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Solar, Powerwalls and Model 3 charger hookup integration.

Hi, I'm a long time reader but first time poster. I have an 8.16kw solar system with 2 Powerwalls. We are expecting delivery of our Model 3 LR in a few weeks. I have the Tesla wall charger and have an electrician who is going to install it. I have read some of the threads here about others integrating these system but I'm still unclear about best practices. My electrician is telling me because the back up sub panel is only 150 amps I can't hook up my charger to the backup side of my system and it needs to go to the grid side. He says that adding the 60 amp breaker will exceed the capability of the panel.



My understanding was that hooking the charger to the backup side is a viable option and the load is dynamically managed by the Tesla app.



Our system produces a lot of power back to the grid. On a typical long solar day my Powerwalls will change before noon and I had planned to use the rest of the afternoon to top off the car. At night the powerwalls power the house. I almost never use the grid this time of year and as I mentioned, bank a lot of power.



I like the idea of being able to charge the car off grid though I know some balancing care must be taken to not empty the Powerwalls. While researching my system I read the the whole system car and all should work off the backup side of the system with some limitations. Is there a limit to my system I'm not understanding or is it something my electrician is not getting. Any advice? Thanks.
 

jjrandorin

Moderator, Model 3, Tesla Energy Forums
Moderator
Nov 28, 2018
15,100
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Riverside Co. CA
My understanding was that hooking the charger to the backup side is a viable option and the load is dynamically managed by the Tesla app.

You are talking about something different than your electrician. No, the tesla app does not dynamically manage the load of charging a car WHILE ON GRID, either a tesla or any other car. You can not exceed load calculations because of any such management (which, again, doesnt exist while on grid).

The only management the tesla app does to charging is while OFF grid, and its to prevent the car from draining the powerwalls, not for the car to not exceed any load calculations.

With that being said, Many times people go at this with the question of "I want to install a 60amp breaker on the backup loads side for my new EV". If the load calculations do not allow that, AND you want the charging on the backup loads side, the question should be "How large of a circuit for the EV can I put on the backup loads side for EV charging?"

Teslas (and Tesla wall connectors) MAX OUT at 60amp circuit / 48 amp continuous, but they dont have to be set to that. They can be set to just about anything, down to 15 amp / 120V. You may not be able to get a 60amp circuit in there, but you may be able to get a 30 or 40amp circuit in there. A 40 amp circuit would give you 32 amp continuous, and would likely be fine for overnight charging.

The car could charge faster, but since your panel doesnt allow a 60amp, and you want it on the backup loads side, see what fits there and make a decision then.


In neither case however does the "tesla manages the load" apply to this, because it only manages charging when off grid, there is no management when on grid of charging.
 
Our system produces a lot of power back to the grid. On a typical long solar day my Powerwalls will change before noon and I had planned to use the rest of the afternoon to top off the car. At night the powerwalls power the house. I almost never use the grid this time of year and as I mentioned, bank a lot of power.

As long as your Gateway can see the grid side, it will use that extra energy to charge the car. There's also an app in beta that helps manage the power flow between the car and the batteries.

 
Ok, thank you for that. I see I was conflating the ability to manage the charger setting to fit the breaker size the calcs allow with balancing the load on the backup side which it does not do.


So yes, that makes complete sense. So if I want to charge off grid I need to put the size breaker the load calcs allow and set the app to not exceed that load. Upside I can charge during a power shortage, downside I can never charge faster then the breaker allows.


So, I have a 200 amp service. Tesla unloaded my panel and put all the breakers in the backup sub panel. Then they put a 125 amp breaker that feeds the sub panel and another 125 amp breaker in the sub panel that loads the rails. Effectively reducing my 200 amp panel to a 125 amp panel. I assume they did this to leave some headroom in the grid side box for say, a 60 amp EV charger or other grid side load? If I wanted to increase my capacity on the sub panel can that be done by increasing those breaker sizes? However, I see all your points, I'm over complicating this. Put the charger on the grid side. I am curious about what you mean by “if the gateway can see the grid side” means. Do you mean that excess power that flows back to the grid will charge the car battery on it's way back to my Net metering bank? The gateway sits between the sub panel and main panel. Or again am I misunderstanding?

