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Elon, I love you... but the PowerWall isn't that great...... yet.

Discussion in 'Tesla Energy' started by wk057, May 1, 2015.

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  1. wk057

    wk057 Senior Tinkerer

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    #1 wk057, May 1, 2015
    Last edited: May 2, 2015
    I was thinking about the Tesla PowerWall a lot today, and a lot of that has been in perspective of my own solar project that many of you have been keeping an eye on. So, prepare for a bit of a ramble...

    I think there is a large distinction that isn't readily made clear by the presentation and the current website for the PowerWall.

    The PowerWall page currently says 10kWh for $3500, or $350/kWh. Sounds pretty decent considering it has integrated BMS, cooling, etc, right? However, the caveat makes this is a mostly useless product for basically everything last night's presentation was about.



    • Caveat: The 10kWh unit is "For backup applications."

    Let's focus on that for a moment. So it's basically a 2kW standby generator with 5 hours of fuel at the rate (10 hours at 1kW, 20 at 1/2 kW, etc). Well, that's probably good if you have a very small mandatory loads panel for your refrigerator and a few lights... but not much, if anything else. It's also only good if you get a lot of power outages and they generally last for short periods. Be without power for more than a day, tops, and a PowerWall will be a PowerlessWall. You could combine this with a half decent solar setup to extend this, but people looking at it for backup purposes (perhaps thinking they could replace a standby generator) are going to be pretty disappointed when their electrician tells then they'll need five or more of them to come close to what a standby genset will handle. Keep in mind, these units only put out 2kW. You can get a 2kW standalone gasoline generator for something like $200 nowadays...

    So for backup power I think it has severely limited applications as a single unit. It would take several to equal a small standby generator and people without solar would have no way to top off the tank, so to speak, and would be dead in the water after the packs were depleted.

    Now I know, Elon means for this to be used with renewables. Makes sense. This would be fine for someone who already has or is getting solar if they want to be able to run their refrigerator and a few lights while the grid is down. I think the backup version, however, has some pretty narrow use cases in the current market.



    • Daily Cycling 7kWh Unit

    So let's look at the real product. The unit that can be cycled daily for balancing your power use and shifting power use to off-peak and all of the other wonderful things that were mentioned in the presentation that have absolutely nothing to do with the $3500 unit.

    The 7kWh unit is the one that can be cycled daily and be used for these things, and costs $3000 or about $429/kWh. $429/kWh is pretty expensive. To get that money back at average USA utility rates of $0.1229 per kWh you'd need to use that $429 kWh about 3500 times just break even on the unit cost. At a daily cycle that means load shifting 7kWh per day at 100% efficiency (site claims 92%) for almost 10 years just to be neutral with the cost of the 7kWh unit. But wait, there's more...

    The PowerWall site specifically says that the unit does not include an inverter. (I ignored this for the previous backup use cases above because it's impractical even without adding this on top.)

    If you have grid tied solar already you may already have a grid-interactive inverter that is capable of accepting power from the PowerWall and feeding it into the grid as you desire. Great. It'll still take you over 10 years, average, just to cover the unit cost.



    • Grid Arbitrage

    For those who do not, or are thinking about just shifting loads from on-peak to off-peak, you'll need an inverter. A "dumb" 2kW grid-interactive inverter isn't all that expensive, but it will lengthen your ROI time none the less. Get one that can actually be smart about how it handles this battery power and you could approach doubling that time.

    This is all assuming the PowerWall unit has within itself the means to charge from the grid. If you need an AC->DC converter, then add in that cost (unknown at this time if the PowerWall has this).



    • Real world numbers

    Now, let's say you've gone through all of this and you're still ready to jump on board.

    The EIA says that the average American residence consumes about 10,908 kWh worth of electricity per year. That comes out to about 30 kWh per day, or an average load of about 1.25kW 24 hours per day, 7 days per week. The load will obviously vary, but we'll use this as a base. Using this, we can determine that it is likely that during the on-peak, off-peak, and solar peak times of day that there will be enough time in any of them to charge or discharge this 7kWh battery pack fully in any of those three time frames for the average American.

