Is Telsa proposing the right thing for my Powerwall installation?

[MorrisonHiker]

Maximum output of the Powerwall is just about 21A, and the HPWC will send up to 48A to the Model 3. If you were on pure backup, this load would trip your Powerwalls into overload even if nothing else in the house was on.

Ah ha! That explains why they set up our wall connectors on an 80 amp breaker with up to 64 Amps split between the three Powerwalls. We did mention we would probably be adding a 4th Powerwall so they wired it so we could bump things up eventually. I guess that would mean we might be able to do 80 Amps shared on a 100 Amp breaker. Then again, we only have 100 Amps going to the garage, so it's more likely we'd only be able to go up to 72 Amps.

 

[cwied]

Got it, I'm tracking.
On Back-up mode PW will charge my car by HPWC but ensure no more than 10kW from 2 PW.

The HPWC would not be on the essential load panel, so would not work during a power outage. However, your 14-50 could be on the essential load panel. This means that you could use your mobile charger to charger your car when the grid is down. I'm assuming you could also switch your HPWC to 60 amps instead of 80 to get whole-house backup, but given that you have the 14-50 for emergencies, that might not be worth it.

 

[Frankman60]

Time Based Control on the Powerwalls can prevent the Powerwalls from discharging during that period.

The plan to install two Powerwalls in the next few months includes whole house backup, including a NEMA 14-50 on a 50 amp breaker that I use to charge my Tesla. Because I will soon have 9 cent per kWh Super Off Peak I want to charge the car from the grid and not the PWs, using the Time Based Control function within the app as you mentioned. But my concern is what happens if there is a grid outage during Super Off Peak when my Tesla is charging? Will the PWs try to take over the charging and will they be overloaded? I should mention that other family members have plug in hybrids that also may be charging during Super Off Peak using a 120 volt charger at approximately 1200 watts each. Would it be better to NOT include the NEMA 14-50 with the other backed up breakers to prevent a PW overload during a grid outage while the cars are charging?

 

[MorrisonHiker]

The plan to install two Powerwalls in the next few months includes whole house backup, including a NEMA 14-50 on a 50 amp breaker that I use to charge my Tesla. Because I will soon have 9 cent per kWh Super Off Peak I want to charge the car from the grid and not the PWs, using the Time Based Control function within the app as you mentioned. But my concern is what happens if there is a grid outage during Super Off Peak when my Tesla is charging? Will the PWs try to take over the charging and will they be overloaded? I should mention that other family members have plug in hybrids that also may be charging during Super Off Peak using a 120 volt charger at approximately 1200 watts each. Would it be better to NOT include the NEMA 14-50 with the other backed up breakers to prevent a PW overload during a grid outage while the cars are charging?

The Powerwalls would be able to handle charging you car during an outage but they be rapidly drained since they have such a small capacity compared to a Tesla's battery.

A while back, someone tweeted Elon and requested better integration between cars and Powerwalls so that they could better communicate. Hopefully they'll add functionality in the future where the Powerwalls will tell cars (and smart appliances?) that the grid is down and suggest they stop charging.

 

[Frankman60]

Smart communication between the Powerwalls and Tesla cars would be great. Doesn’t that smart communication happen when more than one HPWC is connected to the same circuit? I thought I read that the HPWCs communicate with each other so they don’t overload the circuit when charging two Teslas.

Two Powerwalls should be able to handle the NEMA 14-50 at 40 amps, but it is very likely that two 120 volt EV chargers will also be drawing 1.3 kWs each, plus my house will be drawing approx 1.5 kWs at that time. Could the Powerwalls handle that during a grid outage or would they shut down and black out the house?

The Powerwalls would be able to handle charging you car during an outage but they be rapidly drained since they have such a small capacity compared to a Tesla's battery.

A while back, someone tweeted Elon and requested better integration between cars and Powerwalls so that they could better communicate. Hopefully they'll add functionality in the future where the Powerwalls will tell cars (and smart appliances?) that the grid is down and suggest they stop charging.

 

[MorrisonHiker]

Smart communication between the Powerwalls and Tesla cars would be great. Doesn’t that smart communication happen when more than one HPWC is connected to the same circuit? I thought I read that the HPWCs communicate with each other so they don’t overload the circuit when charging two Teslas.

Two Powerwalls should be able to handle the NEMA 14-50 at 40 amps, but it is very likely that two 120 volt EV chargers will also be drawing 1.3 kWs each, plus my house will be drawing approx 1.5 kWs at that time. Could the Powerwalls handle that during a grid outage or would they shut down and black out the house?

