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Help me understand Powerwall2 use case

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

  1. masam

    masam Member

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    I'm interested in getting a Powerwall2 for emergency backup purposes. Is the main purpose of the Powerball to shift TOU or as a backup in case of power outages or some combination of both ? Does it even make sense to have a Powerwall with the sole purpose of backup power ?
    Thanks !
     
  2. wycolo

    wycolo Active Member

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    You need to have a real need for a PW to justify the going price or the effort involved in finding one to buy.
    --
     
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  3. kingjamez

    kingjamez Member

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    Without a large PV system, Power Wall doesn't make financial sense until the price comes down. TOU savings don't yet justify having one.

    -Jim
     
  4. SapphireX

    SapphireX Member

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    Might make sense in some areas where power is almost 'Free' at night and/or have very high
    daytime rates or with PV Solar without net metering but $6,200 for 14kWh Powerwall = $442 / kWh.
    That means that even if you charge it with 'free' power that is 'worth' 0.125 per kWh (Texas)
    you would have to cycle that Powerwall battery every single day for approx ten years
    ($442 / 0.12 = 3,536 / 365 = 9.79 years) just to break even without accounting for
    additional charge / discharge losses. (assuming you charge it daily)

    I think / hope there will be growing trend where the utilities actually start paying the
    customer to charge our cars at night. That might reverse the payback equation and quick.
    Here in Texas we produce tons of wind power that goes wasted at night. During the
    day we use what ever is available (clean and dirty) to meet the real time demand.
    The utilities have to build more and more expensive (peaker) power plants just to
    meet that summertime peak demand being made worse by the popularity of EVs.
    Every EV charging during the day is using power roughly equivalent to home
    electric home oven on 550 deg with the broiler on. That's a lot of daytime peak
    demand power.

    Just a side note .... a single 14kWh Powerwall can only store 56 miles of range (approx)
    for a Tesla. That means that a fully charged 100D is like $43,400 worth of Powerwalls.
    It would be sweet if the Tesla could feed power back to the Powerwall and house.
     
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  5. Tam

    Tam Active Member

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    Let's do another SIMPLE math for those who like to take advantage of the price difference as mentioned by SapphireX :

    Pacific Gas and Electric Company off-peak rate is $0.12225 while partial peak is $0.24986 and peak rate is $0.45389


    [​IMG]



    The price difference between cheapest and most expensive in a day is:

    $0.45389-$0.12225=$0.33164

    So if you can recharge and discharge during those hours, you earn or save 14kWh x $0.33164 or $4.64296 per day.

    $4.64296 x 365 days = $1,694.6804 annually

    In 10 years you'll get $16,946.804 which should well cover your cost of investment.
     
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  6. masam

    masam Member

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    Thanks for all the replies ! This is the best explanation I've seen for the Powerwall usage scenarios.
    I have a ~12kW solar system and was considering getting the Powewall mainly for emergency backup. Unfortunately, my electricity provider does not have TOU pricing. So, it looks like I might be better off with a cheap generator to run a critical circuit or two for the occasional power outage.
     
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  7. pilotSteve

    pilotSteve Member

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    Very interesting. California sure has some sky high electricity rates. Makes (a certain kind of) sense given other aspects of the benefits/costs of living there.

    In SW Washington (Clark County) we are blessed with BPA dams on the Columbia river with 24/7 power at $0.0816 per kWh. Guess I don't feel so bad that solar/powerwall doesn't pencil out for me.
     
  8. mr blue sky

    mr blue sky Member

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    I wonder what happens to peak rate pricing in the future. Already this spring before A/C systems were needed during the summer time there were times when apparently due to solar power production the wholesale energy price went negative (California’s Rising Solar Generation Coincides With Negative Wholesale Electricity Prices).

    If it's cheaper for power during peak solar times then they are going to have to make quite a bit of adjustments to their rate schedules. Possibly at some point if you don't have a battery system they may not even want to interconnect a new set of panels to the grid.
     
  9. ecarfan

    ecarfan Well-Known Member

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    Both.

    If you have frequent power outages than it certainly makes sense to use a Powerwall as a backup.
     
  10. WannabeOwner

    WannabeOwner Active Member

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    Depends on the nature of your power-failure.

    Frequent-ish short power failures, UPS might be best. Powerwall (as I understand it) will not cover the moment of the power break, so all computers etc. will reboot and work will be lost.

    Long duration power cuts - e.g. after a storm. I think generator will be best for that, but if you have Solar PV and can recharge a PowerWall during the day AND have enough PowerWall capacity for 24 hours (or more if you cannot guarantee sunshine at the same time as the extended power failure)

    We have a generator socket on the outside of the house, but I wish we had changed the wiring so that all "essential circuits" (like TV :) ) were on one board, so that we could easily isolate all non-essential circuits to make it trivially ease to reduce usage during a power cut. We have to go round the house turning everything off to get to the point where the generator can cope.
     
  11. bonaire

    bonaire Active Member

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    One user here reported that they indeed did not lose computers at time of "cut". They installed a PW2 as a sort of whole house UPS and the test where the switch-off from the grid allowed no disruption.
     
