Powerwall Alternatives That You Should Consider as Tesla Battery Hype Reaches Fever Pitch

powerwall alternatives

Just 3 of the Powerwall alternatives now available in Australia.

Manufacturers of Powerwall alternatives struggled to be heard last week as the mainstream media went crazy over the beautifully stage managed, first residential Powerwall battery storage installation in Australia. 

The mood was one of expectation and excitement — mixed with a tinge of dread in fossil fuel circles – as it heralded the advent of affordable home batteries from Tesla and its many competitors in the home battery market.

The Pfitzner family from the Western Sydney suburb of Kellyville Ridge was chosen as the first to receive Elon Musk’s Powerwall, which stores up to 7kWh of energy to use during the evening and through the night.

Australia is one of the first countries in the world to be sold the Powerwall, and Chris Williams, Managing Director of Natural Solar — the installers of the first Powerwall in the country — is excited.

“Since being announced as a Tesla Powerwall installer at the tail end of last year, Natural Solar has received an unprecedented number of enquiries about the Tesla Powerwall, indicating the Australian public is well and truly ready for this new frontier in renewable energy,”

“…as one of the first installers to bring the Tesla Powerwall to mainstream Australia is truly exciting and invigorating, and cements our position as leading experts in renewable energy and battery power,” he added.

Home owner Nick Pfitzner told an equally excited media that he has long been a fan of Tesla.

“I’ve been watching Tesla since the Roadster was first released, as I work in IT and was quite a fan of Elon Musk in general. When it was announced last year that they were moving into household energy storage, I jumped onto the official Tesla reservation list,” Pfitzner was reported as saying by the Business Insider.

Despite the media frenzy, the jury’s still out on whether Tesla’s Powerwall is worth the capital outlay. There’s no doubt that it’s a great piece of technology, beautifully packaged and slickly marketed.

However, there are a number of Powerwall alternatives currently on the market that, on paper at least, beat the Powerwall in either price or performance.

Here’s a quick comparison of the Powerwall with three other products currently on the market:

(A quick explanation of terminology – Cycle Life refers to the number of times a battery can be discharged/recharged before it reaches the end of its lifespan. DoD means ‘depth of discharge’ – so if a 10kWh battery has a cycle life of 1,000 cycles at 80% DoD, that means it can be drained 80% and then recharged 1,000 times before it’s sent off to the battery graveyard.)

Note: Except for the Sunverge SIS (which includes installation), these prices are the retail price of the batteries + GST. Installation and freight costs are not included.

Tesla Powerwall
Storage capacity: 6.4kWh
Power: 3.3kW steady, 5kW peak
Cycle Life: 3,000 cycles at 80% DoD
Expected lifetime: 10 years
3.2kW steady, 5kW peak
6,000 cycles at 80% DoD
15-20 years
Samsung ESS AIO
4kW steady
6,000 cycles at 90% DoD
15-20 years
Sunverge SIS
11.6 kWh
5kW steady, 8.5kW peak
8,000 cycles at 85% DoD
20 years

You can see from the Powerwall alternatives above that the LG Chem RESU has more or less exactly the same amount of energy storage and power output as the Powerwall – but it’s $600 cheaper, and has twice the cycle life (meaning it will more or less last twice as long, which means it could give you double the returns of a Powerwall). Much more bang for your buck!

The Samsung and Sunverge products are more expensive, but they are ‘all in one’ units that come with inverters included – a Powerwall + compatible inverter will push its price above the $10,000 mark.

The Sunverge in particular has an extremely impressive expected lifetime – over 20 years! Just imagine what kind of advancements there will be in battery storage technology by 2036.

As I have pointed out in a previous blog post, the Powerwall is undoubtedly a major breakthrough in the world of solar battery storage. However, it’s become clear in the ~9 months since that was posted that the breakthrough was more in the marketing and public awareness side of things rather than the technical.

Tesla succeeded in making lithium-ion solar battery storage sexy to the average Australian, and we take our hats off to them for that.

I’ve also previously pointed out that those considering switching to battery storage (Powerwall or alternatives) should do the research first to see if it will suit their current financial and energy needs.

There’s no doubt that the Powerwall in Australia represents the vanguard of the solar and storage revolution and is a quantum leap in our country’s renewable energy sector – a leap that will see households reduce their reliance on the grid, even withdrawing altogether under some circumstances such as households in rural or isolated areas.


It is exciting times (or the beginning of dreadful days if you happen to support the fossil fuel industry). But is too early to make the plunge? Would you consider a Powerwall or one of the alternatives now? Or will you wait until the technology becomes cheaper and more efficient? (Or even for The Powerwall 2 which Elon Musk just announced

I’d like to hear your thoughts.

