I do not want/need to be backed up - e.g. the spa sub-panel and the EV charging outlets? Would these then remain on the main panel, and backed-up loads still need to be moved to a different sub-panel? In this scenario, would having a separate meter be any help?

Btw, I have 200A service. Found the following options from Square D, are these recommended? Need to find equivalent if I go with separate meter/panel.

1. SC3042M200PF - Square D Homeline 200 Amp 30-Space 42-Circuit Outdoor Ring-Type Semi-Flush Mount Solar-Ready Main Breaker Plug-On Neutral CSED-SC3042M200PF - The Home Depot
2. SC3042M200PS - Square D Homeline 200 Amp 30-Space 42-Circuit Outdoor Ring-Type Surface Mount Solar-Ready Main Breaker Plug-On Neutral CSED-SC3042M200PS - The Home Depot

Good to know about Sense and whole house surge suppressors.

 

[SoundDaTrumpet]

@sarge_in The separate main meter with no distribution is the simplest format for the a whole home backup according to Tesla (cheapest for them). In practice, combination panels allow some loads to be removed from Powerwall backup; otherwise, Tesla would be recommending more Powerwalls.

The combination panel above appears to look okay as it has an accessory lug kit to grab to the conductor bars and can fit 2/0 wire, which isn’t exactly 200A as it falls short at 195A for THHN wire and 175A for the lesser wire. This means the subpanel choices will have to be 175A main breaker for the TEG2 when implemented. I think it will work, but I am not at all a fan of solar specific panels. Tesla may prefer a sub feed lugs of greater ampacity rating available on another panel.

After PTO with Powerwall, I lost (50/50 on this forum) access to usage data. PG&E has no data to do a rate comparison for me. PG&E suggested I buy/install a separate system like Sense. Not worth my time to get an early read on the inevitable switch to EV2 rates.

For surge protection, I did add the simple circuit breaker types at both my service and house panels. It just happens that Siemens & Eaton make such equipment. All brands carries their own non-breaker types too. I am more interested in response time (shortest ground path possible) and have no need for surge capacity/longevity (lightning is infrequent, but brings the greater threat of wildfire).

 

[sarge_in]

Thanks @SoundDaTrumpet , but seems SCE does not allow separate meter...oh well.
If you are aware of any other combination panel that allows 200A main breaker or sub feed lugs, do let me know.

I am leaning the same way...install surge suppressor, but skip Sense.