    Hypothetical: Let's say that the household has a large enough solar array to produce their yearly kWh usage over the course of a year so their net usage is 0 kWh. Nice! Well, now the utility company gets smart (like many are) and isn't paying this household the same per kWh feed in as they are charging for feed out. We'll say they're nice and only charging $0.10 more per kWh used than sold.

    Well this household is in a good position to benefit from the Tesla PowerWall, right? Let's run some numbers.

    Assuming a good solar day, the Tesla PowerWall would be fully discharged at the beginning of the day because night time usage would have exceeded 7kWh on average. During the day enough solar power is generated so that all household loads are powered by local generation, and an 7kWh of what would normally have been put into the grid would be generated to charge the Tesla PowerWall. For the sake of simplicity we'll ignore what may have been generated beyond that and just say it was self consumed (best case).

    The sun goes down and it's go time. Good stuff. Tesla PowerWall uses it's power to offset loads........ up to 2kW. Anything beyond that will still need to buy power from the grid (HVAC, cooking, hot water, EV charging, etc). But, we'll still be able to offset 7kWh worth regardless over the next 3.5+ hours. Now, 7kWh of usage later we are back to being dependent on the grid. No problem... we just saved $0.70 today, right?

    $0.70/day payback on $3000 + inverter + install................. not looking so good for ROI at 12+ years.



    • What would it take to be useful?

    Now these are all just contrived examples. What would it really take for this to make sense?

    To ROI in 7 years (a reasonable long term investment, beyond this it's best to invest elsewhere usually) the unit would have to save about $1.18/day @ $3000. This isn't counting installation and other equipment needed. At a daily cycle, 7kWh per day, those kWh would each need to save at least $0.168 after efficiency losses. This would need to happen every single day, 100% discharge, 100% charge, to payback in 7 years. And this is just to get your money back.

    So, if you have solar and you get paid at least $0.17 less per kWh fed into the grid as you pay per kWh out of the grid, this could work for you. Unfortunately, this is a small subset of solar users so far. A lot of incentives work the opposite and pay more for solar input than charge output. Most just credit the same both ways (net metering) currently. If you're on time of use metering and your on peak is $0.17 more than off peak, and you already have the rest of the equipment needed to make this work for arbitrage, then this could work for you.

    The above, in my research, appear to be a pretty small subset of residences. I'm not sure how this can appeal, financially, to users who don't already have the related and required equipment to make this useful.



    • Going off-grid with PowerWall

    "But wk, what about using these to go off grid like Elon said?"

    First, let's get one thing straight. One of these units is not going to let any normal user go completely off grid. Not one person. Don't kid yourself. 7kWh per day is 23% of what the average American household uses. Double that for daytime solar generation and consumption and you're still at less than half of average. Sure, some people can make that lifestyle work, but that's not mass market. I'm not talking about the people who are willing to make large changes in usage to fit this product into their lives. I'm talking about the regular people who just want something that works for them, not the other way around.

    Then, you have to factor that there are days where there is little to no sun. (My worst solar day since testing has been 4% of expected average output... which is really bad and might as well be nothing.) You're going to need a couple of days of reserve power. At half of the average American usage (15kWh) and two days of reserve (probably bare minimum for an off-grid use case) you're looking at 4 or 5 of these PowerWall units *minimum* at half of average usage. Moving back to the average USA residence and that's more like the maximum of nine units just to have normal use + a "two rainy days in a row" reserve. Oh, and nine of these is $27,000 and is almost as powerful as 100A grid service (usually the feed for small homes with non-electric HVAC and cooking/hot water).

    Long story short, without major energy efficient life style changes most people would not be able to go 100% off grid even with nine PowerWalls. You could get really close, but at some point you'll run out.