So either connect the charger to the backup side with whatever that capacity allows or to the grid side and under no scenario will you be able to charge during a power outage, unless you trickle charge from a 110 outlet?
 

jjrandorin

Moderator, Model 3, Tesla Energy Forums
Moderator
Nov 28, 2018
15,100
19,509
Riverside Co. CA
I am curious about what you mean by “if the gateway can see the grid side” means. Do you mean that excess power that flows back to the grid will charge the car battery on it's way back to my Net metering bank? The gateway sits between the sub panel and main panel. Or again am I misunderstanding?

What @jgleigh likely means by " if the gateway can see the grid side" is that each one of these installations is a construction project, so we cant really know how your setup is vs mine (or theirs) for example.

In order for the wall connector (or any other load) on the non backed up loads side to be able to use powerwall power, the gateway has to be aware of those loads. Thats normally done with putting a current transformer (Transducer?, i cant remember what the "T" in CT is exactly), on the wires that load is on.

If there is a CT there that is measuring the loads, then the gateway can see it, and if the gateway can see it, the powerwalls can supply power to it when the grid is on, even if its on the non backed up loads side. If there is no CT for the non backed up load side, then the gateway cant see it, and the powerwalls cant power it.

Even if there is a CT on the non backed up loads side, they are non backed up loads, so the powerwalls would only be able to power them when the grid is up, not grid down.

In my (unasked for) opinion, the best thing for you to do in this case is to meet the desire you have (which is the same one I had btw) is to:

1. Install the wall connector on the non backed up loads side.
2. Ensure that its being measured by a CT so the powerwalls can see it and provide powerwall power to the car when grid is up.
3. Install a 240v outlet on the backed up loads side that meets your load calculations. A 20/30/40 amp outlet, whatever likely fits the load calcs.

If you do the above, the wall connector would be what you normally use, with a 60amp breaker and 48amp charging, but in a power outage, IF you wanted to charge your car, you would use the mobile connector and the appropriate adapter to charge on the backed up loads side.

You would really only want to charge the car during a power outage to allow the solar to stay on and have that power go somewhere. You likely wouldnt even want to charge at 48amps during a power outage, because of the speed at which that would drain your powerwalls.

Using powerwalls to fill a tesla is like taking a AA battery and using it to fill a D battery. it works, but its not something that makes a lot of sense to do unless you need to for some specific reason (like keeping solar generating, or a need to get power into the car so you can get to a farther supercharger, etc).

Its really kind of a niche case, but planning for niche cases is why many of us have powerwalls in the first place.
 
Thank you jjrandorin, your advice is most welcome and this is really useful information and an education I desperately need and want. Just to be clear, I don't want to charge the car from the Powerwalls, I just want to charge the car after the Powerwalls are full with the excess power that normally is going back to the grid.

With the current debates here in CA about taxing or limiting Net Metering, I wanted to take my power to charge the Model 3 before it’s goes into the DWP's hand. That’s my primary concern. secondly I would like to charge during a power shortage but as you have pointed out, that can be done using your advise.

So, just to keep kicking this can if anyone cares to, let’s say it's noon, my Powerwalls are fully charged and I'm feeding 5 kw back to the grid. Now I start charging the car. What’s happening to the excess power that normally feeds back to the grid, is it charging the car hooked up to the grid side and the excess going to the grid?

I have read about the CT and need to read more to fully understand but it sounds like if the solar panels are feeding excess power back to the grid for Net Metering and the Powerwalls feed the grid to be part of Tesla's Virtual Power Plant the gateway must “see” the grid side. I'm new to all this. I have had the solar and Powerwalls for a year and the model 3 arrives soon so I would like to expand my rudimentary understandings. Particularly while I have an electrician preparing my system for the model 3. You information is very welcome. Thanks.
 

jboy210

Well-Known Member
Supporting Member
Dec 2, 2016
6,963
4,761
Northern California
... I just want to charge the car after the Powerwalls are full with the excess power that normally is going back to the grid.
Under normal operation you will get this with your install. The car will appear as a load like a dryer and pull from the solar and Powerwalls at the same time. Only if the load exceeds that capacity of the Powerwall and solar will the grid be used.

