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Getting Free Powerwall - Will it save me money and will it work

Discussion in 'Tesla Energy' started by Uncgolf, Dec 9, 2015.

  1. Uncgolf

    Uncgolf Member

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    Ok I "Won" the powerwall with the Tesla referral contest. I am not on the power company's time of Use program, but considering switching once I get the powerwall. Paying 11 cents kwh 24/7 now. If I switch I pay 6 cents off peak, 11 cents shoulder and 23 cents peak. I would obviously charge the Tesla in off peak and try to time shift as much as possible, and then use the powerwall as much as possible during peak time. Power bill last year was $2500. NO Solar panels here

    On-peak pricing is in effect during the non-summer season months from 6:00 a.m. – 9:00 a.m. Monday through Friday. Shoulder-peak times include 9:00 a.m. - noon and 5:00 p.m. – 8 p.m., Monday through Friday. All other times are off-peak, including weekends and holidays.
    On-peak pricing is in effect during the summer season months from 1:00 p.m. – 6:00 p.m. Monday – Friday. Shoulder-peak times include 11:00 a.m. - 1:00 p.m. and 6:00 p.m. – 8:00 p.m., Monday through Friday. All other times are off-peak, including weekends and holidays.

    I know I need a converter which cost about $2k. What will I need to get the Powerwall to charge during off peak hours and discharge during peak hours?

    If I could time shift and only use off peak, I would save about $1000 a year. I know I can't get there 100%, but guessing maybe 50% of that.

    Thanks

    Mark
     
  2. Chris TX

    Chris TX Active Member

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    That sounds about right. If you can schedule it to charge only during off-peak house and drain only during peak hours, it's going to pay for that inverter quicker. I've yet to see what kind of app/GUI the PowerWall has.
    I have free electricity from 9PM to 6AM and would LOVE one or two PowerWalls. My power bill would be around $0.
     
  3. Cosmacelf

    Cosmacelf Active Member

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    Let us know when you actually get the Powerwall. I have yet to see a confirmed sighting of one.
     
  4. ChadFeldheimer

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    Given that your annual power bill is $2500 and you are paying $0.11/kwh, that means you use about 23,000kwh each year. Or about 62kwh each day - or $6.85 each day.

    Let's ignore the powerwall for a moment. Suppose on a typical day (averaging in holidays and weekends) you use 15kwh peak, 16kwh shoulder, and 31kwh off-peak (shifting half your usage to off-peak). This means your cost of electricity is 15*.23+16*.11+31*.06 = $7.07. You're paying a little more for time of use billing with this usage pattern, but it's not much. If you end up using more peak, then the powerwall has a taller hill to climb. If you can shift more off-peak, you might consider time-of-use - powerwall or not.

    Now add in the powerwall. Suppose you shift 7kwh (powerwall capacity) from off-peak to peak each day. If the powerwall shifts at 100% efficiency, you will save (.23 - .06) * 7 = $1.19 each day. It will take $2000 / $1.19 = 1680 days, or 4.6 years to pay off the inverter/converter. As stated above, longer if you end up using more than 15kwh peak (on average).

    If my math is correct, powerwall is not very compelling for your time of use rates. Do you have another home with a larger rate difference between peak/off-peak?

    - - - Updated - - -

    Also, congratulations on the "win"! How many referrals did you make? If you're not comfortable saying, no problem. I'm just curious.
     
  5. Config

    Config Member

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    Since it doesn't seem like you're going to save any money, sell it to me and I'll figure out how to wire it up to my PV/solar array system ;-)
     
  6. SabrToothSqrl

    SabrToothSqrl Active Member

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    If it were me, in PA, we have pretty cheap power. I'm around 10-12c total cost kWh. But my power likes to go out during storms.

    Id use the battery backup one to run my well pump and propane furnace, and fridge... Lot easier than my generator hookup hack.
     
  7. Uncgolf

    Uncgolf Member

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    I don't have it yet and have no idea when I will actually get it. I assume in the next 6 months. Installation is included. I referred 11 people, so I got all 3 giveaways. The Giga factory tour, the Powerwall, and the discounted MX.

    ChadFeldheimer, I agree with your calculation, but I think I will be able to charge the battery more than once a day. Charging at off peak rates will save money when I use it during the shoulder hours too. Charge noon to 5 pm in non summer months and discharge 5-8pm.
     
  8. ChadFeldheimer

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    Good point! That's an extra $0.35/day on non-summer, non-holiday weekdays.

    When I calculated 4.6 years from 1680 days, I neglected to exclude weekends and holidays. But this is approximately offset by the extra $0.35/day. I think you're still at ~5 years to breakeven.

    As a related aside, I installed solar PV at a previous home, with a similar breakeven point (~5 years). It turned out I unexpectedly had to relocate after 2 years. Now my tenant enjoys a lower electricity bill =)

    Congrats on the 11 referrals (and counting)! That's a lot! Seems like Tesla might do well to give free powerwalls to everyone that generates 10 referrals (versus just the first to reach 10 in each region). There's probably a reasonable correlation between referring EV's and powerwalls.
     
  9. Uncgolf

    Uncgolf Member

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    I think my pay back will be sooner than the 4.5 years. I would not switch over to Time of use without the Powerwall and I will have significant savings on weekends with TOU and I will save 40% on charging the Tesla. If I can just keep peak use at a minimum by using the battery or cutting back, I should come out ahead.