About Finn Peacock

I'm a Chartered Electrical Engineer, Solar and Energy Efficiency nut, dad, and the founder and CEO of SolarQuotes.com.au. I started SolarQuotes in 2009 and the SolarQuotes blog in 2013 with the belief that it’s more important to be truthful and objective than popular. My last "real job" was working for the CSIRO in their renewable energy division. Since 2009, I’ve helped over 700,000 Aussies get quotes for solar from installers I trust. Read my full bio.


  1. with 365 days in the year and only 3000 cycles at 80% DOD I hope installers are going to advise hyped up Powerwall consumers properly in a suitably sized system so they will only occasionally reach the 80% discharge rate or some of us are going to be very upset and out of pocket in around 3 years, well before it has paid for itself.

    • Finn Peacock says

      And word on the street is that if the internals reach a certain temperature (that will be very easy to reach in most of Australia) then Tesla reduce the warranty to 2 or 5 years depending on the temperature and timeframe. I’d love someone to confirm or deny this…

      • We have installed a few SMA systems paired with the island and deep cycle batteries. So far it has been flawless. The Sunny Island solution of storage gives our clients freedom to maintain, change and upsize their storage easily.
        Other methods restrict the user to that brand of expansion for example the tesla can be modular with only the tesla, similarly enphase and so on. The Tesla is over rated in my opinion, publicising its price before its launch gave everyone a false hope.

        • Tony Smith says

          While this is not a huge issue in grid hybrid, the Sunny Island is good until it decides to start frequency ramping the grid tie inverter. Then (as we’ve seen a lot in off grid), front loader washing machines, dishwashers and air cons say “I want 50Hz – what’s this 48 or 52 Hz crap? See ya!”

          Selectronic is Australian, doesn’t frequency ramp in managed AC coupling mode and has way more functionality than the German stuff.

          • Hi Tony,

            This was very useful. I will look into this in more detail. Thanks!!

          • Jeff Wehl says

            The Sunny Island doesnt frequency ramp in hybrid mode.

            SMA use their own version of MODbus for on grid ramping. You are correct in saying that they use frequency ramping for off-grid, but Ive never had any problems with it.

            It would be pretty much impossible to try and alter Swanbank’s frequency. .

        • I am getting a LG 6.5 battery installed installed with my solar panels in a couple of weeks time. My reason for installing a battery is for my medical equipment plus there is nothing worse than not having a cup of tea during a blackout.They only happen once and a while people say well so far twice this year the power has gone off early of a morning. I am not worried about it paying for itself as I have read all the articals and know that will not happen. Just convenience for my decision and independence too.As long as I can reduce my power bill that is all I am hoping for.

      • There is nothing specific in Tesla’s capacity retention warranty regarding internal temperature, but there are general caveats, such as maintaining the average ambient temperature to 30C. Difficult to do in many circumstances. Again, nothing is stated concerning the consequences. The capacity warranty is given at 2kW output, and not the claimed 3.3kW continuous output, so another ambiguity.
        It may inferred that the warranty may be exceeded, but the reality is that a constant or even frequent daily 3kW discharge, or full capacity use, is unlikely.

        That is one of the reasons why the recent Energia analysis shows that only a few of the systems tested, will provide a return. Fit to circumstances, and capacity decline, are the most siginficant factors. Low intial battery cost is a benefit, but not the determinig factor.

        Taken at face value, Tesla’s capacity retention warranty is similar to Panasonic’s and LG’s warranty. ‘Cycle Life’ is not an adequate metric, even if the effects of time ( calendar life) and temperature are ignored.

        Throughput, the total discharge is better. Like Tesla, LG’s warranty is de-rated according to total discharge, both resulting in 60% at 10 years, if full daily cycling were attempted. On the other hand, capacity decline over that period, means that the total discharge (let alone the often presumed capacity x10years ) can’t be reached within 10 years. In short, 60% at 10 years should be taken in each case. That also applies to Panasonic’s RESU 6.4, where a simplified chart in the operator’s manual indicates that same result.
        Samsung offer 65% at 10 years or 6000 cycles, whichever comes first.
        Sunverge don’t provide sufficient details to allow comaparison, and neither do the others, in many areas.

        Samsung’s Installation Manual provides detailed specifications. From those, I would say their unit is the better choice, but like the others, still unlikely to show a return.

        • Finn Peacock says

          Thanks Thomas – really useful info.

        • Nick Pfitzner says

          From the manual, some technical stuff:

          “3.1.4 Power Capability

          The Powerwall shall be capable of regulating discharge or charge power at rates > 200W, subject to the maximum capabilities shown below:

          PD: The continuous and peak discharge power capability of the Powerwall shall be 3.3kW measured at the DC link for all initial SOEs above 30% to fully discharged, and temperatures above 0°C and below 38°C at BOL.

