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Help needed installing a Powerwall 2 Off Grid DIY

Please excuse my ignorance with the Tesla solar systems. I've been gathering Tesla equipment to upgrade my home off grid power system. I do not use much power; most things are running on propane. The TV and washer/dryer are my big power consumers. I do have a shop and run welders (tig and wire feed) weekly, but I have a Miller welder/generator for those instances. I currently have a very SIMPLE 12v system: four 250-watt solar panels going to a charge controller that maintains five 12v AGM batteries. This system does fine so long as the sun is out, however the batteries no longer hold a charge. So now is the time to replace the system and trash the old batteries. I've been trying to gather installation manuals and get a plan for the system. I'm looking for a SIMPLE completely off grid system (grid power is over $73,000 away). If there is a forum better suited to my questions, please send me the link or web address. If someone is willing to give me a call or a phone number that's even better as it will save so much time. I'm currently building an addition to my shop so its a sun up to sun down, eat and sleep life right now. PLEASE only reply if you have positive input with information that will help me get power the easiest way: another forum on another website had a member tell me the powerwall 2 is a better boat anchor than a battery for my situation! That is serious IGNORANCE and total BS!

Here is what I have:
1. sixteen 250-watt 60 cell
2. Powerwall 2 (battery)
3. 7.6 kW PV Inverter (Rapid Shutdown Equipment)
4. Backup Gateway 2

I originally purchased the Powerwall 2 battery thinking I could run my solar panels to it and run the Powerwall 2 to my inverter. That was a year ago and there was absolutely zero support for a DIY guy a year ago! At least now installation manuals can be found online but I'm still confused.

1. What would be the simplest way to utilize the powerwall 2 for an off-grid system?
2. Do I wire the solar panels in series to get to a target voltage? What is this target voltage? is it DC or AC?
3. Does or can the solar panel voltage directly charge the powerwall, or do I need to use the 7.6kW inverter?
4. Does the Backup Gateway 2 need to be used? Can a generator be used in place of the Grid Supply when the battery is discharged?
5. Is it required to hook the system to the internet and register with tesla?

Thanks in advance!
John
775-777-6081
 
1) I don't think there's a choice of different configurations--you'd configure your system like "Whole House Backup" in the instructions, but possibly with the generator in place of the grid.

5) Not sure, may be required for initial configuration. Certainly required for any warranty. Not sure if 100% off-grid is a supported use case.

4) Yes, the Backup Gateway will be required. It has a computer inside that tell the PW2 it's OK to operate as a micro-grid when the grid is out. You could hook up a generator as the "grid" and if the power quality is good enough (not clear on the details of what that means), then the PW will sync to the generator, but you'd need to configure the PW system for "non-export" as the generator can't absorb power outflows like the grid can. You may need to do this at least temporarily to bootstrap your system and do the initial configuration.

3) PW2 is AC-coupled, which means that the PV panels (DC) can not directly charge the PWs. Your PV inverter generates AC, and then the PW uses that AC to charge its batteries when appropriate.

2) Yes, and it's DC, and the details on that are standard for any string inverter PV install, there's nothing different about being off-grid.

Have not actually done the computations, but the inverter spec sheet will have a maximum allowable voltage. The PV panels spec sheet will give you an open circuit voltage (OCV), along with the parameters to adjust it for temperature. So the OCV at the extreme temperature you could get locally will determine the maximum number of panels per string; you may need to split the 16 into two strings of 8. There will also be a minimum number of panels per string, to achieve the minimum operating voltage the inverter requires. Also a maximum current per inverter input connection, which is based on the PV panel spec sheet short circuit current, again adjusted for temperature extremes.

Cheers, Wayne
 

BGbreeder

Active Member
Jun 19, 2020
1,114
791
Bay Area
You might find a recycled Nissan Leaf battery an easier starting point, as you would have the ability to customize it and control it yourself. With a Powerwall, Tesla is always going to be in the mix, and as @wwhitney points out, I don't think that 100% off grid is supported any longer.

All the best,

BG
 
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