Addon to existing Powerwalls?

cr0ntab

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Feb 26, 2021
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Hey All,

I wanted to get the communities feedback on adding my own DIY powerwall to an existing system.

TLDR; what AC coupled inverter should I look into?

First, what currently exists:

1x 10kw SolarEdge Inverter with 10.89 watts DC of solar connected
2x Tesla Powerwall 2

My big question is around which battery inverter to use. The existing system is completely AC coupled so I'm guessing I have to go down that route to get additional backup power into the system.

From what I've seen (hybrid?) inverters seem to be a pretty good chunk of coin but I haven't been able to figure out what would be the best for my use case.

I am planning on adding another ~2970 watt's DC of solar, and I was going to do that through the existing solaredge 10k inverter. Maybe it makes sense to funnel this all into another hybrid inverter instead?

I haven't figured out how I'm going to tackle "splitting" the solar output between the two batteries, but one step at a time right?

The big driver here is to get cheaper storage. I bought the two powerwalls *before* I learned about this site, but they're pretty expensive for what they are.

I'm hoping I can add more storage for less money.
 
I wanted to get the communities feedback on adding my own DIY powerwall to an existing system.

TLDR; what AC coupled inverter should I look into?

First, what currently exists:

1x 10kw SolarEdge Inverter with 10.89 watts DC of solar connected
2x Tesla Powerwall 2

My big question is around which battery inverter to use. The existing system is completely AC coupled so I'm guessing I have to go down that route to get additional backup power into the system.

From what I've seen (hybrid?) inverters seem to be a pretty good chunk of coin but I haven't been able to figure out what would be the best for my use case.

I am planning on adding another ~2970 watt's DC of solar, and I was going to do that through the existing solaredge 10k inverter. Maybe it makes sense to funnel this all into another hybrid inverter instead?

I haven't figured out how I'm going to tackle "splitting" the solar output between the two batteries, but one step at a time right?

The big driver here is to get cheaper storage. I bought the two powerwalls *before* I learned about this site, but they're pretty expensive for what they are.

I'm hoping I can add more storage for less money.

Your 2 Tesla walls are the final gate keeper. They will nullify all what is going in and out of your home.

So, if you want to add your DIY low cost system, you need to do all of it before it comes to the Tesla walls!

I would do it like this:

Build the additional battery and treat it as a separate system from the existing. Just charge it with own MPPT chargers.

Use a simple Battery-grid tied inverter like the SUN2000 to push that energy out when you need it first. The CTs of that inverter must be placed before the Tesla gate (maybe even just on a very specific circuit like HVAC directly), so that they can discharge before the PW2.

You can use some sort of microcontroller with a few relays, to program the times, when you want to discharge that new pack (like your peak hours with most expensive tariffs).
 
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And the other thing is, you have to understand, that those 2 systems are not talking to each other. So they will both try to achieve the set goal, of not importing power from the grid. The faster system will win. If the Tesla wall is that faster one, it will equalize the current in your house, before the other inverter even notices it.
So it might be necessary, to allow the Tesla wall a small amount of import in its settings. Then the other inverter can always go for that bit.
It will surely be a bit of playing around in menues, get it run as desired.

But if there are times during the day, where your 2 PWs are not able to supply all of required demand, then it of course doesn't matter. Then the other system will just discharge the missing on top of it.

As a second option (probably the fail-save), you could choose one or more energy hungry individual circuits of your house, and feed them through a hybrid inverter like the MPP solar. As long there is battery power available from your new array, it will run as a isolated system, and when the battery is depleted, it will just switch over to the rest of your grid and the Tesla walls will take care of that part as well.
 
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