Top Ten Facts about Tesla’s $350/kWh (DC) PowerWall battery

Special post by Bruce Lin and Matthew Klippenstein

[Edit May 11 – corrected PG&E peak price to $0.36 not $0.40/kWh – $0.36 was used in the calculation]

There’s an MBA joke about scaring your clients by asking them “What’s your China strategy?”.

Today, it’s “What’s your Tesla strategy?” $350/kWh (DC) retail really is that significant. In the few days since the Tesla energy storage announcement, we’ve had a half-dozen people ask what we think about it. As energy systems developers with experience in several different chemistries and system scales, we can make some well-grounded educated guesses on the design and economics.

Here are our top ten conclusions, with plenty of links to reference information.

1. The 92% efficiency figure is misleading

The “92% efficiency” figure quoted by Tesla isn’t as good as it sounds, but it doesn’t matter.

The PowerWall batteries themselves likely run at about 48VDC, and are boosted by an internal DC-DC converter up to 350V-450V. This is to match the DC input of typical inverters. The huge difference in voltages means a significant efficiency hit – one-way efficiencies are probably about 94% to 97%.

You have to add a AC-DC inverter (with 97% efficiency each direction), so your real-world AC-AC round trip efficiency drops to 87%. This is lower, but it doesn’t matter, because losing 13% of your low cost electricity is insignificant in the economics. Amortizing the capital cost of your system, by ensuring long lifetime for the batteries, is far more important.

PowerWall energy flows

2. The battery architecture is designed for long durations, which means low power.

It’s likely that the battery packs are two modules identical to (or very similar to) the Tesla Model S design, in order to achieve production synergies. Each module has 6 groups, and 74 cells in parallel per group. This makes a total of almost 900 cells, with an operating voltage of about 48 VDC which makes certain safety aspects easier to design. The two packs are one-eighth of a Tesla Model S 85 kWh.

92% efficiency can only be achieved by running the battery at extremely low current, to minimize resistance losses. (Our Catalytic battery model suggests this is as low as 0.6A per cell) For example, a normal Tesla car battery probably has a DC-DC round-trip efficiency of less than 80% because people charge quickly (one round of resistance losses), and output high power when driving (a second round of resistance losses).

The tradeoff is that they are putting in many, many cells to supply the 10 kWh – far more than would be necessary for the rated 2 kW of power, or even the peak 3.3 kW. (In a car, those two modules would be pushed to deliver about 30 kW of power) In other words, the PowerWall was designed for energy output, not power output.

By the way, this is the traditional argument of flow battery proponents – if you want to store lots of energy in regular batteries, you wind up being massively oversized for your desired power output; the faster you discharge electrons, the bigger your losses become, and the bigger energy your battery has to become, to deliver the same kWh to the outside world. In essence, you’re paying for a lot of power you can’t use. Alternately, if you did want to run high power, you would need more cells to supply the 5 hours of energy.

This isn’t a show-stopper – the achievement of $350/kWh (DC) is still significant. Customers will just need to be aware that they will need multiple units to serve high power.

3. Thermal regulation may be a hidden performance cost

Another benefit of operating at such low current densities is that there will hardly be any waste heat during charge and discharge (as a result of the lower resistive losses). However, ambient temperatures will be the major thermal issue – cooling during hot days to ensure long lifetime, and heating to prevent the cells from freezing in the winter. We can guarantee the parasitic efficiency of the thermal regulation system is not included in the quoted 92% DC-DC efficiency – electric heating to keep the cells from freezing will be a big efficiency hit for outdoor-mounted units in cold climates. This shouldn’t be an issue for Australian early-adopters, but it will be interesting to see what users in Germany (Tesla’s other early target market) report.

Along these lines, the flat design isn’t solely for aesthetics – it provides for more surface area for passive cooling, to keep the cells running cool and increasing their lifetime. Note that the raised “hump” in the design also discourages stacking of PowerWalls in front of each other.

[Edit May 10 2015: after doing some more calculations, it seems unlikely that this is correct – the heat generation from cycling really is negligible, and it’s more important to keep the system insulated from ambient heat/cold and incident solar heating. So it’s more likely an insulated box than a radiator. Would be very interesting to see one opened up …]

The fact the system runs on liquid cooling isn’t particularly significant. The Model S pack already uses liquid cooling, so why not use the same technology?

4. Your batteries can last hundreds of cycles, if the system is designed right

How come 7 kWh is only $500 cheaper than 10 kWh? Most likely, they have the same two modules, and the 7 kWh unit is merely cycled more shallowly (70% of the depth-of-discharge of the 10 kWh unit). The 10 kWh unit is only rated for weekly cycling, which means far fewer cycles over its lifetime (about 500 rather than roughly 3,500). By comparison, the 100,000 mile warranty on the batteries in the 60 and 85 kWh Model S implies up to 1,700 deep-ish discharges, or a (much) higher number of (much) shallower discharge cycles.

It’s fine if the 7 kWh “daily cycle” system loses capacity over repeated cycles, since it effectively has 30% more margin than the 10 kWh unit. Also, the fact that each cycle is very mild inherently reduces the chance of the undesired, damaging side-reactions that gradually impair batteries’ capacity.

Interestingly, the 10 kWh high-energy system uses lithium nickel-cobalt-aluminum cells (the same chemistry used in the Model S), while the 7 kWh daily-cycle system uses lithium nickel-manganese-cobalt “NMC” cells (actually a more common chemistry, used in the Nissan Leaf and power tools).

Panasonic Tesla Pack - by Theron Trowbridge

Image credit: Theron Trowbridge, CC-BY-NC 2.0

5. Cutting the grid connection will cost two or three times more than you think

2 kW should cover the average 1.2 kW electricity usage of the average American house, but 3.3 kW peak power will not be enough if you have many devices in a large American house, all running at once, and you want to disconnect entirely from the grid.

In fact, a SolarCity VP admitted that a single PowerWall is not enough to disconnect entirely from the grid, as 7 kWh doesn’t really offer enough power to cover all non-daylight hours: “it would require multiple units to take someone off the grid.”

Elon Musk noted in the 2015Q1 earnings call that it does not make economic sense to go off-grid, with PowerWalls. It was claimed that it might be economically in Germany, where feed-in-tariffs are less expensive – if there’s enough demand from our readers, we’ll do an economic analysis of this case next.

6. The all-in price is twice the $350/kWh ‘wow’ number, but still impressive

The core cell cost that enables this $350/kWh cost is believable. Earlier this year Swedish researchers polled a number of battery electric vehicle manufacturers on their proprietary cost structures, and estimated that Nissan and Tesla are currently at $200-$300 per kWh for the battery pack. The PowerWall sale price is an excellent real-world confirmation that manufacturing prices really are in this range.