FWIW, I am in self-Powered mode and just make sure the aggregate of the solar and Powerwall meets the charging + other household needs. I usually start charging around 10:30 or so. Also, I charge multiple times a week to lessen the amount of charge the vehicles need. I work from home and this help ensure I don't pull the Powerwalls down below where the day's solar can refill them with enough power so I avoid using grid power.
 
So, just to keep kicking this can if anyone cares to, let’s say it's noon, my Powerwalls are fully charged and I'm feeding 5 kw back to the grid. Now I start charging the car. What’s happening to the excess power that normally feeds back to the grid, is it charging the car hooked up to the grid side and the excess going to the grid?
Yes - generation and loads have to be balanced, so any solar power being generated has to go somewhere. In this grid-on scenario, if the power is not being consumed by home loads nor being stored into the Powerwalls (because they are full), it has to go out to the grid. Alternatively in a grid-down scenario, the Tesla gateway device (which would disconnect the home from the grid during the outage) would signal the solar inverters to curtail production partiallly or fully until solar generation and loads are balanced.

In the grid-on scenario you asked, there are no CT's needed for this to happen - the CT's are simple passive electrical sensors. Electric power is just flowing "downhill" to the path of least resistance, in this case out to the grid. Heck, no Powerwalls or gateways are needed either, excess solar always flows out to the grid. But excess means after any loads before the meter. So when you have a charger (or anything) on the non-backed loads panel, the solar power will flow there first, before flowing through the meter to the grid.

The CT's are not there to help control the flow of excess solar when on-grid, there's generally no reason to in most cases, only excess Powerwall discharge. The Powerwall can decide to discharge less, same or more than the household loads, depending on regulations, policy, and the mode you want. Everything behind the gateway (including the backed-up loads panel) is already monitored, but the non-backed up loads are not, cannot unless the aforementioned CT's are installed to monitor the loads.

That said, there are a few exceptions to monitor and curtail excess solar - for most, just temporarily pre-PTO, before you're allowed to export solar (you need the Powerwall+ and compatible inverters). For a few (mainly Hawaii), newer installs not allowed to export when on-grid.
 

miimura

Well-Known Member
Aug 21, 2013
7,372
7,355
Los Altos, CA
So yes, that makes complete sense. So if I want to charge off grid I need to put the size breaker the load calcs allow and set the app to not exceed that load. Upside I can charge during a power shortage, downside I can never charge faster then the breaker allows.
I wanted to reply specifically to this point. The load calcs give you the maximum breaker size. The breaker size then needs to be configured in the EV charging equipment. In the Gen3 Tesla Wall Connector, this is done during provisioning. Once that is set, you can never charge a connected vehicle with more than 80% of the breaker size. So, if you have a 30A breaker, you get 24A car charging. There is no app adjustment higher than that.

I would suggest that charging during a power outage is very unlikely to be necessary, so the best place for EV charging circuits is in the main panel. You can use other solutions to tell the car to use surplus solar or use an EVSE like Emporia that has that function. Install a small 20A or 30A 240V outlet if you feel you really need solar and Powerwall vehicle charging during a utility outage.
 
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Reactions: jjrandorin
it sounds like your powerwalls and solar are back feeding your backup panel. is your gateway a sub off the non backup panel? or does it have a feeder to the meter base? but you might be able to install the optional breaker bus in the gateway and install the charger there...
pictures would go a long way here.
powerwalls/gateway should be set for PCS to not trip the gateway breaker, so i you ever pulled more then 125 amps from grid to the backup panel it should make up the difference with power from the batteries.
 
Thank you CrazyRabbit for your input. As far as I know my system as set up by Tesla is like the flow chart they published online other then the charger, not installed yet. Here is a picture of my installation. Unseen is the the 90 amp sub panel that feeds the circuits on the other side of the house but the breaker that feeds it is here in the back up panel.
 

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