    • My setup

    I have avoided comparisons to my own large off-grid solar project to keep this write up as general as possible. However, I feel that it's worth pointing out that my complete setup including everything (100+ solar panels, 8 inverters, installation, and 191kWh worth of Tesla batteries) is costing somewhere around $500/kWh total per kWh of energy storage. Extracting just the parts that the $3000 PowerWall does for that price, those parts cost about $250/kWh (mainly the batteries). Even if my setup were lead acid based (the "ugly" and "smelly" batteries that Elon compared to in his presentation), which it doesn't, the cost would still be significantly less per kWh than a complete system using PowerWalls.

    Not only that, many solar installers already do these battery setups. They may not be in a sleek and fancy case like Tesla's, but your wallet will be better for it.

    I have 191 kWh of storage (that's the same as ~28 of Tesla's 7kWh units......) because that's what it will take to get me through a string of a few rainy days at average or slightly below average usage and to stay off grid as long as possible. Grid electricity is only $0.10 per kWh where I am, so no amount of grid arbitrage would ever pay for itself here. Honestly, my project is beyond what I probably should have invested in it. Unfortunately I was already too far into it to not follow through once that became apparent.



    • Summary, TLDR version

    I'm still confused as to why the unit *doesn't* include a grid interactive inverter (similar to a small Outback Radian)... if it did, at the same price point, I may be writing something entirely different.


    Anyway, the TLDR summary version: PowerWall sounds cool, it just doesn't really seem practical for the masses. It makes no sense for the average person to put one of these daily cycle units on their garage wall, especially considering the cost of the additional equipment needed to make it work as advertised. It sounds good at first glance, but if you really run the numbers for your situation.... it probably doesn't make as much sense as you might think.

    Feel free to post some numbers for your situation (on-peak/off-peak rates and times, solar buy sell prices/etc) and I'll do some crunching for you if you like in the thread and we'll see how far off I am with all of this.

    I hope my dissenting opinion on the PowerWall doesn't bring about immediate hatred. I think that most people who have followed my projects here and elsewhere would know that at least somewhat know what I'm talking about and I wouldn't take such an opinion blindly.

    (Keep in mind this is about the PowerWall residential unit. I do believe the commercial unit (PowerPack) has huge potential.)


    ----

    Update: Posted first round of responses to questions and comments here: Elon, I love you... but the PowerWall isn't that great...... yet. - Page 3

    Update: Did up a spreadsheet demonstrating an on peak/off peak load shift using two powerwalls and 1000kWh per month normal usage (and example provided by another user). Two PowerWalls, 14kWh/day load shift to off-peak - Google Sheets
    (scroll right to see realistic with efficiency calcs)
     
  2. MarcG

    MarcG Active Member

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    Thanks for the long, thoughtful post. I enjoyed reading it, especially that I work in solar myself and do quite a bit of research on such topics - it's crazy how market dependent it can get!

    To address your specific point above, my take is that a Tesla is initially positioning the Powerwall for residential applications that already have solar systems installed, or will be installed concurrently. In these cases, the solar system will already include a string inverter, so the Powerwall would connect DC to it and the PV array (note: this won't work in a microinverter setup, you'll need AC/DC converter to charge the batteries as well as a DC/AC inverter to grid-tie or power the home).

    So, while I totally understand your points and it would make sense to include an inverter in standalone applications, I do like the fact that Powerwall doesn't automatically include the inverter (agree that it should then be priced lower..)
     
  3. Dutchie

    Dutchie Member

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    Thanks for your analysis. It was my hunch too when I saw the presentation last night. The price seems fair at the state of technology at present, however, for the application it wants to use it for the state of technology is not quite there yet. I haven't got to analysing it in detail as you did, but you numbers seems to confirm my initial thought.
     
  4. mwulff

    mwulff Member

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    @wk057 Thanks for the write-up, while I have no doubt that it accurately represents the american market this unit will be sold globally.