The HPWCs communicate via a cable that connects them. There's no real way for the Powerwalls to communicate directly with the HWPCs (that we know of). With the Powerwalls and cars, they would probably have to communicate via Tesla's servers. The Powerwalls would send a notification to Tesla's servers and the cars could check if the Powerwalls are currently supplying power during an outage. If there's an outage, the cars could delay or cancel their scheduled charging or send a notification asking for confirmation that they should charge.

I'm not certain how long two Powerwalls would be able to handle that load. Normally they can provide 5 kW each but can provide up to 7 kW each at peak...but I think that's only for a short period. If I'm reading this correctly, it would only be 10 seconds at 7 kW if off-grid.

The 14-50 outlet would be using 9.6 kW if you charge at 40 Amps...so combined with the two Volts and the household load, it probably couldn't sustain it for long. If you could charge the two Volts at a different time than the Tesla or reduce it down to 24 Amps, they should be able to handle it for a longer period as you'd be below 10 kW total.

 

[Vines]

Just pointing out that the Mobile connector that comes with the TM3 only uses 32A of a 40A continuous circuit, so about 7700W. If you also want to charge another 2600W worth of cars, plus 1500w of house load, your 2 PW will go into overload in 10 seconds I believe.

 

[MorrisonHiker]

Just pointing out that the Mobile connector that comes with the TM3 only uses 32A of a 40A continuous circuit, so about 7700W. If you also want to charge another 2600W worth of cars, plus 1500w of house load, your 2 PW will go into overload in 10 seconds I believe.

You can easily adjust the Amps used by changing the settings in the car. While 32 Amps it the max for a Gen 2 MC, it can be set to charge at any rate at or below 32 Amps.

 

[Frankman60]

Thanks MorrisonKiker and Vines, that is helpful. To be clear, I do not want to use the Powerwalls to charge any of my vehicles with a 9 cent per kWh Super Off Peak rate from my utility. To take advantage of that low rate, all three vehicles must charge between the hours of 12 am and 6 am, and the Plug in hybrids need all of that time, plus an hour or two fo fully charge. But it is possible that a grid outage could occur during that period while all three vehicles are charging.

I like the idea that the Tesla could charge at a lower rate than 40 amps on the NEMA 14-50, to keep the total house load under 10 kWs. So if the two 120 volt chargers use 1.3 kWs each for a total of 2.6 kWs, and the house is idling at 1.5 kWs, that is a total of 4.1 kWs being consumed without charging the Tesla. That leaves 5.9 kWs (10.0 - 4.1) for the Tesla to charge and still stay under the 10 kW capacity of two PWs. What amp setting would I need to adjust the Tesla charging to to keep the draw under 5.9 kWs? Also, at that rate I would need to figure out how many miles of range is added per hour because I only want to charge for 6 hours during the low rate. Presently at 40 amps the miles added per hour are about 29.

This brings me back to my initial thought, should the NEMA 14-50 be excluded from the PW backup? If yes, then how is the installation affected by doing this?

 

[Vines]

As it stands now you could set your Model 3 to charge at a reduced rate through the screen in the car, and it would always charge at that rate while at home. There is no possibility to have the charge rate reactive, so that it reduces its charge rate only during an outage, unless you do this manually.

In most case, especially with California weather I do not recommend putting Tesla chargers on backup. You can still self consume battery power regardless if the 14-50 is on the backup circuit, just it will not be available during grid outages, you would have to charge your model 3 on 120V instead.

For most Tesla car owners (unless you regularly arrive home with very little battery) the vehicle battery is so large that there is little risk your power will go out and at the same time the car doesn't have enough miles to go where you need to go. With other models of car this might not be as true so YMMV.

 

[MorrisonHiker]

Thanks MorrisonKiker and Vines, that is helpful. To be clear, I do not want to use the Powerwalls to charge any of my vehicles with a 9 cent per kWh Super Off Peak rate from my utility. To take advantage of that low rate, all three vehicles must charge between the hours of 12 am and 6 am, and the Plug in hybrids need all of that time, plus an hour or two fo fully charge. But it is possible that a grid outage could occur during that period while all three vehicles are charging.

I like the idea that the Tesla could charge at a lower rate than 40 amps on the NEMA 14-50, to keep the total house load under 10 kWs. So if the two 120 volt chargers use 1.3 kWs each for a total of 2.6 kWs, and the house is idling at 1.5 kWs, that is a total of 4.1 kWs being consumed without charging the Tesla. That leaves 5.9 kWs (10.0 - 4.1) for the Tesla to charge and still stay under the 10 kW capacity of two PWs. What amp setting would I need to adjust the Tesla charging to to keep the draw under 5.9 kWs? Also, at that rate I would need to figure out how many miles of range is added per hour because I only want to charge for 6 hours during the low rate. Presently at 40 amps the miles added per hour are about 29.