          PC: The continuous and peak charge power capability of the Powerwall shall be 3.3kW measured at the DC link for temperatures above 12°C at BOL.

          The charge and discharge limitations of the Powerwall depend on the temperature and SOE of the battery. After a 24 hour ambient soak in the off state the Powerwall shall have the charge and discharge capabilities as shown in the figure below, at BOL. The battery temperature vs. SOE maps below represent the modeled battery system performance and are used to indicate typical battery system performance under varying conditions.”

          And then:

          “3.4 Environmental Requirements

          The Powerwall shall be installed outdoors or indoors such that the ambient air surrounding the Powerwall is within the operating temperature requirements and the average temperature is less than 30°C over the life of the product.

          Performance may be de-rated in extreme ambient temperatures as shown in the temperature-SOE maps above.”

          The key bit is “30°C over the life of the product”. There would be few areas in Australia that would firstly get a powerwall installed, that also meet that requirement. Better battery options for those places I’d say, and better technology coming in general from both the lithium end of the scale through to older technologies and everything in between.

          The two temp-SOE maps are linked here and each labelled appropriately. The green zone is “Optimal State”, yellow zone is “Limited Power State” and red zone “Minimal/Zero Power State”.

          Fortunately, I don’t live in an alpine area or in a desert, so I reckon I’ll do alright 😉

          Though Sydney is feeling rather warm this last, relentless fortnight.

          • Hello Nick,
            Thanks for the information. Your manual contains more detail than is (was) available from Tesla’s website. The last re-issue of the manual was 4-Dec-2016, and warranty, 22-Jan-2016, but are no longer linked.

            In the last releases, general performance, and capacity warranty, are rated at only at 2kW charge/discharge at 25C,without details of the 3.3kW case. Is that information included in your manual or warranty?

            As it stands, retention warranty is described as;
            85%, at 2MWh accumulated discharge or 2 years
            72% at 9mWh accumulated discharge or 5 years
            60% at 18Mwh accumulated discharge or 10 years.

            As I mentioned in my reply to Tim, not signifcantly different in that respect from some of the competitors, listed or not.

            Like many similar calculations, Energia’s analysis will consider battery end-of-life to occur when capacity falls to 70%. Dependent upon use, that point typically occurs between 7 and 12 years. After 70% capacity, Lithium-ion batteries become increasingly inefficent, while storage declines to the point where cost errodes income, if only because the deficit in capacity may need to be bought from the grid.

            Battery storage is expensive. From the column on the left, each data entry is more or less 85% of the former. From that, total throughput (accumulated discharge) can be calculated to be 70% of expected full-capacity cycling over 10 years, or 70% of 23.36MWh = 16.4MWh

            At $8000, the battery would add $8000/16,400kwh = $0.48 to each stored kWh.
            That is one reason why the Energia analysis ( of 3 offered systems) shows very few installations will pay. Another, is that the remaining solar energy ‘subsidises’ the battery storage cost, so again, dependent upon household use patterns.
            Unfortunately, it will be quite some time before Lithium-ion batteries become cheap enough, and durable enough, to warrant inclusion over a wide range of household circumstances.

        • Nick Pfitzner says

          Hi thomas – replying to this comment as I think your reply below hit the maximum depth for the forum on replies.

          The document I’ve been given is dated 20th November at Revision 1.1 so its entirely possible this information is out of date or has since been more comprehensively noted. I might chase that up with Natural Solar.

          It is titled “Daily Powerwall Customer Specification”, and as a result, no warranty information is included, so that is something else I’ll chase up.

          It is supposed to be proprietary and confidential while we’re at it, so I’m unlikely to reproduce the entire thing here – don’t want to lose any good will 😉

          Your point about the value of the battery system is valid. No-one really knows what will happen longer term, and the only parallel I can see is looking at the Tesla Motors vehicle performance. Of course, they operate in a slightly different environment, and this survey is apparently for the Roadster, which I’m told is older technology even within Lithium:


          There will always be people who “jumped too early”, but I’m fairly comfortable with the decision on more than just a financial basis. I accept that efficiency will be lost over time, as with anything we buy. Cognitive dissonance in the consumer sector is not uncommon.

          However, even if the degradation in battery performance puts a spanner into my ROI calculations, it should be offset by any price rises that our governments see fit to introduce to help protect the grid and their mates who run it.

          And, once I get some Reposit GridCredits going, the reward should be free energy, which I can’t get without the battery under the current schemes.