    Many european countries use significantly less energy per capita than the US (documentation). Exceptions are Luxembourg, Sweden, Norway, Iceland and Finland, this can be partially explained by the fact that Sweden and Norway have access to abundant hydro-power and Iceland has more geothermals than they know what to do with. I don't know about Finland and Luxembourg though.

    If you go back and redo the math for a more european consumption of about 8-15 kWh's per day and stiff taxes on grid usage then the Powerwall becomes insanely attractive. I'll do the math for my own usage and a general solar-array and see how long it takes to pay for itself.

    But again thanks for the insights :)
     
  5. Johan

    Johan Took a TSLA bear test. Came back negative.

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    Good analysis wk.

    Like I said in the investment thread: the current sensible business case is large scale industrial/commercial applications.

    Elon confirmed it with his tweet.
    eacc63cdcede3ca53497b2f7338c4682.jpg

    Oh and kWh use varies widely. I just paid for 8000 kWh in two months (March and April).
     
  6. cantdecide

    cantdecide Member

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    yes... and no, but mostly yes I agree with you.

    Firstly you missed another big negative of going off grid... Summer versus winter.
    You aren't going to store energy from summer to use in winter (completely uneconomic).
    So you need to size your solar to work for winter in general. This produces a great deal of extra energy in summer (assuming you don't live in the tropics). If you are doing that, why not sell the excess back to the grid for a profit?

    However, having said all of this, nobody is "average", noone has 2.3 kids (etc)......
    The product isn't for everyone or even the average person (i.e. I fully agree with you) ... almost no product is... but there are a significant number of people who aren't average that this would work for. If 1 in 10 US families are appropriate for this right now, the gigafactory will be busy for quite some time. With increasing solar penetration, the use cases will increase and this can stop solar killing the grid.

    For the 7kwh box...
    1) Hawaii. If your electricity costs $0.50/kwh and your excess solar can't go to the grid then the economics are great. Install solar and batteries for your average usage and reduce your utility bill 90%.
    2) CA. Incremental electricity price for most people is $0.30-$0.40/kwh. California has many people. CA may push everyone to time-of-use. Much of CA has time of use differential of $0.20/kWh. Do the numbers look any different for that?
    3) Other places with expensive electricity...
    4) Many people have solar... Solarcity has already announced this as an option for new systems and would be installed next to the existing inverter. This drops incremental installation and inverter cost to almost nothing.
    5) off-grid... while this doesn't help you to cut the cord, it will be the method of choice if you build remotely and the utility wants to charge you $100k to connect then instead spend $60k on batteries and $30k on solar. [unless another brand of batteries is cheaper and better?] This may even spark more remote building as some things become viable.
    6) The threat of cord cutters, and the threat of small users. Regardless of whether a utility has net metering, this gives everyone the chance to be a small user. This pushes infrastructure costs to everyone else. Utilities are pushing back by trying to charge small users more. If they do that enough, cord cutting can start, threatening to snowball into having the same infrastructure costs over less customers. Overall, this can change the attitude of utilities... they can't make solar a profit center for themselves, or the users go away. Then solar and users win.
    For the 10kwh box:
    1) An off-grid can supplement some 7kwh boxes with some 10kwh rainy-day boxes (cycling once per week).
    For backup:
    1) I just had my backup generator serviced after it failed in the last outage. Almost cost $3k. Can we factor maitanence in? Do the numbers improve?
    2) I'm ok with 2kW if this allows me to use the full power of my solar panels as well while the sun shines. With some smarts in there I could still run my a/c while the sun shines, which I can't on my generator.

    For the utility box (also used by many large businesses).
    When the "duck" curve gets worse, as it will, this box can save the solar industry (e.g. Hawaii now, CA in 2 years).
    Overall wholesale electricity rates become more smooth and more predictable.
     