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  12. gavine

    gavine Petrol Head turned EV Enthusiast

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    With a Powerwall, you can actually USE your solar system during an outage. On sunny days, you should be able to charge up the PW while powering your house at the same time. The PW should then take you through the night. I'm thinking about getting one just for backup purposes with no attention to financial payback.

    Generators are noisy, and they require fuel and maintenance. To me, if you have solar, a generator makes no sense now that Powerwalls exist.
     
  13. WannabeOwner

    WannabeOwner Active Member

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    We have significantly less insolation, and numerous, sequential, "grey days" in Winter, so cannot produce a day's power usage on a good day, let alone a string of bad ones. If we did have enough PV for Winter we would have 10x too much in Summer (which is fine provided we get paid enough to export it and we have enough roof area for the panels!) Power cuts are more common in Winter, than Summer ...

    Generator is less than $500 and I use mine once or twice a year during a power cut. I don't bother if the power cut is short, so a Powerwall would be a bonus in that situation, in particular if it did indeed provide continuous uninterrupted supply

    Average daily household electricity consumption in USA is 32kWh, UK is 13 kWh . Insolation in London is 0.52 kWh/m² per day in December and 4.74 in July. Here is a graph for a UK PV roof - they have data for last 11 years :) its a 1.8 kWp system (which gives approx 1600 kWh p.a. in London latitude), the Y-axis is kWh per week

    [​IMG]

    At 13kWh per day this is only going to provide about 60% of average household usage in Summer and 10% from Nov through Jan.

    I don't think that that is the party-line from Tesla, only quote I could find was "The switchover time and UPS functionality are reliant on the DC to AC inverter. The inverters that are currently Tesla Powerwall compatible do not support UPS functionality. The inverters currently have switchover time in the seconds. In the future as our inverter partners develop UPS functionality, Tesla will notify its channel partners." but that was on an installer's site, not Tesla's own.
     
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  14. ItsNotAboutTheMoney

    ItsNotAboutTheMoney Well-Known Member

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    The main purpose of the Powerwall is to be a battery. Powerwall 2 uses a cell that's suitable for regular cycling.

    So you can use it for emergency power (with a suitable system that will automatically switch between home and grid power), you can use it to take advantage of TOU pricing (as long as your monopoly utility doesn't get in the way) and you can use it store solar power when your PV system is producing an excess, to cover times of the day when it's producing less than you need.

    As for the question of whether you should buy one, well, depends on whether you're thinking of it like an investment or a luxury item.
     
  15. gavine

    gavine Petrol Head turned EV Enthusiast

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    Whole discussion summed-up in one sentence!
     
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  16. brucet999

    brucet999 Active Member

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    #16 brucet999, Jun 10, 2017
    Last edited: Jun 10, 2017
    Thanks for an interesting best-case analysis.

    Wouldn't one need to limit the discharge to 10% SOC to avoid bricking the PowerWall?
    Also only works when one has 14kWh of peak time usage. Probably lots of people do in the East Bay and Central Valley, but maybe not on the coast.
    Would the 7kW discharge rate even cover the load of a whole house AC unit? But i guess it doesn't have to for your example, if PowerWall provided 6 hours at only 2kW during peak rates time, it would provide 12kWh of savings without going below 10% SOC.
    At $6200 cost for Powerwall, it would appear that the break-even would come at something like 4 years without going below 10% SOC.

    I'm going to run the numbers for my planned system in SCE territory where peak rates are "only" $0.334/kWh.
     
  17. Tam

    Tam Active Member

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    Ahhhhhhhh! Here goes the footnotes.

    I was providing a very SIMPLE math to illustrate why would anyone want to spend money for Powerwall 2 as arbitrage.

    But life is not simple and it has many footnotes to prove that arbitrage may not work in all cases!
     
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  18. Tam

    Tam Active Member

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    There have been reports of power outage but Powerwall owners didn't realize it because the switch was so instantaneous and smooth:

    73-year-old Tesla Powerwall owner powers through South Australia’s state-wide blackout without even knowing

    Tesla Powerwall delivers 12 hrs of power during SA blackout

    [​IMG]

    Fossil generator may have an initial low cost of investment but it just can't beat Powerwall convenience if you can afford it.
     
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  19. Ampster

    Ampster Member

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    In reality it would be a little longer than that because the $0.33 differential is only on weekdays during the four months of summer. The rest of the time including weekends in the summer the differential is some where arround $0.12 per kWh. That still could bring it in under 10 years. One caveat is that if you are using it to during day to arbitrage it might be difficult to assume you are charging it with the sun. Therefore the investment tax credit might be difficult to claim. Instead of trying to chose between rationalizing it as an investment or a luxury item I classified my system as a hobby, with back up and tool for load shifting.
     
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  20. bonaire

    bonaire Active Member

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    The logging features are your backup to determine if ITC is legal or not. Those taking the credit just need to have data stored for the charge cycles (24-hour time window logging stored in a server somewhere, perhaps Telsa is doing this for customers or the customer themselves could do it). CA SGIP program also requires something like 50 full discharges per year in order to qualify for the SGIP money (which is very large per KW in Level 1 - $7050 for one powerwall project).
     
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