This brings me back to my initial thought, should the NEMA 14-50 be excluded from the PW backup? If yes, then how is the installation affected by doing this?

You can used Time Based Control to set schedules for your Powerwalls. We have ours set to stop powering the house at 5 am and we charge all cars between 5 am and 9 am. IF the power goes out, then the Powerwalls would take over and the vehicles could possibly charge at that time. While I'd be awake for most of that time period, I might not be awake at 5 am and the cars could possibly drain the Powerwalls. This is one reason we'd like the Powerwalls and cars to be able to communicate. Elon said that feature will be coming, we just don't know how soon.

If you set the Tesla to 24 Amps, that would be 5.76 kW. If you currently get 29 mph, that would mean you'd get about 17.5 miles per hour at 24 Amps instead of 29 mph at 40 Amps.

 

[Frankman60]

If you set the Tesla to 24 Amps, that would be 5.76 kW. If you currently get 29 mph, that would mean you'd get about 17.5 miles per hour at 24 Amps instead of 29 mph at 40 Amps.

The 24 amp setting should be OK to keep everything below 10 kWs, and 17.5 miles per hour added over the 6 hour period (105 miles) should be fine most of the time. But, sometimes the battery will be low and the Tesla will keep charging at this slower rate after Super Off Peak ends at 6 AM. The rate increases 2.5 times after 6 AM so charging at a slower rate to keep demand under 10 kWs will be costing more for electricity. But the last thing I want to happen is invest in two Powerwalls and if the grid goes down after we are sleeping the Powerwalls overload because of the car charging and the house goes dark. That sort of defeats the purpose of backup power.

It seems that the best solution is what Vines recommended about not backing up the NEMA 14-50 with the Powerwalls. If I am not mistaken, that would require another panel to be installed with only the 50 amp NEMA 14-50 breaker in it. My installation plan for whole house backup is to move all breakers from the main panel to another panel because of the way the meter is integrated with my main panel. If I am not mistaken, if one circuit (NEMA 14-50) is not to be backed up, it has to be put in an additional panel which requires more space to put that panel. Does that sound right?

 

[miimura]

The 24 amp setting should be OK to keep everything below 10 kWs, and 17.5 miles per hour added over the 6 hour period (105 miles) should be fine most of the time. But, sometimes the battery will be low and the Tesla will keep charging at this slower rate after Super Off Peak ends at 6 AM. The rate increases 2.5 times after 6 AM so charging at a slower rate to keep demand under 10 kWs will be costing more for electricity. But the last thing I want to happen is invest in two Powerwalls and if the grid goes down after we are sleeping the Powerwalls overload because of the car charging and the house goes dark. That sort of defeats the purpose of backup power.

It seems that the best solution is what Vines recommended about not backing up the NEMA 14-50 with the Powerwalls. If I am not mistaken, that would require another panel to be installed with only the 50 amp NEMA 14-50 breaker in it. My installation plan for whole house backup is to move all breakers from the main panel to another panel because of the way the meter is integrated with my main panel. If I am not mistaken, if one circuit (NEMA 14-50) is not to be backed up, it has to be put in an additional panel which requires more space to put that panel. Does that sound right?

My Powerwall installation left all my 240V loads in the main panel, not backed up, including my two EV charging circuits (240V 50A each). The 125A breaker that previously fed a sub-panel with most of my household 120V loads went to the Backup Gateway. Some minor loads like the laundry room plugs and one set of bathroom lights were re-routed from the main panel to the Generation Panel that was added as part of the installation. The Generation panel has the solar and Powerwall breakers plus those relocated loads. The original sub-panel is connected to the second output on the Backup Gateway switch.

So, I suppose the topology of your electrical service and panels matters in how they do whole home backup. I heard of one installation where they were able to re-route the output of the main breaker immediately after the meter from the main panel to the Backup Gateway. Then the output of the Backup Gateway switch went back to feed the old breakers in the original main panel. A Generation Panel was still used for the Powerwall Breakers and only the solar breakers were relocated out of the main panel to the Generation Panel.