However, the $3,000 or $3,500 cost is for the DC system only – be careful to compare apples to apples when looking at other products. For example, the inverter will add around $1,000 to $2,000 to the cost of the device.

As another real-world point, SolarCity is quoting $7,140 to add the PowerWall to a solar installation (includes inverter, maintenance contract, installation, control system). This doesn’t sound unreasonable, and again $700/kWh for a small home-scale system is very cheap, though as mentioned above, multiple PowerWalls may be needed for many customers. SolarCity is doing well with this sort of deal – the price is for new solar installations only, where SolarCity would have to install an inverter, control system, and wiring anyway, so it’s cheap for them to kill two birds with one stone.

Musk was being slightly misleading in his 2015Q1 earnings call when he said the inverter is part of a solar installation and should not be counted in the cost. Either SolarCity’s $7,000 includes the inverter cost, or the inverter cost is assigned to the solar part of the install, and other factors are raising the cost to $7,000. Either way the true price is more like $700/kWh (AC installed)

7. The PowerWall does not let you make money on arbitrage, and Tesla knows it.

[Edit: clarified that 36 cents/kWh was used in the calculation, not 40 cents/kWh]

Tesla executives confirmed that the economics don’t work in America, and here’s why. As a residential owner, in the best case you’ll make 36 cents/kWh selling to the grid at peak hours, and buy at 10 cents/kWh at night. (Northern California utility PG&E’s Time Of Use residential rate sheet).

Once you include round-trip efficiency losses, that’s about $1.66 gross profit per 7 kWh (DC) cycle.

Note that PG&E gives you that 36 cents/kWh only during the summer (defined as May-October). For the other half of the year, the difference is negligible and uneconomic.

So revenue is $1.66 x 183 days x 10 years = $3,000. You’ll just about recoup the cost of the product, but you won’t profit because of two factors – the cost of inverter and installation, and the time value of money. We’re in a low-interest environment, but setting aside $7,000 today to make back $3,000 over 10 years still leaves you down $4,000 – not a very good deal.

Your utility will be happy that you help them smooth out peaks and troughs this way, but they won’t make it worth your while. In the longer run, enlightened utilities may offer you demand pricing if you promise to keep maximum power low, or pay you to absorb spikes in power. And it’s likely that SolarCity (or the other installers) will take a cut for interfacing with the utility on your behalf, to participate in these kinds of demand-side management activities. It’s still hard to see this win on raw economics for a typical homeowner.

8. The PowerWall is for three different groups – and maybe a fourth

The economics don’t make sense for most customers in North America, but some will find it worthwhile:

    – Customers who highly value staying powered during blackouts, and specifically want a battery. A generator would be cheaper, but the battery will be quieter, quicker to start, and avoid fuel storage.

    – Customers who want to get off the grid entirely, in low-power applications at remote locations where a grid hookup (or upgrade) is prohibitively expensive, or with very expensive power

    – Early adopters who enjoy being able to show off the sleekly-designed PowerWall to their friends. It worked for the Tesla Roadster.

We’ll do a more detailed analysis of levelized cost of electricity (LCOE) and the German use case in a later article.

What this means is that, by itself, the Tesla PowerWall residential unit won’t disrupt the energy industry, as it’s looking like a niche product. The 40,000 early adopters that have reserved a PowerWall add up to 0.4 GW (a fraction of a single fossil power plant, or a very large solar installation, or 4,500 Model S cars). It’s yet not clear that it will expand much beyond that.

Still, it’s cheap, and it’s available (soon). These are huge factors in the energy business, and could lead to further scale.

9. The strategic impact is mainly on competitors, and Tesla’s supply side

The $350/kWh (DC) number is impressive, and Tesla did a good job of shocking industry watchers by quoting the DC-only, no-installation cost. Even the full price of $700/kWh (AC) is very cheap for a small-scale residential product, and research labs and energy storage competitors are going to have to explain their own path to beating that number.

We don’t have much information about the large-scale utility systems yet. We would expect them to be cheaper than $700/kWh all-in, and this already may be enough to gain significant traction. The highly modular approach with small building blocks (100 kW) is interesting – this could be a Google server type approach where a system is built of many cheap, replaceable parts. If there’s enough interest we’ll write more on this later.

Commercial customers can benefit from avoiding demand charges – if they commit to never exceeding a certain maximum power, this can gain them significant savings from their utility suppliers. This will be particularly true for commercial customers with large solar arrays in jurisdictions where feed-in tariffs are lower than utility rates – if they produce surplus electricity, it could be far more lucrative for them to charge their batteries, to minimize their grid draw when utility rates are highest.

On the Tesla side, there may be a supply chain play here. Every time Tesla doesn’t meet purchase agreements or sells fewer cars than projected, it can use these PowerWalls to soak up supply. Used battery packs may be a future part of the equation as well. Used packs are difficult to maintain and recycle, since they represent a safety risk, so rebuilding them into 7 kWh PowerWalls (running at extremely low current to extend their life) may allow Tesla to hit ever lower price points going forward.

Conclusion

There are a lot of details that Tesla would like to paper over in their $350/kWh (DC) announcement – so we hope that you find this analysis useful and perhaps a bit more in-depth than the first wave of articles published elsewhere. Still, Tesla’s PowerWall release is a ground-breaking announcement and a challenge to the rest of the industry, and we look forward to what comes next. Let us know what you think in the comments below

Catalytic Engineering is well-positioned for these sorts of assessments and we’re available for more detailed analysis, competitive intelligence reporting, engineering due diligence projects, etc. in batteries and other system engineering projects – please get in touch.

P.S. For our attentive readers who were keeping track, here’s number 10: The presentation started almost an hour late, and the electrical power for the event was run entirely on the batteries. Not to say that they’re linked, but a good reminder that it’s still early days for this technology and product, especially as Tesla has job postings for dozens of engineers to work on the second (non-prototype) generation of the PowerWall.

85 thoughts on “Top Ten Facts about Tesla’s $350/kWh (DC) PowerWall battery

  1. The Powerwall is for Hawaii. Solarcity is poking a stick in the eye of HECO. It may be for elsewhere, but wow, is it ever for Hawaii.

