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Newbie Powerwall question

Discussion in 'Tesla Energy' started by mattack4000, May 18, 2018.

  1. mattack4000

    mattack4000 Active Member

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    Sorry for another newbie question related to the Powerwall. In my previous attempt to justify a purchase, I just couldn't get the numbers to work anywhere close to what I need to be. With the recent update to the software, I just want to run the numbers again and see if I should toy around with it.

    Before I start, I want to say I have a very tiny solar system with small electrical load. It is a 9 or 10 panel system, so I am thinking it is a 1 or 1.2 kwh system. I am in NorCal, I am guessing it will generate 4kwh total on average per day (correct me if I am wrong). The main power draw to my house is the car and AC during the summer time. The average load for peak hours is 8kwh per day in the summer and 3kwh during partial peak times with my solar panel offsetting some of load, i assume mostly during PP hours.

    Anyway here are my questions related to the Powerwall.

    #1 Is it true my solar panel will have to be disconnected from PG&E, therefore no more net metering? The solar panel's generation will go toward the Powerwall only?

    #2 In theory, let say I program the powerwall to hoard all the power during partial peak times (6AM-5PM). Does that mean I can charge roughly 1/3 to 1/2 of the battery in one day?

    #3 Once the clock hits 5PM, can I simply use the Powerwall to power the house and therefore eliminate almost all peak hours charges?

    #4 At 9PM, I would set the Powerwall to save its battery for the next day where the same cycle again?

    #5 What if the Powerwall is full and the solar panel is still making power?

    Sorry for the rambling, getting sleepy, but curious to know what the answers are
     
  2. cwied

    cwied Member

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    1. No. I think you may be confusing the fact that the Powerwall can only charge from solar. That doesn't mean that solar can't go to the grid, just that you can't charge the Powerwall from the grid.

    2. Without knowing how much solar you produce, it's hard to answer this question.

    3. With the time-based control setting, you could set it up to discharge automatically during peak hours, provided you can store enough solar energy to run your house.

    4. With time-based control, you just program your peak rate periods and the Powerwall will automatically switch on and off at the right time.

    5. Excess power goes back to the grid. In fact, with time-based control the Powerwall will run your house even if solar is producing so that you can export more to the grid to get the highest net metering credit.
     
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  3. arnolddeleon

    arnolddeleon Supporting Member

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    How old is your PV system? 1 to 1.2 kW sounds pretty small, unless it was a DIY system, the overhead of getting it installed would have made the economics pretty challenging if it was that small. Can you double check on the size? The panel count doesn't quite tell us enough. 16 years ago a panel would be around 80 watts. Today 300 watt panels are pretty common.

    Is your only goal with the power wall to save money? Have you compared it with installing more solar?
     
  4. mattack4000

    mattack4000 Active Member

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    This was put in new by the builder in 2015. I don’t have enough roof space for more panel unless I put them east facing. I am sorry you are right, the system is 2kwh by sunpower
     
  5. mattack4000

    mattack4000 Active Member

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    Thank you for the info. Now that i am more awake, I am putting more thinking behind this. There is no way my solar can produce enough power, so I know for a fact I will have to buy power during off peak hours. During the day, I can probably generate enough to offset my peak time usage and possibly offload some during partial peak in the winter. Looking at my peak usage from the past 2 years, I can save roughly $400 a year by trimming peak usage. So I guess the ROI is closer to 20 years, which is way past the life expectancy on the Powerwall I am guessing? Although I could argue having emergency backup is worth something too. Am I looking at this rationally or is it one of those thing where I should just buy it and not think?

     
  6. miimura

    miimura Active Member

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    You should definitely think about the payback. I was going to install a natural gas backup generator, so I have mentally subtracted that from my Powerwall installation cost. In my case, since I got in SGIP Step 1, I will pay back the balance of my costs in about 7 years, just with reduced energy charges. I paid about $1,000 at my last true-up. That year included charging 2 EVs and it was before I was changed over to the Silicon Valley CCA.
     