 

[Vines]

The 24 amp setting should be OK to keep everything below 10 kWs, and 17.5 miles per hour added over the 6 hour period (105 miles) should be fine most of the time. But, sometimes the battery will be low and the Tesla will keep charging at this slower rate after Super Off Peak ends at 6 AM. The rate increases 2.5 times after 6 AM so charging at a slower rate to keep demand under 10 kWs will be costing more for electricity. But the last thing I want to happen is invest in two Powerwalls and if the grid goes down after we are sleeping the Powerwalls overload because of the car charging and the house goes dark. That sort of defeats the purpose of backup power.

It seems that the best solution is what Vines recommended about not backing up the NEMA 14-50 with the Powerwalls. If I am not mistaken, that would require another panel to be installed with only the 50 amp NEMA 14-50 breaker in it. My installation plan for whole house backup is to move all breakers from the main panel to another panel because of the way the meter is integrated with my main panel. If I am not mistaken, if one circuit (NEMA 14-50) is not to be backed up, it has to be put in an additional panel which requires more space to put that panel. Does that sound right?

Not sure if you have checked this detail, but not all main services can take a large 200A breaker directly on the bus. Many of them max out at 100A or 125A for a 2P branch circuit, so it can sometimes be difficult to do whole home backup with a MSP. Therefore if you go by the 100% rule, you could have a 125A branch circuit for your backup loads, and still have 75A left in your main panel to leave your car charger in the main panel distribution bus. However this apporoach will reduce your overall capacity, as the rest of your loads will be reduced to 125A. Will need to do a load calculation to see if thats acceptable, marginal or undersized.

Some MSP allow a 150-200A 4p Breaker on the buss, so thats another option, use a 4P 150A to feed your backed up loads, and the 50A leave on the main bus to have your car charger backed up.

Alternately, backup the car charger, leave it at 20-24A most of the time and crank it up to 32 when you need the extra charge speed. 3rd party apps like Dashboard for Tesla do have the ability to start and stop charging automatically at 6 AM if thats what you want as well.

 

[Frankman60]

Not sure if you have checked this detail, but not all main services can take a large 200A breaker directly on the bus. Many of them max out at 100A or 125A for a 2P branch circuit, so it can sometimes be difficult to do whole home backup with a MSP. Therefore if you go by the 100% rule, you could have a 125A branch circuit for your backup loads, and still have 75A left in your main panel to leave your car charger in the main panel distribution bus. However this apporoach will reduce your overall capacity, as the rest of your loads will be reduced to 125A. Will need to do a load calculation to see if thats acceptable, marginal or undersized.

Some MSP allow a 150-200A 4p Breaker on the buss, so thats another option, use a 4P 150A to feed your backed up loads, and the 50A leave on the main bus to have your car charger backed up.

Alternately, backup the car charger, leave it at 20-24A most of the time and crank it up to 32 when you need the extra charge speed. 3rd party apps like Dashboard for Tesla do have the ability to start and stop charging automatically at 6 AM if thats what you want as well.

Great information, thanks. I like the option about leaving a 50 amp breaker on the main bus just for the NEMA 14-50. I will check with the company doing the installation to see if this can be done in compliance with the local code. I would also really like to upsize the 200 amp main bus because it is presently maxed out. What is involved in upsizing the main bus to something more than 200 amps? I noticed that the contracts says that a 400 amp backup loads panel will be installed. Does that mean that the utility service will now be 400 amps?

 

[Vines]

Great information, thanks. I like the option about leaving a 50 amp breaker on the main bus just for the NEMA 14-50. I will check with the company doing the installation to see if this can be done in compliance with the local code. I would also really like to upsize the 200 amp main bus because it is presently maxed out. What is involved in upsizing the main bus to something more than 200 amps? I noticed that the contracts says that a 400 amp backup loads panel will be installed. Does that mean that the utility service will now be 400 amps?

No, the proposed 400A subpanel will not give you more overall capacity. Typically we use 400A subpanels to adhere to the 120% rule. Your main breaker is still the same at 200A.

What it does is allow us to install more generation sources than we are otherwise allowed. Usually we are maxed at total sources = 120% of the buss rating. So a 200A buss has 240A of capacity. If you have a 200A main breaker (Grid Source) then you have 40A left over (PV or PW sources.) If you downsize the main breaker to 150A then you have 90A for PV+PW.

If you use a 200A breaker, and use it to feed a 400A subpanel then you can backfeed up to 200A.

When you say the main bus is maxed out, what are you referring to? You need more physical room for breakers or more electrical capacity for power hungry Teslas? Depending on your utility rules it may be possible to install a different type of main service panel. Maybe a "Solar Ready" 225 A panel with 200A main breaker? These can sometimes be done without replacing the wire from the transformer. However these other panels would only allow you to install more generation sources, you would still be limited to 200A of supply from the utility. This would be "Upgrading the bus." Its much simpler and easier to do these equipment upgrades since you aren't asking PG&E for any more amps.