    • Good point. Will be interesting to see how that plays out. It seems like somebody’s going to have to pay for storage, whether it’s HECO or the homeowners …

  2. Thank-you very much for this in depth review. We so badly need information like this to try and balance the ‘hype’ that is being pushed about the Powerwall.
    Like you, I have had many friends and co-workers ask me about the product because it seems too good to be true, they have taken was was, at a very high level, presented at opening night, and run with it…. There are just too many holes and ‘gotchyas’ to be so black and white.
    I will be sending this write-up to as many of them as I can.
    Keep up the good work.
    (BTW, my biggest sore point is their total lack of any mention of the mains transfer switch, and any existing solar inverter surge power rating. It is just not that simple to take a house off the grid when there is a blackout – solar inverters are grid tie, the Powerwall will need to provide a start signal to the inverter….. On and on it goes. You get my point I’m sure).

    • I was looking for numbers too and found after a long research a strom-report and cleantechnica calculation on Per-kWh lifetime prices. http://strom-report.de/#tesla
      it says that Powerwall retail price will be minimum of 10 cent per kwh.
      retail price starts at 12 cent.
      the storage is not an economic advantage for people who pay less than that.

      • Hi Martin, thanks for your comment. That cleantechnica report is very good.

        As I understand it, they are assuming the solar energy is free. Also, that surplus energy is not used by the home AND cannot be sold to the grid. This surplus energy is therefore stored in the battery and used later, saving the homeowner from buying that electricity from the grid.

        The cost of the PowerWall gets divided among 15 years x 365 cycles to arrive at a cost of 25 cents/kWh.

        This is contrasted with buying that electricity at 35 cents/kWh

        Thus a net savings of 10 cents/kWh cycled. Electricity price has to be at least 25 cents/kWh to break even in this scenario.

        Seems to me to be quite a narrow and unusual case where you would be unable to use that surplus solar, or sell it to the grid for a revenue of at least 10 cents/kWh (rather than zero).

        More in our follow up article …

  3. Nice read. Thanks for that fine analysis. I would be glad to read more about the calculations for Germany.

    • As an engineer who has worked with diesel, oil-fired, bagasse fired, nuclear powered and solar power systems, I may have a different viewpoint. The path to low-cost power for both the consumer and the generator of power is made up of layers of generation and storage. In my case, I have a Maui home with three sources of power.

      Starting out about 25 years ago, all the power was supplied by Maui electric. Though I had no residential heating or cooling, my electricity cost was over $500/month. Installation of a five panel, two storage tank solar hot water system brought the cost per month down to about $300 per month. Subsequent installation of a solar electric system, deliberately undersized at about 1.2 KW peak power has brought the total electricity cost down to about $150/ month. A solar battery system, to supply the remaining 350 KWhr/mo or roughly 12 KWHr per day would cost an incremental $3000 for an additional 1.5 KW of generation and an additional $14,000 for a storage system. $17,000 to save, perhaps $150/mo does not seem like a good investment to me – even in Hawaii, where my power costs about $0.40/ KwHr.

      From my point of view, getting storage should be an incremental decision. The low hanging fruit, for getting off a central generation system starts with a hot water system. A solar electric system that supplies you, and not the power producer is the next step. The final step, in most cases, and even in Hawaii, is storage. PLease note that this is looked at WITHOUT government subsidies. A government subsidy is a real cost that you will someday pay. If it does not fly without the subsidy, it should not fly at all.

      • Sir i’m about to invest in a prepaid plan, ($6750.00) to sunrun for 0.067 rate. They take all credits & rebates. my elec. bill averages 130.00 per month. do i need to beware of anything? You can call me @808-450-1661 I need to make decision asap.

  4. One more fact:

    Lead acid batteries have been around for quite a while and are significantly cheaper. They may be a lot heavier than Tesla’s batteries but most houses don’t move so weight isn’t a big factor.

    • Hi Lee, traditional lead-acid is an excellent comparison, due to its low cost and familiarity. I spent a bit of time looking for a similar comparison. Here’s an interesting ~10 kWh unit for $6,800 including 120VAC inverter

      The batteries alone weigh 240 kg, as you mention (packaging and product design is definitely an area Tesla has excelled in) .

      Note that the battery spec limits depth of discharge to 50% if 500 cycles are desired , which I think is a key weakness.

      On the face of it, this system would be roughly the same cost, only half the energy for the same durability, and much larger/heavier. Are there better examples out there?

      • Powertech’s analysis is misleading!

        When talking of daily cycling, Li-Ion is difficult to beat in an interactive grid buffering environment where the battery will go top to bottom (70%DOD max) each day.

        In an off-grid environment, a large amount of contingency capacity is needed to allow for those few runs of cloudy days. The battery size and efficiency can be mitigated by over-supplying solar panels. This makes a lead acid a good contender. A typical day sees a shallow cycle (20-25%DOD) and there’s reserve for several dark days. I would expect a lead-acid battery to last 10 years if properly specified and maintained. Also NiFe would possibly be even better – totally abuse tolerant and 50 a year operating life — put it in your will.

        • Mr. Armstrong’s point is valid. Since pv is currently cheaper than storage of any flavor, it makes sense to size the pv above annual peak day energy use. Storage would be sized to mitigate local cloudy weather conditions resulting in considerable static use. (This applies to off-grid applications). Lead acid makes sense. Typical Bay area energy insolation and home energy use profile call for a roughly 10kW solar array with 3kW/100kWh of storage. This results in a tremendous amount of summer overproduction with no battery draw, but just keeps the winter low insolation/high rain season sustained with battery. Not very efficient setup but it works economically in PG&E territory with $1.50/W installed PV and ~$200 kWh storage. Overproduction pushed into water heating is added bonus.

  5. The most significant issue IMO is the near total lack of understanding of any of these issues and numbers on the part of the general public. It’s well and good that insightful and informed techies can make heads and tails of this innovation; given that the average mortgage remains 30 years, and presumably the cost of the system would be included in the 30 year note, a homebuyer would likely -unwittingly – end up paying $20,000 for the system. A compact natural gas genset will always be a better option for emergencies, and if someone really wants to go off the grid, there will always be better, more economical options.

  6. What all purveyors of Lithium-based chemistries get a free pass on (& unrealistically deflated $/kWh) is that recycling – genuine recycling, not just turning the lithium in these cells into cement filler sludge – is for the most part NOT part of their purchase price. That’s an externality the Lithium industry is currently getting away with, unlike lead-acid where nearly 100% recycling has been built into the purchase price & writ large across the world for decades.

