Installing PowerWall 2 in an existing grid-tie solar system

[dennis]

Does anyone here know what you would have to do to install Powerwall 2 in a grid-tie solar system and also use it for the disaster backup scenario? I understand that Powerwall 2 has an included inverter, but that is used to convert the DC output of the batteries to AC. The input to charging the batteries is AC, so that power will come from the existing inverter(s) in the solar array.

In my grid-tie system, the SunPower inverters convert DC from the panels to AC only when they sense the presence of AC power from the grid. This protects the utility company lineman from getting zapped by a distributed power generation source he doesn't know about. This works fine in the load-shifting case because the grid is functioning. But in the disaster case the grid is down and therefore my existing inverters won't allow the solar panels to charge the Powerwall.

My assumption is that I will have to change out one or more inverters to more complex ones that protect the grid but also allow the charging of the Powerwall. Can anyone refer me to specific products that provide this function?

 

[AEdennis]

@dennis Great point... I wonder if my set-up is similar to what you're describing. I'll have to see if someone has the answer to your (and now my question as well.)

 

[martinicus]

+1 same question with my solaredge inverter. I'd imagine it might be fairly involved, as my inverter is hooked up to the circuit breaker. There would have to be some sort of shut-off relay to isolate your house when the grid is down. Would be great to be able to plug the Tesla into the powerwall and charge directly off the DC.

 

[miimura]

When the grid goes down, you need to disconnect from the utility. This applies to generators and batteries equally. Usually, this disconnect is called a transfer switch. It can be automatic or manual. The existing PowerWall installations that provided backup power had a transfer switch and a auto-transformer. The additional transformer is needed because the house (split phase USA grid) has to have a neutral with power that is +/-120VAC from the grounded neutral. The SolarEdge inverter is a pure 240V device, so it needs the autotransformer to balance the power about the neutral. I suspect that the integrated inverter in PowerWall 2.0 may actually be dual 120V inverters so they don't need the extra transformer. In any case, you will be without power for 2-10 seconds while the system activates the automatic transfer switch and re-starts the inverter in island mode.

With some battery inverter systems, you can run them so that they are not interrupted when the grid goes down. However, the sizing and load panel arrangement has to be designed for this. For example, you could have the batteries and inverter only supply the critical loads panel and not be able to feed back into the grid. Other high power non-critical loads would be connected directly to the grid main panel. This would be a setup to maximize self-consumption and only use the grid when the batteries were low and to do things like charge EVs off-peak only. A separate input to the battery inverter system would basically just be a battery charger, not a grid interactive inverter connection. I don't really expect the PowerWall 2.0 to be this flexible.

EV charging directly from the stationary battery pack requires a completely different device to regulate the charging current between the two different battery pack voltages.

 

[gavine]

I'm thinking they might offer a PW2 option without the inverter

 

[miimura]

I'm thinking they might offer a PW2 option without the inverter

It would make more sense to me to offer a version with no DC solar input than one that had no inverter.

 

[gavine]

It would make more sense to me to offer a version with no DC solar input than one that had no inverter.

Why? If you have solar panels and an inverter, couldn't you put the PW (sans inverter) between the panels and inverter? The inverter wouldn't know (or care) if the DC power was coming from the panels or PW.

 

[miimura]

Why? If you have solar panels and an inverter, couldn't you put the PW (sans inverter) between the panels and inverter? The inverter wouldn't know (or care) if the DC power was coming from the panels or PW.

Sure, if you have an inverter that already works with PowerWall 1.0 that makes sense. However, it doesn't make any sense for Tesla to put any more effort into working with other inverter vendors to implement their charge/discharge strategies. A grid tied inverter is very simple - maximize the energy pushed into the grid. Period.

 

[gavine]

Sure, if you have an inverter that already works with PowerWall 1.0 that makes sense. However, it doesn't make any sense for Tesla to put any more effort into working with other inverter vendors to implement their charge/discharge strategies. A grid tied inverter is very simple - maximize the energy pushed into the grid. Period.

Right. I forgot that PW isn't compatible with all inverters. Thanks for the clarification.

 

[David.Harris]

OK so thinking outside the box for a second, here's a 'no brainer' feature request - why can't the Powerwall 2 function just like an online UPS when wired for backup thru an ATS to my sensitive load panel.

Homes and businesses nowadays have so much electronics that I REALLY don't want to spend >$25k on a 28kVA system of multiple batteries and 30 solar panels if my 'sensitive load' line drops for 10 secs each time the grid goes down. Clearly the grid returned excess needs to be cut but why does the whole system need to drop.

I'd like all sensitive loads to run 'online' from my stored batteries, which are then in turn charged by my solar panels and generator if need be, and when excess solar is generated its returned to the grid.

It's not a question of safety, because UPS's especially higher end ones >3kVA that are traditionally hardwired have been doing it for years.

The demand for this solution would be substantially higher as it would also fully wrap up the off-grid use case. Hybrid really has to go the next level to be really compelling, and better still - as a Tesla integrated component in PW3+.

Otherwise, please tell me I'm missing something - am happy to be proven wrong . . .

 

[miimura]

A hybrid inverter like an Outback Radian can do the UPS function exactly as you would like. They can also be ganged to build large systems of up to 10 units for 80kW continuous output.

The reason that a PowerWall can't do this is that it only has a single AC connection and must drop with the grid, flip the transfer switch, then restart in island mode. A Radian on the other hand has a dedicated AC inverter output and two additional AC connections for grid interactivity and generator input. When the battery is full and your solar is still producing, you can export to the grid and still have your loads uninterruptible. Alternatively, you could connect the grid to your generator input and permit the installation with no back feed. That means that you can only draw from the grid and you would not need permission for solar interconnect. Overall, a much more flexible and powerful system. However, the installation is much more complicated. The upcoming SkyBox system from Outback Power is supposed to address that issue and simplify installation.

 

[alfmatsenius]

Australia boasts more than 1.5 million homes with solar panels to date, with many more systems being installed each month. Now the Tesla Powerwall is available, many home owners are thinking about adding battery storage.