  7. mattack4000

    mattack4000 Active Member

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    Does the cost of the PW and installation qualify for the 30% federal tax credit?

    What is this SGIP credit? I already have solar, so no new generation

    THanks for the information, figure the Tesla guys are useless..
     
  8. miimura

    miimura Active Member

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    Powerwalls do qualify for the 30% Investment Tax Credit if they are paired with on-site solar.
    SGIP is a California rebate for battery storage systems. However, it is very late to get in on that program. Tesla has used up their allocation in this program. You could get an independent installer to do the Powerwall(s) for you and get the rebate, but the cost will likely be very close to Tesla installing them without the rebate.
    SGIP |
     
  9. mattack4000

    mattack4000 Active Member

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    What is the current wait on one?

    Also let say it is a hot day. My solar isnt generating enough to charge the battery or power my ac. Would the grid then kick in and power the house while the solar will either offset it or keep charging the battery?

    Thanks
     
  10. Raechris

    Raechris Member

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    That is my understanding.

    4-6 months I was told.
     
  11. SoundDaTrumpet

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    mattack4000, (a) to qualify for the 30% federal tax credit, you need to charge only by solar. California SGIP this green power source rule does not apply. However SGIP's language expects you to charge only by solar if you have solar (beyond reproach, legally conservative). (b) Using the return on investment calculation, I have 5 years before payback. The calculation is based on excess generation between 7AM-2PM for summer and for winter multiplied by the rate arbitrate (peak rate in cents minus shoulder rate in cents x 80% round trip inverter efficiency) inverse divided by my cost. Calculation is not perfect because I didn't account for the weekends having shorter peak hours and no shoulder hours; if accounted for, shortens payback period. My payback period can get as low as 4 years if I change appliances (neglecting equipment cost) from gas to electric. (d) Sounds good and all but according to the real time data, my install is one of the least expensive. You can easily see 10+ year payback paying retail. (e) Those paying fees to use the utility as a bank/battery (net metering) benefit more from Powerwall by a few cents per kWh. (f) You must be on some rate plan with huge multiples between peak and off-peak (talking dimes not pennies differences).
     
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  12. boaterva

    boaterva Supporting Member

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    Not sure if this is a 'regionality', but I've been told (and I believe Erik and David quoted the same) of 6-8 months. And that's presales, lol.

    What you really do, I was told, and have done, is just reserve them (a la Model 3) on the PW page. If you order them all together, they appear not to be able to install anything until everything is available. There is no problem, ITC-wise, of doing the Solar, and then adding the PWs later. As long as they charge from Solar, as was said above.
     
  13. mattack4000

    mattack4000 Active Member

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    I have done more deep dives on this topic. 2 Powerwall is simply not a good idea for me financially as my solar system is too small and there is no way I can top it off even on the best of days. If I want one for the sake of having one, I think my task is to get one and use the PW to offset all my peak time usage (about .40/kwh while using the grid to pay for my off peak and partial peak usage. With that financial model, I am seeing a 8-10 years breakeven point with the 30% tax credit in mind. I would also price the ability of having backup power is worth $750 personally. If I add that, I can trim another year off my breakeven number.

    I do have one question. I know the battery can push 5kw. If Iet say I have an emgerency and have to charge the vehicle, would the gateway be small enough to take half the power from the grid and half from the battery seamlessly to perform the task or would I trip the breaker?


     
  14. cwied

    cwied Member

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    Yes, if the demand exceeds the Powerwall's capacity, the extra will come from the grid as long as it's up. It's only if the grid is down that there would be an issue, but with only one Powerwall it's unlikely your car charger would be backed up, so I don't think you need to worry about it.
     
  15. mattack4000

    mattack4000 Active Member

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    It’s actually just my ac as that draws 4kw. I don’t charge my car at peak times
     

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