          I’m actually thinking about splitting technologies – have a lithium battery AND a Redflow Zn-Br to get the best of both worlds.

          Maybe in a few years…

          • Erik Christiansen says

            Having some form of second battery may not be entirely wasted when using a Redflow ZBM, as not only does it tolerate regular 100% discharge, the documentation I’ve been reading states that it requires it every 120 hrs of zinc pump running. That’s to maximise the electrode surface area – fundamentally to retain capacity for as long as possible, I figure.

            There are MODBUS write registers in the MMS to allow this maintenance cycling to occur at the end of a discharge, or automatically, if I understand the brief words. So maybe dump everything into the solar HWS boost element early on a sunny morning, followed by recharging (max 2.5 kW rate, though.) The battery voltage is zero after the electrode stripping.

            There is mention of a “pre-charge stage including a current-limiting device for a minimum of 6.5A DC at < 60V DC" to bootstrap the system if the charging system can't start at zero volts. (I have a GSL MPPT60-2 3.4 kW
            solar charger which auto detects 12/24/48v batteries. I expect it would be confused in this situation.) A less intelligent charger would be easier to integrate with a ZBM if I understand that requirement adequately.

            It does need good ventilation. While lead-acid can emit a bit of hydrogen, the info indicates precaution against a whiff of bromine in the event of a fault condition. I have a vague recollection of the domestic version, apparently out by the end of the month, coming with some sort of additional enclosure – presumably so it can be installed on a verandah.

            It's good for 10C to 45C, with auto shutdown at 50C, so my concern would be frosty Gippsland nights if it's on the verandah. Might have to put it in the garage.

            One interesting attribute is that the battery can be refurbished. The "electrode stack can be replaced when it reaches end-of-life" at half the cost of a new unit. The zinc and bromine pumps are custom-developed brushless DC motors, so there's only bearings to wear.

            I've read that charging at greater than a 2.5 kW rate is one way to void the warranty, so I might ask them whether that can be safely managed by profiling the charging voltage.

            Where on-grid solar power adopters can economise by using the grid as a battery, I could limit my off-grid battery requirement by just arcing up the 240v generator if there's no sun for two days.

          • Nick. I appreciate your position, and your candour, but would like to hear of any responses from Origin, if you get any, please. Sorry to be so negative,
            but I can’t make the battery pay except where some specific circumstances are idealised. Storage cost is high. Payback would be around 17 years. If the system shows shorter, I suspect that battery costs are being hidden by the overall cost. Perhaps some more questions for Origin?

            The car’s battery. Different in so many ways. The chart in the reference suggest perhaps -5% at 35,500 miles or 130-150 cycles. A lot of batteries
            can do that. Many equate large and small batteries. A large capacity battery in a car, will have a high average state of charge, and won’t see full-cycling very often. Because there are no other cars with 85kWh batteries, direct comparison does not occur, but only with smaller batteries.
            The real challengem is the small battery, where every kWh counts, and deep-cycling is frequent. The car’s battery may look to be something special, but it’s primary advantage is large capacity. The downsides are a physically big battery that limits design, high assembly costs, and the need for very low cell cost.

            How well do Tesla do, with the small deep-cycled battery? No better than anyone else, but at a higher system price, compared to simpler 48V.
            Be aware that capacity decline may mean more grid purchase. If tarrifs rise, the declining battery may not insure against that.

            If Tesla’s home battery were an exception, that would show, but the specs say otherwise. The power output chart, indicates 3.3kw is available over a narrow window, just like most consumer grade lithium-ion cells.

            There have been many studies of grid-connected home batteries over the years, from academics, technical journals and government bodies around the world. The general conclusion is that battery cost must be around $100/kwh, (and also lower inverter costs) before being economical in the majority
            of circumstances. No battery comes close. I wonder, do Tesla not read?

            Other manufacturers are riding the wave that only Tesla’s image could have created. Nobody would care, if it were Benq or Sunverge.

            But, there is a big problem looming. Australia generates >200,000GWh annually from fossil fuels. There is talk of closing those plants by 2030 -just 14 years away. That means building renewable generation, capable of 40GWh year, for each and every day until 2030. How’s that going?
            CSIRO conducted a survey concerning cost-reflective tarrifs. Only 7% responded to the questionaire. Talk is cheap, but there is little action or interest beyond knee-jerk solutions, or assigning blame to the ‘greedy’ grid. Batteries fill that need. That is my major problem with Tesla. They started something, but provided no evidence, but only self-serving hype and slogans.

            One million home batteries would provide a total annual output of 2,336GWh. That’s expensive, and less than 1% of what is needed, even if all were fully charged each day – but only for the years until the battery wears out.
            That’s not a solution, but another problem.