  7. mwulff

    mwulff Member

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    #7 mwulff, May 2, 2015
    Last edited: May 2, 2015
    Ok, I went back and did a lot of data lookups. For this analysis I crunched 2 months of power usage data (Juli and January) and I looked at our daily usage pattern. The bottom line is that the Powerwall would allow us to go off-grid for about 8-9 months of the year resulting in significant cost savings.

    I also looked at our daytime vs. nighttime consumption and found that we never exceed 7 kWh of nighttime usage, typical number is about 4 kWh. So the Powerwall has the capacity to shift enough solar power from midday to the night. Basically we can cover all of our power needs with solar because of the Powerwall.

    And now for the numbers. First up daytime power consumption vs nighttime. In this context daytime means sunrise to sunset.

    Representing summer:

    5th of May: daytime: 6.48 kWh, nighttime: 3.18 kWh.

    And in the winter corner:

    5th of January: daytime: 6.37 kWh, nighttime: 6.28 kWh.

    I chose these days as examples because they were worst-case within that particular month. It is clear that we would never exceed the amount of power that the Powerwall can provide. Also I was unable to find a single instance of our house drawing more than 1,05 kWh of power at any one hour. So the 2 kW specification + 3kW peak is well within what would be needed.

    I used a small 4.32 kWp solar-array as an example. This is the kind of array that would fit on our small roof. We could probably fit a 6 kWp array but that would just make all the numbers even better :).

    Next up is savings pr. month. In the summer would be around $160 in electricity, 30-40% of that comes from the powerwall shifting the power resulting in powerwall savings of about $56 pr month in summer. In the wintertime the savings are $8 pr month. On average we are looking at around $33 pr month in savings from powershifting. Multiply up to a year and we have about $400 pr year in savings from the powerwall alone and it pays for itself in ~8 years. Basically it will be under warranty the whole time and if you buy the extended warranty you should be good for another 10 years.

    This is all based on current prices for electricity and the price is expected to go up over the next 10 years and the payout from selling power to the grid is expected to fall about 50-60%. At that point the powerwall becomes a complete no-brainer here.

    I simulated expected changes to power-pricing and the monthly saving went up to $50 per month for the powerwall , now it would pay for itself in 5 years.

    In closing here are some stats:

    January:
    Total power used: 362 kWh.
    Solar power generated: 72.9 kWh.
    Max daily usage: 17
    Min daily usage: 7

    July:
    Total power used: 265 kWh
    Solar power generated: 576 kWh
    Max daily usage: 14 kWh
    Min daily usage: 4 kWh.

    Now I'm off to figure what is so power hungry in our house, 17 kWh is just embarrassing. Watch out, something is about to get turned off for good.

    PS. If anybody wants a copy of my spreadsheet let me know. Most of it is in danish, you have been warned :)
     
  8. matbl

    matbl Member

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    Ehh? 1.05 KW max? You don't have a stove? Or a vacuum cleaner? Or a washing machine?
    (And obviously not a Tesla MS)
     
  9. mwulff

    mwulff Member

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    Actually we do, and I did state it wrong. We never used more than 1,05 kWh at any given hour. Our peak load in Kw can go to about 2,4 kW with the stove on, well within the peak load of the Powerwall. I will update my post to reflect the error
     
  10. Johan

    Johan Took a TSLA bear test. Came back negative.

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    Yes, totally unrealistic for most medium to heavy energy intensive developed world households.

    One 7kWh daily cycler will be perfect in places like India or Africa coupled with a solar array averaging 25kWh or so. At night you need to run some lights (LED), computer/media, phone and food refidgerator.
     
  11. mwulff

    mwulff Member

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    @Johan: I assure you that the numbers I used are completely realistic in Denmark and not that uncommon. Of course I don't count Model S charging along with the house. That will obviously never charge from a Powerwall..

    You are welcome to my usage data if you don't believe me. I would consider Denmark to be the developed world.
     
  12. thegruf

    thegruf Member

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    @wk057 - fantastic post

    Prior to the lauch I already queried here the ROI of such a unit,
    However can I add another perspective to this.