  7. Virtually no one here is taking the global reality of climate disruption due to global warming due to human emissions of greenhouse gases into consideration! While commenters write about their desires for convenience, governments wrestle with how much they can promise to do about it, in the face of their business and media communities, like ours, minimizing it as much as they can get away with, in order to continue business as usual, in order to keep the incumbent players in their current relative positions of power. I know it’s human nature, but it’s entirely unacceptable. Rise above egos, folks! Commit to the good of the whole!
    In point of fact, global warming is perhaps the key driver for Elon Musk’s involvement in electric cars and batteries, and it is why he opened his patent portfolios, so that others can join a race to disrupt, upend, and replace the fossil fuel industry and the existing engine plants of the auto and truck manufacturers. He recognizes that hunting in a pack is a better strategy than hunting alone, because it gives the prey less opportunity for escape, and he welcomes the help of competition, which will of necessity focus on niche markets he may not cover.
    He also understands scale at a profound level, as implied by a recent story about Tesla viewing the six billion dollar Gigafactory battery plant as a product (to be replicated many times) rather than a one-off project. He is looking at the math of how many batteries need to be made to replace a billion engines in one or two generations. A billion cars, trucks and buses on the road globally divided by 30 years equals 33,333,333 battery packs, of 20 to 200+ kWh capacity, per year, and the volume in the early years will be a tiny fraction of that, so the total capacity to be built must be substantially greater; failure is not an option during the sixth mass extinction.
    We also need to build, extrapolating from an estimate by the National Renewable Energy Laboratories, storage for about six hours of global electricity production, but the cloud-based energy management systems, using data mining to learn usage patterns and calculate true availability, will make it possible to rely on vehicles and charging stations selling stored energy back to the grid, for a significant portion of the total. I include charging stations because they need stationary storage to be able to rapidly charge the vehicles whenever they request it, at the lowest rates per kWh.
    Taking the excellent discussion by Catalytic Engineering, and the many questions in comments about Musk’s business model, together, what is needed is to see the larger context. Most of the oil, gas and coal that is burned for electricity is wasted to combustion, conversion, transmission and distribution losses. Most of the oil that is used to make gasoline and diesel transportation fuel is wasted through low efficiency in the internal combustion engine. The extraction, refining and distribution processes that bring fuel to power plants and vehicles also use a large, and ever-increasing portion of the total energy content of what is extracted. What a waste!
    Meanwhile, renewable energy and battery storage keep getting more efficient and less costly. They are already at parity in most of the world, and will be beyond parity globally in most of the remainder within two years.
    The combination of the Risky Business Report, with its ongoing releases of State-level in-depth reports, the findings of major investment houses, the renewable energy trade press, and the fossil fuel divestment movement, is gradually getting through to institutional and individual investors. What are those findings? They use more conservative language, but in essence, they are, “Hey, guess what, folks! If you stay invested in fossil fuels, the asset base that supports your valuations is going to become stranded, and you’re going to lose your money! But look over here! Rapidly growing energy efficiency, renewable energy, and battery industries are going to enter a phase of explosive growth [in about two years]! You can put your money here!”
    I have personally been told, going back to a statement by the founders of Dynaship in October, 1975, and occasionally since then by others, that investors are too dumb to be able to consider two new ideas at the same time. Failure to simultaneously consider the crossing cost lines of the fossil and green energy industries, in the context of the need to leave over 80% of the known reserves in the ground to have a prayer of bringing a halt to the current catastrophe, at the same time, is being that dumb, and it will inevitably lead to the investment portfolios of those so disqualified going the way of the dodo, as they nod their heads in agreement with each other all the while.

    • I, for one, do take your point about the “global reality of climate disruption due to global warming due to human emissions of greenhouse gases” in to consideration.

      However, this must also be balanced with the economic realities as well. I know of not one nation who is willing to stop producing electricity in order to reverse global warming. In order to achieve a decreased dependence on fossil fuel generated electricity, there must an incentive that fits the economic realities as well.

      The Powerwall may not take us all the way there. It is, I believe, a first step for an industry that is still learning to walk, one that brings us closer than we were.

    • Thanks for posting this comment. I too was falling into the trap of considering only what was convenient for me, as I have been researching going off the grid I had a purely selfish motive – I don’t want to be at the mercy of the power companies – Of course your reasoned discussion puts all of this into perspective! Thanks again. – Neil

    • I think your premise is akin to religious fanaticism. Science that needs to be massaged, deleted, withheld or lied about is not science. More carbon may be good or may be bad but a hairline width of difference by “US” in the greenhouse gas levels for the cost the ecofacisists are willing to impose on “US” is what totalatarian dreams are made of. Read the 10th amendment. Live from there and leave me and my “Carbon Footprint” alone, I like it’s size just fine. Your pimple on the ass of history’s timeline is marked by an amount of hubris and narcissism that will be found to be the biggest fraud since “We are all equally at risk for AIDS”.

      • Like you, I also think that human caused global warming is total BS, but I like the concept of cleaner air (less carbon monoxide, nitrous oxide, soot, smog, etc.) that can come about by burning less fossil fuels. In BC, Canada, our power is 100% solar (hydro) and we pay $.08 Canadian per kWh up to a certain threshold and $.12 Canadian per kWh above that so this solar system solution is totally uneconomical for me. The only reason this would make sense for me is for a UPS solution in the case of a total breakdown in society where the grid went down due to war / EMP / X flare / global financial meltdown and gasoline for my 2 backup generators was unavailable. It would be interesting to know what the so called carbon footprint would be for the creation and distribution of these battery systems.

  8. I’ve been thinking about the system design a bit more.

    The system has negligible thermal losses – with 96% one-way efficiency, the worst case heat generation from cells and power electronics is 130W, basically two incandescent light bulbs.

    (In contrast, two modules in a Model S would be generating 30-40 kW, and at 90% discharge efficiency would be putting out 3-4 kW of heat)

    So operating the batteries doesn’t produce much heat. Instead, it’s the ambient temperature that causes the thermal problem. Must prevent the cells from freezing during charge, and from overheating during operation.

    So if ambient temperature is the issue, then design the system to be a very well-insulated box (rather than as a radiator), with only a few small openings for fan cooling on hot days. Possibly not as hard a problem as I initially thought, even on cold days.

  9. I’m quite pleased with the analysis which didn’t set out to poohpooh the Power Wall or the significance of the innovation.
    The points mentioned herein are too help interested people count the full cost before investing.
    Tesla is first a battery maker.
    Fortunately, I reside in England, so hopefully the kinks would have been worked out before it off available here.
    Thanks again.

  10. Thanks for the thoughtful analysis.

    Umm, not to read to much into this, but your illustration at the top shows a Xantrex inverter re-charging the Tesla PowerWall… do you think that will in fact be a reality?

    I have a XW Series (now Schneider) inverter at home and was curious whether older inverter-chargers like these will likely be compatible with Lithium chemistry.

  11. There is another fact not mentioned. Weight. They weigh 100kg/220lb each. It will take at a minimum two people to lift one of these units from the truck and then to the wall where they will hang. Weight is the limiting factor in the form otherwise they would have been made much bigger.

