whole house backup with 2 Powerwalls

[marnao]

I currently have an order for solar + powerwall.. at the same time I've begun finishing my basement. I initially didn't think they were going to have to add a subpanel to support this work. That's since changed and my electrician wants to install a 100 amp subpanel to support the new circuits as my 200 amp panel is relatively full. It's a new house and it seems like there's a separate circuit for literally everything so it's easy to fill up a panel.

The current design is a "whole house backup" and I obtained the design plans from Tesla to see exactly how they planned to wire everything up. It appears they're going to put the gateway between my main panel and service. They are also going to add another 100 amp panel for the powerwall and solar. This hangs off the gateway as well.

My question is.. I assume there's no issue with adding a 100 amp subpanel to my main panel? One of the Tesla advisors told me something like "as long as you don't have any breakers above 60 amps you're fine" but I assume that doesn't apply to a subpanel breaker. I was planning to add a 2nd 50 amp outlet to our garage for a NEMA 14-50 outlet. We would of course never use these in an outage scenario as they would drain the Powerwalls very quickly.

I am trying to get an answer from Tesla on this as well but I've been told it could be a couple of days. And of course I can't talk to anyone who knows what they're doing my message has to be relayed like a bad game of telephone.

If anyone has any insights and has had a similar experience please let me know!

 

[jimmythefrack]

The super simple answer without knowing or having the design is, 200 is the top main breaker for your whole house, as long as you don't draw more than 200 amps total from the power company its fine because above 200 you'd blow the breaker. Meaning yes you can have 100 amp sub panel and a 50 amp plug and if all was simple and easy and you were drawing the full 100 and the full 50 you would be at 150 amps still below the 200. That being said they're not going to install something that an electrical inspector wouldn't approve either. Hope that helps a little

 

[GenSao]

Yes.

Based on your description I am assuming you are getting the Tesla Energy Gateway 2 with the optional internal panel board to be used as your generation panel. See below image and typical wiring layout.

 

[marnao]

It's actually Gateway 1. There's a separate panel for the solar and Powerwalls. I've attached the design.

 

[Vines]

When the GW has a forked design like that you are limited in the loads you can also put in this panel.

This lower ampacity wire is a Feeder tap, per 240.21B, and therefore needs to terminate in a single OCPD (fuse in disconnect)

So what the service advisor is telling you is partially correct. The max draw that panel can currently support is 100A based on the fuse. if you put a 60A load in there, the panel is maxed at 100A draw, considering the PW can charge from the grid.

Also, if you do add load breakers beyond 25A to this panel you are violating the 100% rule, so a 60A is out of the question if you wish to remain code complaint. Adding a 60A breaker only in my opinion does not create an unsafe condition, but the NEC doesn't recognize this as safe.

 

[GenSao]

It's actually Gateway 1. There's a separate panel for the solar and Powerwalls. I've attached the design.

Ahh... Ok. I misread the following, "They are also going to add another 100 amp panel for the powerwall and solar. This hangs off the gateway as well." The Backup Gateway 2 allows the generation panel to "hangs off" directly inside the Backup Gateway 2.

 

[fresnoboy]

Interesting. I'd be interested in getting powerwalls, but I have a 400A feed and backup generator. I could fit the main house feed and one of the HVAC units in a 200A panel, but it would have to be stubbed from the main 400A panel. Because of the backup generator, I assume I can't use it for emergency power (I want everything powered from the generator, not just the main house panel) if it would try and backfeed the generator.

I can't use the gateway as the main transfer switch because they only go up to 200A.

 

[marnao]

When the GW has a forked design like that you are limited in the loads you can also put in this panel.

This lower ampacity wire is a Feeder tap, per 240.21B, and therefore needs to terminate in a single OCPD (fuse in disconnect)

So what the service advisor is telling you is partially correct. The max draw that panel can currently support is 100A based on the fuse. if you put a 60A load in there, the panel is maxed at 100A draw, considering the PW can charge from the grid.

Also, if you do add load breakers beyond 25A to this panel you are violating the 100% rule, so a 60A is out of the question if you wish to remain code complaint. Adding a 60A breaker only in my opinion does not create an unsafe condition, but the NEC doesn't recognize this as safe.

I guess my main question is... at what point do I need 3 Powerwalls instead of 2? They were going to do a whole-house backup with my nearly-full 200 amp panel.. Now I am adding a 100 amp sub-panel with additional loads. Is 2 still enough? How do they determine this?

Shockingly I am still waiting for a call back from Tesla haha

 

[BrettS]

There are two factors to consider when you’re determining how many powerwalls you need.

The first is the maximum current draw. Each powerwall can support a sustained power draw of 5kW with a short surge up to 7kW. At 240V that’s about 20A continuous and about 30A peak. With two powerwalls you double those numbers, with three powerwalls you triple them, etc. So it doesn’t really matter how full your panel is. You could have one circuit that draws a lot of power and need three powerwalls or you could have 20 circuits that just draw a little bit of power and be fine with one powerwall. You need to look at how much how much power your house draws at once. Do you have electric appliances? Might you have several big appliances on at the same time?

The second consideration is how long the powerwalls will last. Each powerwall provides 13.5kWh of power. If your house is drawing 5kW then a single powerwall will give you about 2.7 hours of runtime. Two powerwalls would give you 5.4 hours and three powerwalls would give you 8.1 hours. So even if your power draw is such that two powerwalls will be able to power your house, two powerwalls might not last as long as you want and you could increase that runtime by adding a third.

 

[GenSao]

I guess my main question is... at what point do I need 3 Powerwalls instead of 2? They were going to do a whole-house backup with my nearly-full 200 amp panel.. Now I am adding a 100 amp sub-panel with additional loads. Is 2 still enough? How do they determine this?

Shockingly I am still waiting for a call back from Tesla haha

If the 100 A sub panel is connected to your (E) 200 A main panel (Panel V), then you'll be fine. The 100A breaker for the sub panel does not count; the individual breakers do. I believe there is a bit of a confusion, as you do not plan on adding loads to the generation panel (Panel X).

You'll only need three Powerwalls if: 1) More storage is desired for longer backup, TOU rate arbitrage, and/or be more grid independent; or 2) More power is needed for say a bigger AC (such as a AC with a scroll compressor with a LRA of 140+).

 

[marnao]

Thanks for the inputs everyone. I was hoping the sub panel would not affect things and it kinda sounds like it won't. I don't think it's going to appreciably add to our load in a backup scenario despite having a bunch more breakers. For instance we'll have 2x50 amp circuits for our cars but we would never attempt to charge them during an outage.

Our heavy duty electric appliances include a wall oven/microwave combo and 2x 2 ton AC systems each with an LRA of 60. We have gas heat, stove, dryer, etc.

 

[ElectricCyclist]

Thanks for the inputs everyone. I was hoping the sub panel would not affect things and it kinda sounds like it won't. I don't think it's going to appreciably add to our load in a backup scenario despite having a bunch more breakers. For instance we'll have 2x50 amp circuits for our cars but we would never attempt to charge them during an outage.

Our heavy duty electric appliances include a wall oven/microwave combo and 2x 2 ton AC systems each with an LRA of 60. We have gas heat, stove, dryer, etc.

With some of the latest firmware I believe the powerwalls gateway would be able to interact with the cars if they are Teslas to allow you to charge then to a certain point if you need to but you could just unplug them when an outage occurs if you wanted to be safe.