    Tesla cannot produce a mind boggling large number of these units.
    This is Tesla's first generation unit and is the first product realization of Elon's vision in this direction.

    Much of this will be gauging price point vs manufacturing capacity vs market.

    If it made perfect sense to most everybody the demand would be crazy (and all the competitors would pile in immediately - probably with inferior product)
    so imho it is pitched rather well at early adopters who will pay a premuim even if it doesnt necessarily make 100% financial sense.

    More interestingly though over time is how utilities, states and countries view this, every step in managing peak demand reduces requirement for spare(99% of the time) generation capacity.
    Not building a new power station ($$$bn) is quite significant.

    In this sense ELon's visionary approach really is market disruptive, not for the first time
     
  13. Johan

    Johan Took a TSLA bear test. Came back negative.

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    Do you cook with gas or electric stove? Do you have a water boiler? Or other kitchen appliances? Are none of these rated above 1000W? How do you light your home? How do you wash clothes? Do you own a vacuum cleaner and if so, does it have less than 1000W of power? Do you have any floor heating anywhere? Where does your hot water come from? How do you heat your house in general in the winter?
     
  14. arg

    arg Member

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    My guess on reading that part of the spec was that they are aiming this as an upgrade for people who already have solar and hence already have an inverter - if the PowerWall can just tap in to the DC between panels and inverter, that makes for an easy install and avoids having to scrap your existing inverter(s) which in any case need to be sized to match the solar installation - possibly larger than the PowerWall capacity. Not sure how the off-grid backup fits into this though.

    I recently had a similar (lead-acid based) system installed for my mother, where the system we bought integrated the inverter - and we had to junk the existing inverter (the installer grudgingly gave a small trade-in). Interestingly, this system, although integrated, has separate outputs from the inverter box for the grid-tied and grid-failure loads - so we end up having to have a large contactor to switch the loads from one to the other. In fact in this setup (5kW power, 17kWh of battery) instead of a 'standby panel', we actually power the main panel from the backup supply and have a 'heavy loads' panel with a few circuits to be lost when the grid power fails.

    Just as a comparison, this system was GBP 3500 ($US5000) for the inverter/charger box and GBP 2500 ($US 3700) for the batteries, so about $10K for a system about double the rating in both power and capacity compared to the PowerWall, so by the time some margin has got added to the PowerWall it's only a little cheaper than current competetors, It was also expensive to install - about 2 man-days involved in all the cabling, regardless of whether the price I paid was reasonable. If PowerWall can drop in just tapping the DC between panels and inverter, that would have been just a 20-minute job in this setup - but not providing the grid-fail backup of course.

    And there's the issue: even with the perverse incentives we have in the UK this system doesn't remotely make sense as a financial RoI. We get paid for what we generate, and get charged for what we import from the grid, but using our self-generated power locally is 'free' (the amount we use has no effect on either the generation payment or the usage charge). So storing 'free' power when the sun is shining and using it later (or to clip peaks exceeding the solar capacity) saves 100% of the cost of that power. Unfortunately, the actual value is small in relation to the cost of the system.

    The only reason for installing my mother's system was for the grid-failure backup - it's a somewhat rural area so does have more power cuts than I get in the city, and my mother is convinced that a combination of the Russians cutting off gas supplies and/or the incompetence of our government will lead to a less reliable electricity supply in the coming years. It's infuriating to have solar on your roof yet no power you can use during a grid failure. So, the battery installation is a luxury purchase that's nice to have, not a financially-driven decision. A bit like buying a Model S really...
     
  15. J1mbo

    J1mbo Member

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    IMO Powerwall is more of a marketing tool to promote the "utility scale" product, PowerPack, which has the potential to be a real game changer.

    As per the OP, there are few real world use cases where a single 2kw Powerwall can make sense. I am guessing that most people are still paying back their solar installations through fee-in tarrifs and reduction in electricty consumption - adding the installed cost of a Powerwall will push the ROI back something like 10 years per unit.

