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Help me understand Powerwall2 use case

Discussion in 'Tesla Energy' started by masam, Jun 9, 2017.

  1. Ampster

    Ampster Active Member

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    How long a period does one need to show that data? Months? Years?
     
  2. bonaire

    bonaire Active Member

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    For Fed ITC - 7-year statute of limitations for auditing (I guess...) Always be leaning toward "the truth" when doing credits. Fed ITC is not for dual-use arbitrage but rather for green energy proliferation. However, CA state seems to care more about SGIP programs to handle peak demand support so the SGIP itself is allowing for arbitrage - thing is, the 50 cycles per year is pretty slim - I would have wanted 200 per year minimum.

    SGIP - I think 2-3 years of 50 cycles per year.
    SGIP Program information here: Self-Generation Incentive Program
     
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  3. miimura

    miimura Active Member

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    I would presume that you would have to include all the data from installation until the end of the tax year that you take the credit in order to determine how much of the credit you can take. Since you take the credit entirely in the first year, I would have to wonder on what basis they could ask you for further data beyond that tax year.
     
  4. bonaire

    bonaire Active Member

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    There is more extensive treatment to keep from abusing the Credit beyond the first year. The credit wasn't thought through fully and as we know, the full ITC can be taken for solar pv installed "on any roof" with nasty shading and poor output. I have seen a few horrible installs over the years, especially in NJ where value of SRECs were and are still high.

    Just don't take the ITC if you buy a PW2 for battery backup or arbitrage. It isn't right.
     
  5. MichaelVorst

    MichaelVorst Member

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    For places with high electricity costs, eg Australia, PW2 can save you a fair bit of money, pay for itself in 5-6 years, and deal with blackouts too. I've just done a detailed review of my install here:

    www.mikesgear.com
     
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  6. gavine

    gavine Petrol Head turned EV Enthusiast

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    Great write-up. Very informative and well-written.
     
  7. AZSam

    AZSam Member

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    Regarding the switchover time during a power failure, from what I have seen of the Energy Gateway pictures posted on another thread, it looks like it is not a typical transfer switch that would be used on something like a generator backup. It isn't a double pole double throw switch/relay that allows selecting either the Powerwall or the normal power line. It simply disconnects from the powerline when the power line is down. It can do this by simply using the power line to supply the coil current to the double pole single throw relay.
    It's then up to the Powerwall and inverter to get up to speed to supply the required power. This can be very quick with proper design. You may not even notice the changeover.
     
  8. gavine

    gavine Petrol Head turned EV Enthusiast

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    Like @ItsNotAboutTheMoney contemplated, it can be considered a luxury item.
     
  9. Reciprocity

    Reciprocity Active Member

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    #29 Reciprocity, Jun 12, 2017
    Last edited: Jun 12, 2017
    To me, the main use for PW2 is in places without net metering or bad rates for net metering. The costs are not yet there for arbitrage where you are buying cheap electricity and night and selling it back during the day. In places with no net metering, you would usually have to have a smaller system or waste the extra electricity generated. If you could consistent Charge your EV during those times, that would work great, but its hard because most people work. I think costs will come way down on PW2 as they ramp production and expand production. EV packs for model 3 should be around $125/KWh cost at the pack level. I could see the price dropping for PW2 to $3600 or so over the next couple of years.

    I am not in the market for a PW2 because we have good net metering where I am at, so I dont know this and I probably should; do you get any fed tax credit for PW2 like with solar?

    In addition to costs coming down, there are use cases where the Utilities will want to control your battery and will pay you for the option. I think its Vermont that is partnering with Tesla to test this as a solution where the Utilities can tell your PW2 or car when to charge and your PW2 when to discharge and for this right, they pay you some kind of access fee or cheaper rates for your car. Tesla is going to be one of the biggest utilities in the world in the not to distant future, and they will have the ability to leverage that to help remove more dirty energy sources from the grid.
     
  10. Lloyd

    Lloyd Well-Known Member

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    #30 Lloyd, Jun 12, 2017
    Last edited: Jun 12, 2017
    The power differential of $.33 per KWh is only available during the summertime. Wintertime differential is only $.20 which then stretches your payback time to ~14 years.

    Additionally PGE is phasing out the old time of use rates. The new rate is almost a flat schedule which negates the benefits of the powerwall altogether, and making solar more difficult to get a return for the consumer.

    Pacific Gas & Electric - Tariffs




    NEW Residential Time-of-Day
    Rate Schedule E-TOU
    Option A
    (3-8 p.m.) Time-of-Day Winter and Summer Peak and Off-Peak Energy Charges $0.32854 - Summer Peak $0.39336 ($0.08830) ($17.40) n/a
    Off-Peak $0.31778 ($0.08830)
    Winter Peak $0.27539 ($0.08830)
    Off-Peak $0.26109 ($0.08830)
    NEW Residential Time-of-Day
    Rate Schedule E-TOU
    Option B
    (4-9 p.m.) Time-of-Day Winter and Summer Peak and Off-Peak Energy Charges $0.32854 - Summer Peak $0.36335 - ($17.40) n/a
    Off-Peak $0.26029 -
    Winter Peak $0.22588 -
    Off-Peak $0.20708 -
    NEW Residential CARE Time-of-Day
    Rate Schedule EL-TOU
    Option A
    (3-8 p.m.) CARE Time-of-Day Winter and Summer Peak and Off-Peak Energy Charges $0.16427 - Summer Peak $0.23685 ($0.04779) ($17.40) n/a
    Off-Peak $0.18923 ($0.04779)
    Winter Peak $0.16253 ($0.04779)
    Off-Peak $0.15352 ($0.04779)
    NEW Residential CARE Time-of-Day
    Rate Schedule EL-TOU
    Option B
    (4-9 p.m.) CARE Time-of-Day Winter and Summer Peak and Off-Peak Energy Charges $0.16427 - Summer Peak $0.22163 - ($17.40) n/a
    Off-Peak $0.15670 -
    Winter Peak $0.13502 -
    Off-Peak $0.12317 -
     
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  11. WannabeOwner

    WannabeOwner Active Member

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    I read somewhere that it was 30ms - which is probably short enough that a PC's power supply will survive.
     