    The first program was everyone that referred 5 people got the Powerwall. They changed it.
     
  10. Kbra

    Kbra Member

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    I'm not sure how your utility works but for us in CA there's tier rates even on TOU. So my off peak rate would be ~0.12 per kwh but thats only for the first say 200kwh (i dont recall the real number) then it starts climbing.

    So depending what your total usage is your calc will change a lot if you get hit with tiers.
     
  11. johnbarbour

    johnbarbour New Member

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    I'd look into 2 other things as well.

    If your current plan has a 'demand' component to the bill, it might offer some savings comparable to shifting use. 6/11 isn't a very big spread.

    Also, I'd go ahead and assume that you will only have about 80% of the capacity available. discharging more than 80% of the battery will shorten it's already limited cycle life (~1500). Also, the capacity will degrade with each full discharge.

    Lastly, the power rating is 3.3 kW, there is a god chance your demand gets a bit higher than that(some ac units are 3.3 alone), making the tou plan really expensive if you make a mistake.

    Install the thing and let it run for awhile.
     
  12. stopcrazypp

    stopcrazypp Well-Known Member

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    Before people get too excited, from everything I am reading, the Powerwall expects a 350-450VDC connection for output and charging. It does not come with a AC-DC rectifier, so you'll need one if you are going to do time-shifting, charging it up off-peak. This would be separate from the ~$2000 DC-AC inverter which is also needed to use the electricity. Then you need a system to switch smartly between charging and discharging.
     
  13. Uncgolf

    Uncgolf Member

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    stopcrazypp, what's the ball park of everything one would need to time shift? I never understood how the battery would know when to charge and discharge.

    Thanks
     
  14. stopcrazypp

    stopcrazypp Well-Known Member

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    I have no exact idea the costs. From discussions in the official Tesla forums, a 400V AC-DC rectifier will also cost ~$2000, but not sure where they got that estimate.

    As for switching, in a solar system usually that is the job of the inverter. It knows when there is output from solar and it can use that to handle the charging (which is done only by DC power from the solar panels) and discharging. The Powerwall has a control line which the inverter can use to communicate and tell it when to charge or discharge.

    You can see it in one of the inverters compatible with the Powerwall.
    http://www.solaredge.us/groups/us/products/storedge

    However, I don't think there are many solar installations that use grid electricity and energy storage to do time-shifting because in places with feed-in-tariff, it is typically not allowed. Right now the Powerwall is designed primarily for solar systems as far as I can tell. It is still early however, so perhaps there will be some third party solutions that can combine the rectifier, inverter, and the smart system to handle charging into one unit.
     
  15. Ampster

    Ampster Member

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    #15 Ampster, Dec 20, 2015
    Last edited: Dec 20, 2015
    My system of batteries and inverter are behind the meter and do not connect to the grid. I have a separate gridtied PV Solar system with microinverters. My NEM agreement does not say anything about time shifting and my belief is that it is none of their business what I do behind the meter. I am required to get a building permit for the inverter since it powers some loads. I do not have a feed in tariff so that may be the difference. My hybrid inverter has a timer to tell it when to shift loads off the grid. I use a separate timer and charger to charge the batteries at super off peak rates. It is not the most efficienty but in summer the rate differential is $0.36/kWhr.
     
  16. stopcrazypp

    stopcrazypp Well-Known Member

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    #16 stopcrazypp, Dec 20, 2015
    Last edited: Dec 20, 2015
    Right, for net metering it doesn't really matter as much what happens behind the meter.

    It matters for feed-in-tariff because they can't really tell whether you are just selling them electricity from your energy storage or from your solar panels, when the whole point of feed-in-tariff is to award renewable energy a higher price. If you use energy storage to just take off-peak electricity and sell on-peak back to the power company at a profit, obviously that breaks the intended purpose. So usually there is an outright ban on energy storage when feed-in-tariffs are involved.

    However, as you note, even your energy storage system is not grid tied and can't take energy from the grid (edit: actually should be can't give energy to the grid, but can take). From my research, this is a large majority of the case today. There might be solutions that come out for the powerwall that changes that.
     
  17. miimura

    miimura Active Member

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    What you said is actually backwards from my understanding of his system. I believe it should be "Your energy storage system is not grid tied and can't push battery energy into the grid." He said he can charge the batteries at Off-Peak times and use it with his own loads later.

    By far the majority of solar systems are simple grid tied with no storage. The majority of systems that include storage are completely off-grid.
     
  18. stopcrazypp

    stopcrazypp Well-Known Member

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    Oh right, I misinterpreted the part about not being tied to the grid. His system can't sell to the grid, but it can charge batteries using the offpeak rates. Basically this was what the OP wanted, but currently the powerwall doesn't appear to be designed for that.
     
  19. Ampster

    Ampster Member

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    #19 Ampster, Dec 20, 2015
    Last edited: Dec 21, 2015
    The Powerwall is just a battery with some communications capability. The things that need the programming in order to load shift are the inverter and charger, which are separate from the PowerWall. Some inverters can also work as chargers. Mine could, but it didn't have a way to program charging at certain times, so i went with a charger and timer. The Powerwall operates at above 360 volts and currently there are only a few inverters that operate at that voltage. As more Powerwalls are available, the supply of compatible inverters will hopefully increase.
     

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