          • Nick Pfitzner says

            Origin didn’t install my battery – was Natural Solar and I understand their implementation was a little bit better in terms of bang-for-buck over Origin. I think I got 5kW of panels for about $500 less than the price Origin installed 4kW.

            Your concerns over the engineering are entirely valid. I don’t even know for sure, but I find 17 years on the high side – I’ll detail why in a post on my blog once I’ve got more data rather than theory.

            I also have concerns over the waste problem looking by production of all these batteries, but have a reasonable amount of trust in R&D progressing suitably and the technology becoming cheaper. We’ll never know unless we move forward with it, because the existing battery technologies have failed to light the consumer fire.

            You mention the $100/kWh mark, and I happened to see this today:


    • Erik Christiansen says

      But 3000 cycles / 365 = 8.2 years, not so far short of the 10 yr expectation. that sunny days, holidays, etc. might not bring us close. Now 80% of 6.4 kWh is 5.12 kWh each day.

      If I discharged a Redflow ZBM by 5.12 kWh, that’s also 80% of available capacity. It’s _warranted_ for a 10 MWh life, so 5.12 kWh/day = 4.3 yrs, and expected is 20 MWh, i.e. 8.6 yrs. There’s not a significant difference, between the expectations, I think.

      The chairman of Redflow, who admits to having 5 Tesla cars, says his batteries are significantly more durable in domestic use. I’m having trouble seeing that, on the above basis.

      Looks rather like everyone wants to be a technology leader, but it’s still a scratch race, and they’re not off the blocks yet.

  2. Erik Christiansen says

    For an off-grid weekender (eventually a tree-change), I was very impressed with: http://redflow.com/products/zbm/

    but after some counting on my fingers, the warranted lifetime energy throughput of 10 MWh is just 2.74 kWh average per night, and even the “expected” 20 MWh is 5.48, which is just 68% DoD average for 10 years.

    Against that, it handles frequent 100% DoD with aplomb, according to the specs, and can be left that way without harm. (Presumably due to separation of the two electrolytes.)

    Perhaps that works out better in practice if the battery bank is scaled to serve for several days without much sun. Then the ability to do one 100% DoD for half a week would give a 12.5 yr warranted life, or 25 yr expected.

    On p58 of ReNew issue 128, a system integrator chose to limit DoD on LiFePO4 batteries to 45% to get them to last 4000 cycles (10 years). Is the Powerwall a different chemistry, to allow 80% DoD for the same lifetime?

    Whatever I buy, it’ll last long enough for much better technology to replace it at end of life. And since a grid connection was quoted at $60k about 30 years ago, it’s hard not to be in front.

    • Re Redflow
      Redflow have not yet published complete performance data, but upon what is offered in the manual, not so promising. A problem for many, will be that 10kWh is too much. If fully-cycled for 10 years, throughput would be 36.5MWh, unit cost is then $0.22/kWh. If the average stored energy were only 5kwh/day, so 18.25MWh, unit cost doubles to $0.44/kWh.

      A few things caught my eye. Quiescent consumption is 1.5A@40V = 60W. If that demand were continuous, as quiescent suggests, 1.4kWh/day in parasitic
      consumption. There is also ‘auxilliary power’ of 180W. There is a “mimumum efficiency performance requirement’ of 68% (energy in/energy out)”, which would suggest a mimimum throughput of 6.8kWh/cycle, or a disguised way of expressing cycle efficiency.
      Cell cycle efficiency is known to be around 74%. Add the parastic load, and that becomes 60%. Perhaps 68% is the aggregate of the consumption of both cycles, where differing auxillary and parastic loads result in 68%.

      Power output is 3kW, which may limit use. Many loads can exceed that.

      Fluid volume is 100l. Since hot water has been mentioned, 10kWh is the same energy as stored in that same volume of water, raised by 86 degrees C.
      If you need 100l/day of hot water, that’s a cheaper storage option.
      In the right circumstances, an insulated tank, supported by an on-demand immersion heater, can work out quite well.

      In their comparison table of the Powerwall and lead-acid, Redflow don’t mention quite a few less-flattering things. Storage batteries are not so easy to select, especially when so many manufacturers hide their laundry.

      • Erik Christiansen says

        On my reading, it is the ZBM2 which holds 10 kWh and is expected to last 40 MWh of throughput. If that’s too much, the ZBM holds 8 kWh and is expected to last 20 MWh of throughput. I think that’s the one which was last reported at US$8,000 or so, which seems to be similar to other offerings of similar capacity. Let’s see what appears at the end of the month.