    Would be great if they offered a PowerPack for domestic installations. A neat rack mounted 50kWh solution seems to make a lot more sense that a bank of PowerWall units.
     
  16. mwulff

    mwulff Member

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    I corrected my post to correctly reflect that we can use about 2-3 kW at peak loads. That includes stove and waterboiler. The house i heated by "remote heating" which means that we use surplus water from the local power plant for heating. This is probably a little unique to Denmark, but it provides efficient heat and warm water.
     
  17. lolachampcar

    lolachampcar Active Member

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    #17 lolachampcar, May 2, 2015
    Last edited: May 2, 2015
    The only thing the PowerWall brings to the table in the way of technical changes is the operating voltage. As WK points out, there are already a lot of existing battery options out there at competitive pricing even if they take up your whole garage.

    I too see this as a Gen1 solution and expect the real product to be Gen3. What it does do is move the idea of a battery to the 350-400 Vdc range which allows direct charging from PV string arrays. This DC voltage is right in line with the median DC output expected by string inverters. The rest of the electronics to properly direct available power to the right place (PV->Bat, Bat->Inverter, Grid->house) is child's play once you have done MPT, frequency sync and the like (standard functions in todays inverters). Now inverter companies have a reason to do these functions and have access to a simple bolt it to the wall solution. The simplicity of the battery solution also addresses the installation issue which tends to be a huge cost driver in non-PV savvy areas. For example, my DiY PV installation here in S. Florida was $2.2/W while using both roofing and electrical sub-contractors. It would have been over $4/W using a turn key PV house. That is a lot of extra for permitting, racking and margin.

    Gen1 just seeks to change the way we do storage batteries so that more will do batteries. It's the Roadster paving the way for Model 3. Musk is nothing if not predictable.
     
  18. dsm363

    dsm363 Roadster + Sig Model S

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    #18 dsm363, May 2, 2015
    Last edited: May 2, 2015
    I was thinking about getting this and didn't know what the difference between the 7 and 10kWh versions was so awesome post. Thanks.

    For summer the cost goes to a little over 40 cents in late afternoon to 20 cents from 6-9PM the back to 6 cents at night. Winter rates are between 5-8 cents all day. I don't have solar but may someday.

    Would some combo of a backup battery (no generator on site) with a few 7kWh packs make any sense?
     
  19. thegruf

    thegruf Member

    Joined:
    Mar 24, 2015
    Messages:
    933
    Location:
    UK
    there is another key difference with Tesla solution compared to other battery based offerings.

    Li Ion - lifetime should be 10yrs+
    Lead Acid batteries I believe only have half this lifetime (correct me if I am wrong here)
     
  20. Evbwcaer

    Evbwcaer Member

    Joined:
    Jun 21, 2014
    Messages:
    467
    Location:
    Minnesota
    In Minnesota, Xcel customers, can get on their time of day plan, basically 2 cents at night. Normal customers are about 12 cents 24/7/365. Time of day subscribers pay about 16 cents 9a-9p. If you can save 10 cents per kwh, and use 1000kwh/month, you're looking at $100/month in savings.

    I calculated a 4.5%, 5 year loan for $8,000 for two Powerwalls plus install, 93.22/month.

    So, for the first 5 years you are ahead ($7/month/12month/5years) $420.

    Assume you get another 5 years after that, 10 year warranty, with no loan payment and still saving $100 month. (5*12*100) = $6,000.

    $6,420 ahead after 10 years. If the batteries are still doing their job it only gets better.

    I do realize that a 1000kwh/month user will have a hard time staying at 14kwh 9am/9pm, but they could probably make it happen. Weekends and holidays are at the 2 cent rate 24 hours.

    You guys tell me if I have made errors, because this looks compelling to me...I should say that I am a bit of a Powerwall skeptic.
     

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