  12. 1. Your whole efficiency calculation is based on pure speculation that the batteries run at 48 DVC and boosted by an internal DC-DC converter up to 350V-450V.

    2. Tesla has already stated that the 7 kWh and 10 kWh are different units with different chemistries, and to say that they are the same amount of cells using a different DoD is incredibly speculative. The difference in life cycle is probably due to the cell chemistry and how they are setup (the 10 kWh battery is closer to the Model S per the quarterly call).

    3. In response to #5, yes you will need multiple batteries, but going off grid actually makes sense in places like Hawaii where electricity is more than $0.35 per kWh on average, also, you need to consider state incentives for buying the stationary storage. California SGIP gives $1.80/W and 20% off installation but doesn’t allow you to pay less than 40% of the total (admittedly, this will likely change).

    4. Saying $700/kWh is misleading because it doesn’t scale. If you get two 10 kWh units installed, its not going to cost you $14,000 because the extra labor is nominal and you don’t need another inverter. To get 2 10 kWh units, it will cost you $7140+$3500 = $10,640. Also, if you want to really be “wowed” by Tesla’s cell costs, consider that the utility and industrial level batteries cost $250/kWh and still make a profit since Tesla plans to initially have a low positive margin and most demand will be from utility/industrial partners.

    5. In reference to #7, you forget that you’re basing your efficiency calculations on pure speculation and presenting it as fact. Plus, your math seems to be wrong because you included ‘”round trip” efficiency of 87% twice. If the round trip efficiency is actually 87% as you speculate, that means that you will need a bit more than 8 kWh to charge the battery (and this round trip efficiency also takes into account discharging already, since it is “round trip”) at $0.10, it will cost $0.81 to charge the battery, and discharging 7 kWh at $0.40 will result in $2.80. So you actually make about $2.00 per cycle. Also, why did you take 183 days and 10 years? The battery is supposed to do 5,000 cycles, so use that number. 5000 cycles times $2 per cycle is $10,000 saved in California. Lets chop off $3,000 or so because we were only using summer rates, and voila, you recouped the cost of the full installation of the 10 kWh battery! But the 7 kWh is probably $500 less, so you actually made $500! But wait, there’s more: that pesky California SGIP kicks in some incentives to bring your $6640 ($7140-$500) cost down to $2656! So you actually made $4,344 ($7000-$2656) over 10-15 years. So in California, the 7 kWh seems to be perfect for grid arbitrage. Also, I don’t know why you’re so sure that the $0.30 cent difference between peak and non peak rates in northern California is the largest in the country. I highly doubt it, and if there are places with a larger difference, Tesla’s battery is economical there too.

    • Also, the more electricity you use, the more you save. Let’s say you use a lot of electricity and you need 3 7 kWh Tesla batteries at a cost of $6640+$3000+$3000 = $12,640. California kicks in incentives, bringing it down to $5,056. Then you charge 21/0.87 = 24.14 kWh worth of electricity at $0.10 per kWh for a total cost of $2.41, and then you sell back to the grid 21 kWh worth at $0.40 per kWh or $8.40 total. So the gross profit is $5.99 per cycle. The lifetime is 5000 cycles (5000*5.99 = $29,950), so you saved $29,950-$5056 = $24,894 over 10-15 years.

  13. Hi Tech Talker, thanks for commenting. I really appreciate the responses and especially the calculations. I’ve tried to be as open as possible about the assumptions exactly so we can have this sort of discussion.

    I’m quite confident in the design assessment:

    – We’ve been told that the PowerWall has an internal DC-DC converter and the VDC figure is on the spec sheet. It doesn’t really matter what we speculate, as the DC-DC converter’s losses are entirely included in the 92% DC-DC on the spec sheet. As for the AC-DC losses, yes, that 97% figure is an estimate, but after having worked with 250 kW inverters, I’d very surprised if SolarCity was getting better than that. (e.g. SolarEdge inverters at 97.5%)

    – The 87% is only counted once. Remember that 7 kWh is the DC rating. The 10 kWh unit diagram in the original article breaks it out – it shows round trip efficiency is 9.7 kWh AC out / 11.2 kWh AC in = 87%.

    – I’m very willing to bet that the module designs are the same number of cells, for input voltage reasons if nothing else. It also represents a significant cost savings to have the same design. The difference between NMC and NCA energy capacity isn’t 70% so some, if not most of the difference, has to be shallower depth of discharge. There’s no good engineering reason for Tesla to deliberately put fewer cells in the daily cycler – they would rather produce a 10 kWh daily cycle unit if they could.

    • If the battery is built from two of the car’s modules, ( so around 50v) then efficiency will be even less than indicated by the published or estimated figures.
      Charging from a higher voltage array, requires a DC/DC converter to step down to 50V. On discharge, to step up to 350-400V to suit the inverter.

      But, the DC/DC converter’s efficiency, like the AC inverter’s, is load dependent. There is always a minimum power input ( what the inverter uses without a load). As output level falls, efficiency falls relative to that obtained at full output, (which will be the quoted figure).
      When hoping to show that the battery is economical, by extension of the discharge period ( to cover the nightly period, for example), low power output is often employed. When the battery is attached to a 4KW inverter, the overall efficiency at 200W can easily fall to 75%.

      The Powerwalls are all but unique in their low power to capacity ratio. Few 8kWhr batteries offer only 2kW output. Most will be at least 4KW. Wouldn’t it further add to the Powerwall’s ‘wow factor’ if the battery could also deliver 4KW? As the body of the article mentions, Tesla can’t increase power output, without increasing capacity – because they are using a cell designed for use in laptops, and not bulk storage.

      Tesla use that one small cell format, from cars to home storage to grid storage. If that small cell were effective in all of those cases, there would only be that one cell type on the market. Tesla obtain only that one cell at low cost, so can only offer that cell.

      • I agree, we don’t have much detail on the performance of the DC-DC power electronics system , and there can be a lot of losses when you’re not operating at the ideal point (efficiency vs. voltage curve, efficiency vs. power curve).

        One advantage of including the DC-DC converter as part of the Powerwall is that they can play with the solar-side DC voltage as well (note there’s a Tesla diagram showing the solar panel DC bus directly connected to the Powerwall’s solar-side DC bus). There might be some efficiency tricks there, where they tune that voltage to maximize the efficiency of power flow from solar panel into battery.

  14. COST CALCULATIONS

    Tech Talker, I appreciate that you’re checking the math – I went through it again to double-check, and there was an error in the arbitrage description. The PG&E summer high price used in the calculation was $0.36 per kWh ($1.66 per cycle), not $0.40 per kWh ($1.93 per cycle, close to your $2.00). I had rounded up in the text to $0.40 and shouldn’t have – I’ve updated the article to clarify that the price used was $0.36/kWh. Sorry to the readers for the error and thanks for finding that.