        Efficiency when new appears to be 80%, i.e. 10 kWh in for 8 kWh out, which is why charging rate is limited to 2.5 kW, despite the integral cooling fan, I expect. My reading of the 68% is “Minimum Energy Efficiency Performance Requirement: 68.0% gross (energy out/energy in)”, which I interpret to mean “Don’t expect to make a warranty claim unless you’re substantially under that within the warranted 10 MWh throughput”. Otherwise I would not expect it to be stated as a “Requirement”.

        It is interesting to speculate on the basis of what is read, but it is worthwhile to consider the available evidence before doing so. Speculation about some sort of potential or possible 1.4 kWh parasitic consumption does not hold up well against this statement of performance:

        ” ZBM as a pre-charged generator:
        One of the most exciting operational modes of the ZBM is the storage mode in which the battery act as a pre-charged electrical generator. The ZBM
        can be charged and left unattended for months, with no self-discharge
        taking place and therefore no need of trickle charge, equalisation or
        conditioning. During this period the voltage on the battery terminal can
        decay down to zero Volts even if the battery can retain up to 80% of its
        original state of charge. In case of need, the battery can be restarted
        with a voltage of 36V and a power of 100W supplied for 20 seconds.
        After this time the battery can self-sustain and supply the entire energy
        stored (up to 8kWh for one ZBM). If more ZBMs are connected in parallel,
        once restarted the first battery can power up all the others and restart
        them in a few seconds.”

        From this I infer that the zinc and bromine electrolyte pumps, and the fan, are stopped when not required. The eventual loss of terminal potential is most likely due to the electrolyte not circulating when in stasis, and the 20 second jump-start is for the pumps to whiz up and push fresh electrolyte into the electrode cell. Try leaving a lead-acid battery bank disused and unmaintained for six months, and see what it’ll do after 20 seconds of boot-up sequence.

        If 8 kWh is “too much capacity”, but 3 kW discharge rate is not enough, then the Redflow documentation expressly says their battery is not suited to that application. They speak of “low energy, high power”, as a poor fit if I recall correctly.

        If I need to run the dishwasher, washing machine, 1 kW vaccuum cleaner, airconditioner, and microwave simultaneously, I’ll do it when array power supplies all of that. In a solar house, it is serious mismanagement to run a dishwasher or washing machine after dark. If I run my lathe or milling machine then, each is only 1.5 kW max, and less than half of that with the depth of cut I use, since I’m not in a hurry, and don’t need to see the metal come off smoking.

        I’m not sure yet precisely how to hook up a 5 kW inverter so it’ll only consume 3 kW from the battery, but avail itself of full capacity if the array is providing it. But limiting the ZBM charging rate to 2.5 kW could be done with a simple PWM controller (needing only one power MOSFET in the design), I think. How much LC filtering to design into the output circuit would depend on how much ripple can be allowed. When I’m further down the track, I’ll ask them. I expect it’s a great deal. (If we’re only talking about power limitation, then the filtering will be more to reduce EMI.)

        To my mind, it is that charging constraint, and the need for good ventilation (while not exposing the battery to temperatures below 10C) which make it a little more awkward to design into a domestic situation, than into telecommunications installations, where it is currently finding use.

        • Some test data that may help. The manufacturer claims;
          “Gross energy efficiency of the ZBM2 is approximately 80% without ancillary components. However, the cumulative effect of inverters, cooling systems, and additional power electronics can reduce overall AC-AC efficiency
          to significantly lower levels”

          Sandia Labs tested The 10kW ZBM2 SDK unit. Specified as 10kWh/5kW, 60A max charge.

          From the block diagram of fig 4, Sandia measured the DC energy input at the enclosure input and the output measured at the constant power 5kW load, so is a full load ‘black box’ system test. Table 10 gives the results.

          Input is 15.3kWh, output 10.5kwh, producing the 68.8% efficency of Table 10.
          (And a lot of heat)
          That’s DC-DC, so 68.8% is the ‘gross DC-DC efficiency’. Naturally, if AC-AC were measured, lower still. The efficiency charts of the test may appear to suggest a higher figure, but the energy efficiency is the product of Coulombic and Voltage Efficiencies.
          Charge 250Ah
          Discharge 215Ah

          Charge voltage 61.1V
          Discharge Voltage 49.3V

          Charge energy
          250mAh*61.1V = 15.27kwh
          215Ah*49.3V = 10.59kwh

          Energy efficiency
          = 10.59/15.27 = 69.4%

          The product of voltage and Coulombic efficiencies is 80.6% x 86% = 69.3%. Voltage efficiency is this battery’s Achilles’ heel.
          Rate efficiencies can be found in Table 6, where energy efficiency is 75 to 78%, provided 60A is not exceeded. Inital current at 5kw is around 95A. Like so many batteries, better efficiency is obtained at lower than maximum output.
          In this case ~65% of 5kW. That would be perhaps 2kW for the domestic unit.

