expanding an existing solar setup, continuous load

[tomahawk44]

I'm working (I'm not lead but I don't want to speak out of turn if my line of thought is totally off kilter) to help expand three different solar setups (all residential, 200A service 200a panels). They all have similar conditions in that they have existing solar setups. Each location either maxed out of solar panel roof space or they have no interest in adding more. The only thing we are doing is adding more battery capacity. Two of them are pretty straight forward because we we're just expanding like with like, more of the same brand batteries EG4 in one location and Enphase for another. We don't have to add additional inverters.

The third I'm less sure about has a solar setup with Powerwall 3s, 1 powerwall leader + 3 powerwall expansions. It's setup with the leader connected to main panel and the tesla backup switch between the meter and the main panel (which gives them a whole house backup). Because they are at max expansion capacity for their first leader, I'm pretty sure this means we need to add new leader and probably ac combine both of them to a gateway. However, the HO wants to use EG4 server batteries, which works out for us because we rather not have to deal with the liability of fiddling or modifing the existing setup if possible. The Tesla design spec is to cascade 3rd party solutions at the panel the Powerwalls are tied into (or put an independent system if the gateway/switch is after the main panel and the PW is on critical load panel). This means we're probably looking at something like

current system
meter----> backup switch-----> main panel--(60A breaker, 48A nameplate)---> powerwall leader----> powerwall expansion

Due to existing continuous loads (which total 40A ish nameplate), I believe the plan was to put an ac combiner panel after the main panel (either 100A or 60A), then have the load panel after the inverters and move existing loads from the main panel to the load panel.

so branching off
meter----> backup switch-----> main panel--(60A breaker, 48A nameplate)---> powerwall leader----> powerwall expansion

main panel ----> ac combiner ---(derate breaker and inverters if necessary)--> 3rd party inverters-----> batteries

inverters---> inverter combiner ----> load panel

Does this look right? I'm assuming the thought is that by nesting continuous loads to their own panel downstream of the inverters this avoids counting the ac combiner and existing continuous loads together.

If this is the thought, is this the only way to address the issue?

 

[jaggedben]

It's pretty hard to understand exactly the setup you're describing and the relevant details.

Does Tesla support AC coupling with Powerwall 3 and third party inverters? I don't understand what you mean by 'cascade'. I'm not a Powerwall 3 installer.

Regarding your question at the end about nesting continuous loads, you don't have to add sources to loads when sizing a feeder, you can generally use whichever is greater. But you do have to consider them when qualifying a panel that they are both connected in.

 

[curt swartz]

Why not just add additional Tesla batteries. Doesn't make sense to make the system complicated both electrically and app wise. If the homeowner has any issues it going to be a cluster-F trying to get any support from Tesla or EG4.

 

[tomahawk44]

The current setup is something like this I believe.



From this doc - https://service.tesla.com/docs/Publ...UID-49E1A269-2EBC-4C2C-8AA1-44A08704065C.html

This is the design note about third party system with the Powerwall 3.


@curt swartz
To answer the question about why not more PW3, I think it's an issue to do with limited mounting locations. PW3 has greater clearance requirements.




@jaggedben

Any thoughts?

The major inefficiency obviously is one that Tesla noted themselves. In an outage, the Tesla system will make the third party system nested under it think the grid is still alive. The third party ESS can charge from the PW unless the disconnect is engaged for the nested system. The Tesla system should run out of juice first which will cause it to shut down to protect itself. If the user doesn't engage a disconnect for the downstream ESS, the nested system under it will detect the Tesla shutdown as a grid shutdown and engage ATS, presumably why they call this a cascade design.

 

[jaggedben]

Not sure what the disconnect in front of the third party load center is intended for, but I don't think you want to nest backup loads if the intention is to run the third-party system in a grid tied mode. A third party MID won't know when the Tesla Backup Switch disconnects for off-grid, and will try to backfeed the Tesla. At the very least, the Powerwall 3 has to support AC coupling. But even then you don't necessarily know how the MID

Even if the third party system is setup as a grid-tied only, you still need AC coupling support from the existing. In this setup I could see the additonal load center possibly being useful for self-consumption if that's desired, but otherwise I don't see the point.

Bottom line is I don't think you get away with not messing with the existing setup. Might as well price in the liability or not do it.

 

[tomahawk44]

Ah, that's a good point worth considering. This would only work of the PW disables charging from grid when it disconnects (I think).

PWs can AC-couple, one of their selling points was that they could integrate into existing systems easily. What I'm not so sure about if is the reverse is true. That pdf design note states

Third party backup systems (including generators, vehicles, Powerwall 1, third party battery storage systems, UPS, etc.) are compatible with Powerwall systems in the following configurations. An important note on combining backup systems is that only one system can perform site control at any given time. When the backup systems are installed in a cascading configuration, the Tesla Site Controller (Backup Gateway 2, Controller Powerwall+, or Powerwall 3) performs site control for the entire system until Powerwall is depleted, at which point the third party backup system controls only its system. In the configuration where the backup systems are installed independently of each other, the Powerwall and the third party backup system each control their own system.

The following is the note for what they call the cascade design.

In the example above, Powerwall backs up all the loads within the Tesla backup system. Once Powerwall has been depleted, the third party backup system backs up the loads labeled as third party. This is the most common configuration for systems with third party backup.

Presumably these configurations are common then, I'm just trying to understand how it's getting past safety checks, compliance, etc... The design doc doesn't explicitly state that the 3rd party is limited to grid-tie... though that doesn't mean it's not a restriction.... I assume that a 3rd party hybrid inverter would work with the caveat that the PW3 is designed to its own charging from the grid when it's preforming the site control, thus preventing the downstream 3rd party inverter from charging it in a weird negative feedback loop where they just try to charge each other.

 

[scott746]

It seems like a pretty complicated scheme for a residential setup. Are you sure the Tesla system will communicate correctly with the new inverters?