    Some points on the payback on peak shaving for those who are interested in the details:

    – Note you don’t actually sell 7 kWh AC per cycle. You sell (7 kWh DC x 97% inverter DC-to-AC efficiency = 6.79 kWh AC). Not a big difference but this explains where the numbers come from.

    – Warrantied lifetime is only 10 years, and I would rather go with what Tesla promises in a contract, than assume an extra 50% capacity is possible based on a press release. It’s possible you’ll get more than 10 years, and that would significantly help the bottom line. On the other hand, I’ve also assumed that Tesla has specified the 10-year performance so that the degraded batteries still just meet the capacity and efficiency specs at the end of those 10 years.

    – You have to cut the revenue by 50%, not 30%, to reflect that you only get summer rates half the year. It’s actually worse than we described in the article – you only get the big peak on the week days. So the revenue should be cut by closer to 65%, i.e. 1,300 cycles over 10 years. Have a look at the PG&E prices, especially Residential Time-Of-Use. There’s only one schedule that gives the big range of $0.36 summer peak, $0.11 off-peak.

    I think we’ve been extremely generous in choosing Northern California with its large price differences – even Hawaii’s rate structure is not so wide – but would be happy to be proven wrong.

    So to use Tech Talker’s $5.99 per cycle but 1,300 cycles over the life gives $7,790 revenue ($6,470 if we use the lower $0.36 summer peak rate). It’s nowhere near the estimated unsubsidized capital cost of $12,640 but as Tech Talker point out, if you can get a 60% SGIP subsidy that would make the difference (assuming Bloom Energy hasn’t eaten up all the credits, and the CPUC doesn’t change the regulatory regime)

    I’m made more confident that arbitrage won’t work because SolarCity will only sell the backup 10 kWh unit, not the daily cycling unit; the SolarCity spokesman specifically said that peak shaving is not suitable for North America. I suspect they might not be as generous as you and I were in our assumptions.

    Thanks for all the comments though and please keep them coming. Me, I can’t wait until we can take apart one of the PowerWalls.

  15. This article didn’t go into the main point – ie if I generate my own electricity (solar, wind, biomass CHP, whateva) – is this economically viable and under what circumstances. ie how much would electricity cost on the grid and for myself for this to fly?

  16. What would be the effect on insurance costs of a home or a building using the power wall? Considering the fact that any accidents or short circuits in the system could potentially destroy the entire construction besides causing severe health hazards? Modern buildings may be protected against firs due to electrical short circuits and the fire departments are equipped to handle such fires. But, if there is a fire due to a lithium battery burn out would the present construction specifications be adequate ? Would the fire safety departments need additional investments, equipment, procedures and training to handle such fires? How would all this affect insurance costs?

    • This begs the question, why use lithium ion when Musk could have chosen Lithium Iron Phosphate (LiFePo4)? They can last up to 10,000 cycles, they only take up 15% more space, and they don’t have thermal runaway characteristics (don’t explode and catch fire). They are also WAY WAY WAY more environmentally friendly.

      • Wait a minute… lithium ion batteries aren’t good for the environment? I thought they were made from tofu and coffee ground waste products. You mean we will have to mine lithium and metal with gas powered engines, and create plastics from oil, and build billion dollar plants on formerly pristine land to forward this taxpayer subsidized agenda? …..did u know that energy companies have been using pumps to fill reservoirs during off peak and releasing the water during peak hours for only a ten percent loss for decades?

      • Dear all, I am looking at mobile power for powering soft serve machines made by spaceman in china, my need is to have approx 7 kw power in a truck for street selling of my frozen yogurt.
        Without using the usual conventional diesel generator for the req power, I want clean power, battery charged on my solar power stystem for which I was looking at this tesla power wall.
        I will grateful to all who can give me advice for my project.
        My own brand, made in Sri Lanka, ” Frutty Fro Yo” is a healthy based frozen and drinking yogurt. My making process uses solar power as well.
        Being a healthy product, I want clean power to support my plan of eletric trucks with solar charged battery power. Consumes approx average 14 amps.

  17. The installation costs seem amazingly high. In Australia an inverter alone would add around 10% to the cost. The actual electronics to do the integration are trivial from a cost perpective if done on a mass scale which leaves installation labour. Is it really going to cost $2000 to get a battery added to an existing solar system? In Australia we install rooftop PV for half the installation costs in the USA. And believe me Australia is not cheap. Too much time has been spent analysing the battery and not enough on how Tesla gets from 3500 in the factory to over 7000 installed.

    • The exaggerated installation cost, and profit gained from the forced sale of ‘compatible’ inverters, are the means by which the unrealistic $3000 battery can be marketed.
      It’s good to keep in mind that Tesla have to shift large quantities of cells to gain their low cost – the cost to the purchaser and the environment are buried.

      • In addition, there are many forums where critics have to deal with Tesla supporters. This forum deals with battery engineering, where rational analysis shows the battery does not do well, even when all latitude is given to the marketing claims.

        • Robert, thanks for understanding the approach. We’ve tried to be as transparent as possible in the assumptions, and give the benefit of doubt whenever possible. We should see in a few months how the real world units are doing (inverter cost, etc.)

          However, I also wouldn’t count out the cost advantages of massive quantities – I would not be surprised if Tesla really can manufacture a 7 kWh-DC unit for less than $3,000.

  18. The Tesla Power Wall seems a nicely engineered and designed product at a very competitive price. Nothing revolutionary here; adding up standard size cells to blocks and attaching a DC-DC converter, some fans and controls. Been done before, but it hasn’t set the world on fire.
    There’s a good reason for that: There isn’t a wide real world application for this. Who should put that thing on the garage wall? The home owner with PV on the roof? He has already the cheapest “battery”: Selling his excess solar power during the day when the rate is high and using the grid at night when the rates are low. Arbitration on electricity rates? Really? You would spend some $7 to $10K because you *may* save a few hundred bucks in 10 years? You better invest that money in baseball cards or what-have-you. Or would you invest $12K (battery, island mode inverter, transfer switch) to have a stand-by generator with 2 kW capacity that you use one or two times a year for a few minutes? It will not even power your well pump. You can buy a 5kW generator for $500 at Home Depot that will power your well pump.
    The only real-world application I see that makes some sense is for total off-grid houses that are optimized for low power use. The 92% cycle efficiency of the Tesla Battery is impressive, but in off-grid applications the max. power capacity is more important than the cycle efficiency (you want to start that washing machine, even if you have to add another PV panel). So the Power Wall doesn’t fit there squarely either….SO WHERE DOES IT FIT?