          From fig 13, current can be seen to rise as battery voltage falls with lowering SoC, because a constant power load is being maintained. In household use, for the same load, the inverter’s input current will also rise as the battery’s voltage falls. That additonal current limit will also place the maximum practical output at 2kW for the domestic unit, but check the details when specs are published. If Redline are not forthcoming with test data they surely have, ignore their battery. Without Sandia, how would the consummer know of the ZBM’s limitations?

          The battery has some features that may make it quite useful in some circumstances. Like the telecoms market, for which it was designed.
          There are limits, though, and it’s up to you if you decide to accept them.

          PWM would be difficult. The explanation is rather long, but if you have any questions, please ask. Probably better to buy Redline’s charger, rather than risk voiding the warranty. Inverter load sharing should be an in-built function, unless not intended for grid tie or combined solar and battery use?

          • Erik Christiansen says

            Thomas, those Sandia test results for the Redflow ZBM2 are interesting, even though they are for a battery bank without cooling fan. (But that wasn’t the greatest parasitic consumer, if I correctly recall the earlier post.) Sandia say “Initial characterization tests have shown that the ZBM meets the manufacturer’s specifications.”, but the results are lower than the 80% figure we’ve read. It looks like the 68% figure ensconced under “Requirements”, deep in one of Redflow’s documents, ought to be more prominent.

            Looking at the oddly brief manufacturer’s “data sheet”:


            The battery is charged at much less than 60A, over as long a period as one could ever expect from a PV array, significantly reducing the charging losses.
            (The charging voltage is less than 60v)
            The discharge rate in the first graph is equally modest, similarly reducing discharge losses, but the 4th graph shows the effect of high and low discharge rates. I.e. flog any battery and you won’t get full capacity. Certainly, lead-acid couldn’t compete at the high end, I expect. But I’m more interested in the low end, e.g running a fridge overnight – only 400W, and only a fraction of the time. At the C15 rate, I’ll still get full rated capacity. It’s hard to ask for more than that.

            So we’re down to how many extra kWh the array has to deliver for battery losses. In summer, who cares? A big array delivers heaps. In winter, there’s less excess, if any. An overdimensioned array is then even more attractive.

            In a solar house, I figure the washing machine only runs on sunny mornings, and the clothes dryer is 100% solar/wind powered, being lines between two poles. The aircon is only needed on sunny summer days. If there’s high consumption, it is supplied from the array, with energy flow _into_ the battery at that time. (Maybe except for using the welder while a cloud passes.)

            A lithium battery doesn’t need an on-board computer and such good ventilation. I’ll have to have a better look at them.
            But the Samsung installation manual p19 shows the levelling tolerance on mounting is just +/- 0.5degrees – just crazy! If that’s not blatant warranty avoidance, then what is it?)

            And: the samsung data sheet


            only shows PV-to-grid efficiency, not battery efficiency.

          • Erik, Redflow now have the ‘Zcell’ $17,500 to $19,500. You may be pleased to know, that includes the inverter…
            The battery has its good points, but I think the price is way too high.

            That Lithium battery is an unknown quantity. Anyway, if the same charger is used for several batteries, what may be a suitable charging current for one battery, may not be for another, so the BMS may trip. Batteries that have a data bus solve that problem.
            I am trying to get some life-cycle test data for a few of the more promising LiFePO4 offerings. I will post them if I get them.
            Choice is still so limited, so perhaps lead-acid again.

          • Erik, Indeed, the specs are certainly vague, and need to be scrutinised. Previous specs contained more detail, but the link you posted no longer worked. When looking for that spec, the website appeared with vague specs, but earlier manual contained;

            “Gross energy efficiency of the ZBM2 is approximately 80% without ancillary components. However, the cumulative effect of inverters, cooling systems,
            and additional power electronics can reduce overall AC-AC efficiency to significantly lower levels”

            Whereas Sandia’s result is ~68% DC-DC (at full load), agreeing with the 68% you have seen. As far as I can tell, Sandia’s result includes the parasitic loads under the conditions of the full load test they performed. There may be additional parastic losses or at other times, but if you have enough solar, then as you say, who cares?
            Efficiency is better at lower rates, but all batteries can make that claim.
            If low discharge rate meets your application, then full capacity would be available. The battery is not really suitable for continous high rate use, nor was it designed to be. It’s the cell’s rate of flow that produces the low voltage efficiency. Ventilation would certainly be necessary if used at full load, if only because >4kWh of heat is produced. More signs that it is not intended to be used as a high discharge battery, but designed around back-up in remote telecoms installations. The replacement of diesel generators, that sort of thing. Sandia’s report considers the economics of that situation.