Increasing the service size from a 200A feed to a 400A feed is doable, but certainly another significant cost. If your feed is underground from the transformer, you may or may not have room in the existing conduit to pull new wire. Most often an underground feed from the transformer means you will need a new trench, wire and conduit. This needs to be done in conjunction with your utility and will usually be 15K+ Replacing the service panel itself with a 400A unit will be 3k to 5k depending on options. If the main feed is overhead you will have to check with your utility. Here in PGE territory, overhead drops are pretty easy to increase in size, and quite cheap or free sometimes, however it will take time, and investigation to see if your transformer has the capacity. This is usually referred to as a "Service Upgrade"

 

[Frankman60]

No, the proposed 400A subpanel will not give you more overall capacity. Typically we use 400A subpanels to adhere to the 120% rule. Your main breaker is still the same at 200A.

What it does is allow us to install more generation sources than we are otherwise allowed. Usually we are maxed at total sources = 120% of the buss rating. So a 200A buss has 240A of capacity. If you have a 200A main breaker (Grid Source) then you have 40A left over (PV or PW sources.) If you downsize the main breaker to 150A then you have 90A for PV+PW.

If you use a 200A breaker, and use it to feed a 400A subpanel then you can backfeed up to 200A.

When you say the main bus is maxed out, what are you referring to? You need more physical room for breakers or more electrical capacity for power hungry Teslas? Depending on your utility rules it may be possible to install a different type of main service panel. Maybe a "Solar Ready" 225 A panel with 200A main breaker? These can sometimes be done without replacing the wire from the transformer. However these other panels would only allow you to install more generation sources, you would still be limited to 200A of supply from the utility. This would be "Upgrading the bus." Its much simpler and easier to do these equipment upgrades since you aren't asking PG&E for any more amps.

Increasing the service size from a 200A feed to a 400A feed is doable, but certainly another significant cost. If your feed is underground from the transformer, you may or may not have room in the existing conduit to pull new wire. Most often an underground feed from the transformer means you will need a new trench, wire and conduit. This needs to be done in conjunction with your utility and will usually be 15K+ Replacing the service panel itself with a 400A unit will be 3k to 5k depending on options. If the main feed is overhead you will have to check with your utility. Here in PGE territory, overhead drops are pretty easy to increase in size, and quite cheap or free sometimes, however it will take time, and investigation to see if your transformer has the capacity. This is usually referred to as a "Service Upgrade"

I had a feeling that upgrading the utility service to 400 amps was not part of the project, but I was confused about why a 400 amp subpanel was specified by the contractor. Now I understand why the contractor is using a 400 amp subpanel for two PWs, thanks.

What I meant by the main panel being maxed out is that there isn’t any room left for additional breakers. But it seems that there will be more room in a 400 amp subpanel so that should solve that issue.

If the local code allows me to have a 150-200A 4P breaker on the main bus, and leave a 50 amp breaker in the main panel just for the NEMA 14-50 (not backed up), is there a downside to doing this? I would achieve my goal of not backing up the Tesla charging but am I sacrificing something else to do this?

 

[Vines]

Its not a local code issue, its an issue with the service panel itself. Read the sticker, somewhere it will specify the max branch circuit allowed on the buss. The panels that allow this special 4p breaker will have language that specifies the part number for it.

If you use a 150A 4p to feed your backup system, and a 50A left on the distribution bus, you are only giving up the flexibility that all the loads in the backup system need to work out with the load calculations of the 150A breaker. Its a pretty small difference imo. In other words if you try to add another large load to the backup system, the load calculations need to work out with 150A. Difference is on the orderof 20-30 amps of "calculated load" you leave on the table.

When your company runs the load calculations, if they come out with for example 147A (I'd call this marginal for a 150A breaker), this basically tells you you can't safely add additional loads to the electrical system. In that example, if you had you run the calculation with the car charger on the backup system, and the main breaker at 200A, its likely the calculations result would be 175A-ish so you could theoretically add a small 25A load. Its a pretty small loss.

If they come up with 173A, then this plan will not work as a 150A breaker is undersized.

Also the 100% rule states that the addition of all breakers nameplate rating must not exceed the bus rating. So if you leave a 50A on the bus for your 14-50, then its a 150A max breaker for the backup system.

 

[Vines]

This panel does not accept 4p breakers

 

[Vines]

This panel does accept 4p breakers