  19. The article completely ignores all those people who already have a solar system & therefore already have a DC/AC inverter.
    It also ignores the fact that Solar PV mostly generates electricity outside the times when most households use the most power – so that arbitrage is certainly worth having.
    In the UK the off peak electricity is less than half the cost of peak electricity. Simply moving off peak power to be used at peak times would save around $900pa so payback in less than 10 years

  20. This article was initially about peak shaving without solar PV, and in that case a new bidirectional inverter would be needed

    For homes with solar PV, at first I thought the existing inverters would be adequate, but it has emerged that specific Fronius or SolarEdge inverters are the only ones compatible with the Powerwall.

    Sources: PV-Magazine and Tesla Powerwall presskit.

  21. We have been off the grid for the past 13 years. using wind as our primary generation. Storage has always been a problem. One of the big problems I see with the Tesla wall is the voltage. Most off grid systems large enough to power a home are 120- 140 dc. These systems work like the system in your automobile, they replace the energy used, during the day the controller will turn off the wind or solar system by going to a dump load. A few days out of the year you will not generate enough energy to meet your demand, that is where the battery steps in supplying 8 to 24 hours of stored energy. Solar works great in the day time but a hybrid will give you power 95% of the time. I have been taking readings for the past 5 years and without the central air 4 KWH is about what you will use.

  22. Hi Billy, good to see some data. So for your 8-24 hour stored energy, does that equal the 4 kWh typical daily energy you mentioned? That seems consistent with a 10 kWh backup size PowerWall, if the DC voltage levels can eventually be matched. What kind of prices are you seeing with your existing batteries?

  23. At the Tesla AGM Elon Musk said something similar to his 2015Q1 conference call comment, reiterating that the 10 kWh DC unit is for backup power and peace of mind (i.e. intangible, non-economic benefits). The “daily cycler” 7 kWh DC unit is for going off-grid completely, rather than trying to arbitrage day vs. night.

    From a Fox News article:

    In the U.S. the daily cycling application is going to be very limited,” he added, noting that it doesn’t make economic sense. “In the U.S. you’d really have to care a lot about renewable energy…you’d have to care about it more than the economics in order to do the daily cycler thing,” he said.

    He also added that the peak power output is going to be increased from 3.3 kW to 7 kW. I would assume this is a firmware change that allows the batteries to discharge closer to their full rate (perhaps some power electronics modifications), rather than a cell or pack change. As our article explains, the existing packs can easily discharge the higher power – it just means that toward the end of discharge, or end of life, the rated limit (7 kWh DC) will be hit sooner.

  24. Having not been able to get my hands on a spec for this unit, I am wondering if this may have a good application for low voltage DC installations such as remote bus stops or shelters where peak demand is low but consistent. I would assume there is a way to tap at 24 or 48 volt DC which is great for LED lighting. Does it still need a charge controller or is that internal?

    A nice looking outdoor rated package would certainly cut down on enclosure costs and there are tons of applications even in urban settings where getting electricity to the last leg is either cost prohibitive or unsightly.

  25. Anyone who has already invested in an electric car can connect a 2KW inverter to his 12 V battery which is backed by the 400V car battery, for 250$ and use that as a back-up for electric outage

  26. Observation of the spec sheet raised a concern about the Teslas real world performance.
    I note it has an output of only 5 amps. At 400 volts this equals 2 kWh. A 240 volts as we have here in Australia that would only equate to 1.2kWh. That would not be enough to make a cup of coffee with a typical electric jug of 2400 watts.

    Competing units here typically have a capacity of 20amps which means they can run several appliances at the same time. Perhaps I am not understanding something but it does appear to be pathetically weak and talk of offgrid is laughable!

  27. Chevy-Volt and TESLA: The Chevy-Volt has a 16.5 kWh battery, but it uses a maximum of about 10.8 kWh (about 65% of its capacity), because the battery controls are set to charge to about 90% of capacity and discharge to about 25% of capacity. GM does this to minimize costs of its 8-yr/100,000 mile manufacturer’s warrantee. That warrantee is for manufacturing DEFECTS, does NOT cover performance. According to GM, the battery is expected to have a performance loss of 20% over its 8-yr WARRANTEE life, and more beyond that 8-yr life. The 10.8 kWh gives the Chevy-Volt an ELECTRIC range of about 38 miles on a normal day, say about 70 F, less on very cold and on very warm days, less as the battery ages.

    TESLA has a 10 kWh, Li-Ion, wall-hung, battery unit. I assume TESLA is as capable as GM, i.e., no magic, no hype. There are battery charging losses and discharging losses, and AC to DC and DC to AC conversion losses. The TESLA 10-year warrantee is for manufacturing defects, does NOT cover performance!! The INSTALLED cost of the 10 kWh unit = $3,500 + S & H + Contractor markup of about 10 percent + $2,000 for an AC to DC inverter + Misc. hardware + Installation by 2 electricians, say 16 hours @ $60/hr = $7,100, or $7,140 per this URL.

    http://www.bloomberg.com/news/articles/2015-05-01/solarcity-taking-order

    Assuming a 65% charge/discharge, and a 90% AC to DC inverter efficiency, and allocating half of the 8% DC-to-DC loss to the charging side (the unit has a round-trip DC-to-DC efficiency of 92%, per spec sheet), it would take 0.65 x 10/(0.9 x 0.96) = 7.523 AC kWh of off-peak grid energy to charge up the unit. During on-peak hours, one would get back 0.65 x 10 x 0.96 x 0.90 = 5.616 AC kWh to use in the house, for a minimum energy loss per cycle of (1 – 5.616/7.523) x 100% = 25.4%!!

    If we GENEROUSLY assume the battery would have NO performance loss over its 10-yr WARRANTEE life, and one cycle per day, i.e., 3,650 cycles, and night-time cost of charging at 10 c/kWh and day-time avoided cost at 18 c/kWh, then 3,650 x (5.616 x 18 – 7.723 x 10) = $943.76 would be the gain over 10 years. The cost of financing, PLUS any costs for O&M, PLUS any capacity degradation due to cycling, PLUS the cost of depreciation are ignored.

    The above is a best-case analysis. Actual results are much worse, i.e., terrible.

