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Powerwall economics and tax incentives

Discussion in 'Tesla Energy' started by schueppert, May 18, 2015.

  1. schueppert

    schueppert Member

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    I'm interested in Powerwall for backup; in fact, I was one of the first to click and "order" one. After further research, it strikes me if you are going to buy Powerwall for backup you almost certainly should add a small solar system, even here in Houston where there are no local incentives and I can buy all the 100% wind generated electricity I want for 12c per kWh. Here's why.

    Let's say a 10 kWh Powerwall backup system costs $7k including inverter and installation. For that I get 10 kWh of power if the grid fails, then nothing. What if I add a small solar system to the mix? A 3 kW system would ordinarily come to about $9k (@ $3 per watt), but since I'm already paying for the inverter and electrical installation let's say it adds $7k to the bill, so $14k total. Now comes the fun part. The whole system, including the battery, now qualifies for a 30% federal tax credit on residential solar systems. So the net cost is about $10k. And I now have a system that (1) generates about $400 of electricity per year, and (2) delivers 10 kWh of power per day if the gird fails. Way more attractive to me.

    A couple important details I learned while researching this. To fully be covered by the federal tax credit on solar systems, a battery must only be charged from the solar. This isn't really an issue for a backup application, but it may in part explain some of the strange comments people have been hearing from Solar City. Also, the federal tax credit expires at the end of 2016 and I don't have much confidence it will be renewed so I need to try to get this project completed before then: fingers crossed that Tesla can deliver!
     
  2. JohnQ

    JohnQ Active Member

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    I don't know, it's still an long pay back period for the investment depending on how you value the backup storage. A 10 year payback on the solar savings is $4k meaning you're valuing the backup at $6k. If your time horizon is 15 years then the backup is valued at $4k. Still pretty steep.

    I haven't looked at the details but can you charge the battery from solar if the grid is down? My understanding is that you can't. So, you get 10kWh to play with until the grid comes back up, not per day of continuous grid failure.
     
  3. schueppert

    schueppert Member

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    Oh yeah, still no way this works for me on economics alone. But adding the solar does seem to lower the cost and increase the value of the backup.

    I too have seen the speculation that it may not be possible to charge the battery when the grid is down. I'm not convinced, but I guess we will see. I would clearly not be interested in the solar piece if that is indeed the case.
     
  4. Cosmacelf

    Cosmacelf Active Member

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    Depends on the inverter. There are hybrid inverters out there that will allow you to do this (charge the battery when grid is down) with lead acid batteries. The inverter manufacturers are busy building inverters specifically designed for Powerwall (at least SolarEdge is). The presumption is that it will allow this functionality.
     
  5. Ampster

    Ampster Member

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    Yes, it is my understanding that you CAN charge the PowerWall through the built in DC to DC converter. It would not matter if the grid were down for that part since the inverter would not play a role in charging the batteries. That is an important part of qualifying for the Federal Tax Credit, ie that solar has to charge the batteries.
     
  6. Cosmacelf

    Cosmacelf Active Member

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    Yeah, Ampster gave a better answer than I did. Since the Powerwall is on the same side of the inverter as the solar panels are, the inverter really doesn't come into play.
     
  7. Ampster

    Ampster Member

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    Hey Cosmacelf, thanks for the complement but I left out the little detail that something has to operate as a charge controller so the batteries don't get overcharged. Maybe the BMS or DC to DC converter will do that and then the inverter will send the balance of the power to the loads or to the grid. Presumably that is what the new Solar Edge inverter will do.
     
  8. schueppert

    schueppert Member

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    One of the nice things about SolarEdge's architecture is that the optimizers are already an accurate voltage regulator controlled by the inverter. And that's really all you need for a charge controller. So I think their plan is to just plug Powerwall into one of the DC ports on the inverter with the panels going into the other port. Add a bit of software and I think they are good to go, potentially even on existing units (which they have said will support Powerwall). The AC side is a bit more complicated.
     

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