Confusion on Partial Backup

[metakonger]

Hello,

Would appreciate some clarity on the implications of a partial backup. I'm recently scheduled for an install in about a month, but I got a notification from my advisor that I need to implement a partial backup because i have 400 amp main panel. They said they would install the gateway on a sub panel which excludes AC and car charging, both of which are on the main panel. My problem is that it seems like i can't get a clear answer on whether those things are only excluded when the Powerwall acts as a backup when the grid is down. or whether it's also excluded during normal times when access to grid power is available. My advisor is indicating that the Powerwall will not be able to provide energy to the main panel during evening times although access to the grid is on.
This seems to contradict the information on the thread at: what is a partial home backup?

Is my situation different from what's described in the above thread? my priorities are to offset energy use from my AC and car charging at all times, so this recent news would be a huge blocker.

Thank you!

My design:

 

[yblaser]

You need to make sure they install CTs in the MSP to monitor the non backed up loads. If they do the system will be able to see their power draw and can discharge the PWs to compensate when on grid.

Given the number of loads the only potential issue I can see is whether their is space to add a pair of CTs on the 200A feed to the non backed up loads or multiple CTs would need after the load breakers. I only have 200A service so potentially I can be missing some subtleties here.

 

[jgleigh]

I have this exact setup and it works great. As yblaser said, you just have to ensure they add a CT on your non-backed up 200A leg so that the gateway can see the power flow.

They may be worried since you only have 2 Powerwalls and several large loads in your main panel. But even then, the Powerwalls will provide what they can and the grid will provide any additional needed power.

 

[jhn_]

The 27kw in two Powerwalls is going to be drained in less than two hours if you are charging a Tesla at 48A and not even be able to charge 50% of the EV battery. If you have TOU it might be best to keep that Powerwall power to offset peaks from other uses and charge the EV from grid during off peak. So maybe you want the EV charger to only be on grid?

I’m speaking from similar experience starting out our install putting the EV connector on the whole house backup with multiple Powerwalls. Only to want it grid only later and needing to change.

 

[Jeremy3292]

So maybe you want the EV charger to only be on grid?

I have heard this is the best idea but maybe also consider installing a 14-50 receptacle (32 amp) on the backup side just in case you need it.

I’m speaking from similar experience starting out our install putting the EV connector on the whole house backup with multiple Powerwalls. Only to want it grid only later and needing to change.

What exact issues were you having with the EV charger on the backup side? I assume this is strictly related to when the grid is down, as the solar is being used to power the house and recharge the Powerwalls? There would be plenty of solar output during the day to charge your car when the grid is on, no?

 

[miimura]

I have heard this is the best idea but maybe also consider installing a 14-50 receptacle (32 amp) on the backup side just in case you need it.

What exact issues were you having with the EV charger on the backup side? I assume this is strictly related to when the grid is down, as the solar is being used to power the house and recharge the Powerwalls? There would be plenty of solar output during the day to charge your car when the grid is on, no?

If you have a non-Tesla vehicle, it can drain your Powerwalls if the grid goes down while the car is charging or is scheduled to charge. Tesla has a feature that allows you to limit Tesla vehicle charging when the Powerwalls are below a configurable state of charge level. If you only have Tesla vehicles, then you don't really have to worry about how your EVSE is connected to your electrical system.

 

[Jeremy3292]

If you have a non-Tesla vehicle, it can drain your Powerwalls if the grid goes down while the car is charging or is scheduled to charge. Tesla has a feature that allows you to limit Tesla vehicle charging when the Powerwalls are below a configurable state of charge level. If you only have Tesla vehicles, then you don't really have to worry about how your EVSE is connected to your electrical system.

So there would be no reason to have the wall connector on the non-backup load side?

 

[Vines]

So there would be no reason to have the wall connector on the non-backup load side?

Tesla recommends that Wall chargers be installed on the backup side. I cant really see a reason why to make it not backed up unless you dont drive a Tesla. In fact when I have a grid outage it is nice to be able to dump excess solar generation into the batteries, rather than just using it or losing it.

 

[jhn_]

I have heard this is the best idea but maybe also consider installing a 14-50 receptacle (32 amp) on the backup side just in case you need it.

What exact issues were you having with the EV charger on the backup side? I assume this is strictly related to when the grid is down, as the solar is being used to power the house and recharge the Powerwalls? There would be plenty of solar output during the day to charge your car when the grid is on, no?

Depends on your use case I guess. In my case the EV is not home while the sun is shining. I’m also in the northeast so there is not enough sun during the day on our east-west facing arrays to supply the house and EV for the winter months. We also don’t have TOU and have 1:1 net metering.

Without greater software controls over how a Tesla EV charges while the grid is up, I would recommend a 14-50 or Tesla connector on either side of the home backup. This gives complete control over home and when you use your Powerwalls.

 

[metakonger]

You need to make sure they install CTs in the MSP to monitor the non backed up loads. If they do the system will be able to see their power draw and can discharge the PWs to compensate when on grid.

Given the number of loads the only potential issue I can see is whether their is space to add a pair of CTs on the 200A feed to the non backed up loads or multiple CTs would need after the load breakers. I only have 200A service so potentially I can be missing some subtleties here.

thank you all for all the helpful replies! will contact adviser regarding CT. And seems like I also need to adjust my expectations from what 2 Powerwalls can provide.

 

[GregBallantyne]

thank you all for all the helpful replies! will contact adviser regarding CT. And seems like I also need to adjust my expectations from what 2 Powerwalls can provide.

I went from 2 PW to 3 PW on my installation before going operational, after I learned more about what expectations should be. And I don't have the loads you've got - only a 200A load center. But one my goals going in was whole house backup, in an outage I don't want to give up anything unless it will be long. So no backup panel. I won't charge my car during an outage, unless I isolate that building and power it with a generator.
BTW I'm also making plans to add more PW beyond the initial 3.....