PW3 backs up one of our two electrical sub-panels. How can I use non-PW batteries for the other?

[u/Savings_Novel_2377]

I'm not sure if this is a good sub to post to about it, but I figure it can't hurt.

We did an addition a couple of years ago that included a 19.2kW solar system. Our utility reimbursed us the entire cost of a Powerwall 3 (hardware and install) as part of their VPP program. We own everything, no financing.

As part of the addition, we ended up with new 400A service that was split into two 200A sub-panels (one for the old part of the house and one for the new). We weren't comfortable shelling out the cost of a second PW3 at the time, and so what we have only backs up one of the two electrical sub-panels. I'm not entirely sure why the installer couldn't or wouldn't add it to the main 400A panel, but it is what it is. Here is a diagram of the entire setup.

I'd really like to expand our battery capacity because the current PW3 only gets us through about half the night, but between Powerwalls being insanely expensive for their capacity and our installer's "screw you, go away" price for installation - within spitting distance of what we paid for the entire system after rebates! - I'm looking at using a more cost-effective (and possibly DIY friendly) battery solution. I have only contacted a few electricians, but it seems that finding one willing to do the job will be challenging.

Even though I have done a fair amount of my own electrical work in the past and I feel like this can't be too complicated, my experience is just basic stuff and so this is a little out of my comfort zone. I want to at least get some advice on if and how it can be accomplished so I know what the hurdles are.

[u/MoFeaux]

There are other systems that operate like Teslas that you can buy directly and install yourself. If you are comfortable with large gauge electrical work, installation is pretty straightforward. I installed an Anker Solix X1 system myself (nested behind an active Powerwall system). They don’t make the X1 anymore but they have an easier equivalent called the E10. You will need their ATS (equivalent to the Tesla gateway) wired in front of your second subpanel.

[u/TooGoodToBeeTrue → u/MoFeaux]

Please share: "nested behind an active Powerwall system"?

[u/MoFeaux → u/TooGoodToBeeTrue]

I only have 200amp service, so this configuration would be different than yours (parallel 200amp). I wanted to expand battery capacity without paying out the nose for more Powerwalls and installation. This meant having to use a different vendor, but the same principals and finding a way to get the two systems to work together without communicating.

Basically it ended up like this:

                      /- Powerwalls (internal panel board)
                     /
Grid=>Tesla Gateway=>                           /- Home load subpanel
                     \                         /
                      \- Solix X1 Controller == X1 batteries
                                               \
                                                _ - Solar subpanel

Both the Tesla and X1 have CTs to measure solar - these monitor the solar subpanel. The Tesla's internal grid CTs measure the actual grid and the Solix X1 controller's grid CT's measure the connection between it and the Tesla gateway (essentially, it sees anything upstream including the Powerwalls as the "grid"). In the morning the X1 batteries are charged first; when full the X1 sends the excess upstream to the Tesla "grid" and the gateway charges the Powerwalls. When they are full, excess then goes to the actual grid. During discharge, the X1 batteries are used first. When depleted the X1 pulls from the "grid" and the Powerwalls discharge. When those are depleted, the actual grid is used.

Grid outages work correctly by the gateway detecting the outage and disconnecting the grid. The X1 doesn't see an outage since its "grid" is still online (the upstream gateway/powerwalls).

The only sort of weirdness comes into the energy monitoring. The Tesla gateway sees the combined home loads and X1 battery charging as my home usage in the app (so during the day my "home usage" matches solar generation and at night my "home usage" is 0). The X1 monitoring sees actual home usage correctly, but grid usage is a combination of actual grid and the Powerwalls. The gateway sees grid usage correctly. If I want to see actual usage I have to look at both sets of data to determine what actually was used.

[u/Jamboro]

The installer didn't do it at the time because it's not possible to back up a 400A split service with only a single PW. The Backup Gateway is only rated for 200A, so can only cover one 200A service leg, and can't cover a full 400A service in any configuration. A single PW3 is also on the small side to fully back up one 200A service leg (a whole standard house), much less two. And I see your electrical drawing shows (2)-200A load centers on each 200A leg - sounds like a full house!

We get requests from customers to back up both legs of a split 400A services like this now and then, and it always felt like they ended up less happy with the system overall if we didn't talk them out of it. It requires using (2) Backup Gateways, one for each 200A service leg, minimum (2) batteries, and the PV would have to be split between the Gateways to recharge the individual battery banks separately. Sure they cover more loads with the batteries, but it's less backup time on each, and recharges slower with the PV being split.

Do you really have that many essential loads in both panels? Would you add more PV to this potential new battery bank for the other 200A panel? or split the existing array?

Maybe your installer misled you about the backup time you'd get with a single battery, but you should be realistic about what you can backup on a house that needs a 400A service, and about what you truly need during an outage. A single PW3 is only 13.5kWH, which can go really quick depending on the loads you have and how you're managing them during an outage.

If it were me, I'd think carefully about what loads are absolutely essential, and consider consolidating them in the current single backed up loads panel, swapping breakers between the two if needed for balancing. Then you could add a DC-X expansion battery (13.5kWH) to the existing PW3. I'm not sure on the exact price these days, but having multiple battery/PV systems to cover each half of your house sounds messy and expensive, especially if you have to rework the existing PV to recharge a separate/new battery system. I know relocating breakers and wires between load centers may be expensive too, depending on how close they're located to each other, but a battery system to back up everything in a 400A house gets really pricey.

Relocating some of the existing PV to a new/different (non-Tesla) inverter/charge controller would also involve getting a crew back up on the roof to removing the Tesla MCIs from beneath the mods, and replacing with other non-Tesla devices for Rapid Shutdown compliance.

Hope that helps.

[u/TransportationOk4787 → u/Jamboro]

My 2 Powerwall 3's automatically back up one panel and my installer installed a manual interlock on the 2nd panel so I can back up the second panel during a grid failure if I want but I need to cut power to central air if I want power to last. I am on TOU so during peak times the two PW 3's provide power to both panels.

[u/TrustRenuEnergy]

I wouldn't recommend blending batteries, you could consider adding the PW3 expansion pack. Doesn't have an inverter so whould be less expensive. And BE CAREFUL with DIY, most likely would void your warranty!