  28. Has anybody looked at the benefits of a “New House Build” where the intention from the onset is “an Off-grid” solution ? …….Photo Voltaic’s instead of being added to the roof…are the roof. So you have no national grid connection and therefore no costs. Why is there an imposed limit of a 4kw/h system on domestic properties (16 x 250w panels) where roofs can have much more ? Is there a difference between single and 3 phase electricity other than the voltage. If you have the storage capacity in batteries what is to stop you generating 10-15kw/h and storing that ? In the UK you have to “register” your solar array with your utility provider and they pay you an additional 50% of the feed in tariff on what you generate…….but they don’t measure what you put back into the national grid…So you are being paid by the utility company to lower their “Carbon footprint” based on 50% of what you generate using the panels but there is no enforcement to put excess back into the national grid……Has this additional revenue been calculated into the costs for life of the system ?

  29. In principle, I am a great supporter of the Powerwall, however unless I am missing something, I am concerned about the fire regulations that would apply to installing the Powerwall on internal walls of houses. For example, I understand that the building regulations in Australia require storage of batteries over 24v to be housed in fire resistant enclosures. I note that the Powerwall is rated at 350-400v. Can anyone enlighten me here? Is this something Tesla has got covered?

  30. The system I have is by ephase system that micro inverters for each panel. It converts to 220 at each panel. The power plan I have is on peek. 9 to 9. And peek credits are generated Jan 1 to Dec 31.My solar system in Phoenix,AZ provides most of my on peek power during summer. It fall short near the end of the last month of summer. The on peek 14 cents per KW and off peek is 8 per KW. It sound like I could extend my on peek credits with this device. The other way is to install another 8 panels at $1000 each plus $300 each inverter. Add the cost to install them. Makes me feel as powerwall is a better choice.

    • If you have sufficient steady breezes during the day it might make more sense to install a wind turbine as they produce much more power than solar panels. They are not as expensive as you might think. Smaller turbines produce about 500 watts at 24V and have the advantage of being able to produce anytime there is a breeze, including at night.

      If it’s really costing you $1000 per panel then wind power is something you really should consider. Many of them can be roof mounted. They don’t all have to be pole or tower mounted in the yard if zoning issues are a concern. If however you live in a HOA community you need to check the HOA rules with regard to this unless State rules supercede them.

      This is just a guestimate, but you can probably install a single 500 watt wind turbine with inverter and installation for under $2,000 Significantly less if you can do the install yourselves.

      Just do a Google search on wind turbines to see what is available. Heck! Even Walmart sells them on the cheap.

  31. A quick question.
    Several times in the article they mentioned ‘high’ power and ‘low’ power.
    I’m not sure what they mean by that. Are we talking about 240 vs 120?
    Are we talking about appliances?

    Would someone please give me a little clarification on this?

    Thank you

    • I believe they are referring to maximum amperage/wattage and not voltage. This will affect what you can run or total items you can run and for how long. In simpler terms it just means the bigger unit provides more total power to run more things or the same things for a longer period of time.

  32. Is it a flat installation cost? If someone decided they needed 2 or even 3 powerwalls, would the total cost be $7,000 each? Would it cost 21,000 for 3 powerwalls,or 13,500 for 3?

      • I would imagine that once you have the required inverter in place you would only need pay for the additional powerwalls and for them to be tied in. If you have excess power generation from solar or wind the extra walls make sense, but not if you are just going to charge them from the grid as a backup. Unless of course your provider has surge pricing during peak usage hours. In that case it might make sense to charge them during off peak hours and go off grid during peak time.

        Barring that it might make more sense to install a home backup generator system. Some systems are fairly silent and many can be optioned out to run on both natural gas/propane and gasoline depending on what is available.

  33. Okay , maybe this does not sound too practical for the average Joe that will buy the system as a stand alone and leave it at that. It may however be the last leg of an already existing system that will enable a complete off grid situation.

    In my case I already have a 4 x 2 series/parallel (8 batteries) deep cycle marine battery storage system. When fully charged it is capable of 1,200+ Amps (57KW or 57,000 watts) of peak on demand power at 48V DC and about 5 hours reserve at a 25 amp (1,200 watts) continuous draw. This is currently enough backup power to allow me to go fully off grid for 8 hours of normal usage excluding central A/C. Maybe 3 hours with the A/C set as cold as it can go. The reserve will last longer if the A/C is run continuously rather than being allowed to cycle.

    BTW: The reason most backup systems (including portable generators) are not capable of running your central A/C system is because of the “surge power” required to startup the compressor. It can require as much as 18,000 watts of surge power to start up. Same is true for heat pump systems. Unless you are in Florida or a desert State A/C should not be considered for a backup power situation. If you absolutely must have it I suggest keeping a single 6,000 watt window unit in storage that you can pop in place in a bedroom if temperatures are extreme.

    I generate electricity for my system using solar and wind power. Wind being much more effective on a good day than solar. Adding the Tesla Power Wall to my system would allow me to go fully off grid most of the time day and night. Sure I could achieve the same thing by doubling my existing battery system, but 16 lead acid batteries, their high maintenance and the additional ventilation required for the additional explosive hydrogen/oxygen gas produced is not appealing to me.

    Yes, that is the main drawback to lead acid batteries. They produce hydrogen and oxygen in a perfect explosive combination under normal conditions. If not properly vented they are an accident waiting to happen. If a cell or cells go bad or in an overcharge situation they also produce hydrogen sulfide gas (smells like rotten eggs) that is extremely poisonous. This is the same gas released by volcanic activity that kills unsuspecting wildlife occasionally in Yellowstone Park and elsewhere. Not too much of a problem if your system is kept in a vented outdoor structure.

    Unfortunately many people like to keep these systems in a garage or basement so they are not affected by environmental conditions (too cold, too hot, too humid) that affect efficiency. If you are going to do that you really need to consider walling off (with concrete block) a section of your garage or basement and ensure it is at least passively vented to the outside. That way if something does go wrong it is contained and sealed off from the rest of your home.

  34. Why doesn’t Tesla make a Power Wall with 60-90 KWH, like the batteries they use in their cars, at a reasonable cost? The new Chevy Bolt 60 KWH battery is supposed to cost only $3500. That would be enough to power a small home at say 50kwh/day average use, I would think.

    • $3500/60KWH= $58.33 / KWH just for the Chevy Bolt battery. Now were talking! I hope these Chevy Bolt batteries can be adapted for off-grid home storage batteries.

  35. I would use this unit to store solar power from panels on my 32′ Airstream trailer. Could I store it flat under the bed?
    Currently I have two panels on top that produce over 100 amps. How many more panels could I install (space permitting) and efficiently charge up the Powerwall?

  36. I am looking to have back up power when the grid is down, and I don’t want a noisy internal combustion engined thing in my backyard that needs fuel and maintenance. I am thinking three PowerWall modules should keep our home operating for a few days. My question: What is the lifespan of these panels for this type of application?

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