  12. miimura

    miimura Active Member

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    Anybody on this board likely has an EV, even if not a Tesla. The E-TOU and EL-TOU are terrible for EV drivers. You are much better off on the PG&E EV rate schedule. Here is my estimation of annual arbitrage savings when on the PG&E EV rate schedule. The assumption is that the battery is charged by solar during the Part-Peak hours on weekdays, Off-Peak hours on weekends and is discharged a fixed 10kWh/day during the Peak rate period, regardless of your actual power use. Accounting for weekends this way (2/7ths of the number of days in the season) gives you an extra $100/year in arbitrage. The Powerwall will have to push the energy out faster on the weekend because weekday Peak is 2-9pm (7 hours) while the weekend Peak is 3-7pm (4 hours). Anyway, this is completely dependent on the way Tesla sets up the strategy in the firmware, which it appears customers have no control over.

    Long story short, you could arbitrage $700/year purely charging on solar and discharging 10 of 13 kWh per PowerWall per day.

    PowerWall Arbitrage Estimation.jpg
     
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  13. miimura

    miimura Active Member

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    I never thought about it, but you're right, it's probably just a single throw disconnect, not double throw.

    The more tricky part is re-engaging the grid connection. They will have to prevent the return of grid power from arbitrarily kicking the switch back on. First, you want to make sure that the grid is stable before you re-connect. Second, you have to make sure the waveform is in sync before you re-connect. So, the gateway will have to sense the voltage, voltage stability, and phase so that it can re-sync the PowerWall inverter to the grid before it allows the switch to re-engage the grid power. If they do match the voltage and phase, there will be no discontinuity at all and therefore no arcing in the switch.
     
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  14. Lloyd

    Lloyd Well-Known Member

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    Likely the EV rates will follow the TOU rates! They will not be this good forever!!
     
  15. Tam

    Tam Well-Known Member

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  16. miimura

    miimura Active Member

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  17. montreid

    montreid Member

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    TOU schedules comparisons really depends on how much PV generation matches/overproduces and overall usage. We moved over to EV schedule in SDGE since our PV system is undersized at 3.8kw only with 2 EVs.

    That said, forget PW2. I want V2H. Why invest in another battery when we have 50+KwH sitting idle in the driveway at the shoulder time when the PV dies out and out utilization spikes.

    All our OHM alerts these days are the shoulder periods since we have so much solar production now with the duckbill phenomena.

    V2H solution for those with PV/Central Inverter already is easy gain. Of course Tesla PW2 would die because of that (hence no push from Tesla to support V2H).
     
  18. hmmm

    hmmm Member

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    Funny you ask...I was sitting at Peets near the San Mateo supercharger the other day and overheard 2 Tesla interns (I think) on the phone discussing marketing copy and messaging for the PW/SR combo to clear people's confusion, improve understanding of what does what, how they are complementary etc. They came up with something good I thought for the latter: Solar by day, storage by night.
     
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  19. bonaire

    bonaire Active Member

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    #39 bonaire, Jun 23, 2017
    Last edited: Jun 23, 2017
    V2H should be a big industry - I don't get why it is not yet.

    Instead of NCA batteries in cars, put the PW2 NMC batteries in cars in a small enough quantity and at a low enough price so that using it as car and V2H (not daily but during power failures or occasional arbitrage) would be fine. NMC can take 3500+ recycles. And Jeff Dahn has shown that new chemistry doping of NMC could offer 90% capacity after many more thousands than that - say 8000+ cycles. V2H needs such batteries only if the cycling is commonplace. But a car with 50-100 kWh on board used as V2H on occasion should be a regular consumer use case.

    Heck Tesla should offer an "NMC Upgrade" battery pack in cars and introduce the "game changing" (I hate that phrase) V2H solution to allow a MS 90 to operate a house with enough stored energy as six powerwalls. Powerflow circuits need to be determined but it could be clipped into a Storedge inverter as if it were another PowerWall. Perhaps the design of the high voltage DC PW2 was intended as a practice solution to eventually using the HV main battery pack of cars in a future V2H? The PW2 AC has a 50V internal battery voltage so it can be used in many existing hybrid-inverter solutions (which don't use high voltage DC battery subsystems - typically they use 48V nominal).

    Powerwall 2 Full Specs Reveal Cheap Storage And Limited Warranty
     
  20. montreid

    montreid Member

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    From my understanding Tesla engineers mentioned new batteries are powering PW2 and they appear to be NCA batteries that will power the Model 3. The final chemistry balance maybe different (I don't think so) since the warranty of 70% at 10years a a commitment of power consumption to 2800 cycles (7.5+ years if cycled fully daily) if fully discharged is consistent with tesla car battery performance.

    This would make sense to use the exact batteries, then modules and ultimately packs to cost efficiencies out of the Gigafactory.

    V2H is my HOPE.

    That said, with SGIP, we're exploring adding PW2 heavily subsidized and a break even year 4 with our Netmetering+TOU scales. That allows for 3+yrs under full warranty if cycled daily still. So, may work out until V2H matures (or someone learns to hack a tesla and the tesla PW2 gateway to discharge when plugged in)
     

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