            LG have renamed their product, 6.4KW Resu-ex. To put a battery on the open market, manufacturers must supply related documents that are not openly published. From those, LG use a Lithium Polymer Cell, where chemistry is ‘proprietary’, but from what I know of their cells, is likely to be LMO. It’s not of the 18650 type, but a prismatic. Their public claim is ‘6000 cycles’, which is correct, if annual discharge is 1960kW, over 300 days of the year.
            Then, end of life is 60% at 20 years (6000 cycles). If >2MWh/year, can become 80% at 10 years. More frequent than daily cycling, 60% at 10 years.

            A difference is that the LG has better ‘calender life’ at the same 2kW output, so can endure 20 years. Samsung may be likewise, because they use a similar LMO cell design. The Samsung mounting angle is a bit overwrought, but it would be easy to avoid violating that specification, I expect. Odd, though.
            How many hidden warranty problems are there from all manufacturers? Who will be around in 10 years’ time?

            Samsung’s is complete though. From panel to grid. More details can be found in this version. Pages ~107 have some efficiency charts.

            Despite the better performance than many, neither LG nor Samsung offer much hope of return of expenditure.

            Panasonic’s NCA18650, were, and still are, a high capacity cell for consummer goods like laptops, and not long term storage cell. Claims of ‘high-tech BMS’, or ‘special know-how’ will not change the basic cell. Can be worse, though, and Tesla have succeeded in doing that. Promises of a better version 2, and disguised specs, are consequences of that failure. It was announced that unlike the car, the “daily’ battery would use NMC and not NCA. Via suggestion, it was *assumed* NMC would have longer life, but that is not the case. NMC is cheaper, but generally shorter-lived than NCA under the same conditions.
            Must be busy at Tesla, as they try to make ends meet.

            Industrial batteries, like cells, are rated to 80% under common test conditions. Detailed engineering calculations determine useable life. It’s the home battery bandwagon that has adopted 60% and vague promises of cycle life. Redline entered the home market after a Tesla-owning CEO recently arrived, and then, the glossy website followed. If you see what I mean.

            Lead Acid is viable. It works. Owners of commercial UPS will not even listen to offers of Lithium Ion. They replace their cells when “budget allows”. To my personal knowledge, some pay USD$80/kWh. (similar to truck fleet owners’ Group 31 battery prices). They don’t care about the rest.
            But, Lead Acid quality varies a great deal, and price is an unreliable guide. Some batteries aimed at off-grid applictions, are worse than average truck batteries. Once again, like so many other products, marketing makes the difference.
            Golf cart batteries can work well, because they are medium discharge, but will readily tolerate some higher use. Worth a try?

          • Erik Christiansen says

            How many hidden warranty problems are there from all manufacturers?<

            Looking at possible alternatives to a Redflow ZBM, I've taken a gander at LiFePO4:


            There's no data there on lifetime vs DoD, none of the wealth of info Redflow provides. There is one scary admission, though; you have to use a separate charger per battery, or the integrated "protection" module may pop. And you have to use their special chargers too. You can bet a warranty claim would be hard if you don't.

            Since I only need overnight power for a fridge, DC-powered computer, and LED lights, plus same for a dark cloudy day, I may even buy more lead-acids after the current ones give up. I did pay $125/kWh for deep-cycle gel, but as you can only use half, the 100%-DoD-is-OK ZBM is attractive, except for price.

  3. Peter Wise says

    Good you have ‘outed’ Musk again. I along with others have tried to put to bed the ‘foamers’ excited about his revolution based on hype rather than substance. I am however disappointed you have not scratched below the surface on this and your other offerings above in terms of the economics. It’s not really that hard.
    To get a product to assess I have discounted the LG product as it does not offer an integrated solution. The Sunverge website offered nothing by way of technical specs for their product. This leaves Samsung whose website was pretty good.
    In crunching the numbers we need to nail down exactly what capacity is offered. Samsung were upfront and explained the 7.2 KWh needed to be discounted for the 90% DOD. Their own figures dropped the capacity to 6.48. They further provided a graph showing capacity deterioration with age. If we average this to 90% (generous) we get a capacity of 5.8kWH .
    Now it gets interesting. Whilst 6000 cycles is talked about, the warranty is only 5 years. In your references above to the Powerwall you talk about using the battery power in the evenings and night. As such over 5 years we have 1825 opportunities to use our 6kWhrs. This means that if we manage to fully charge our battery EVERY day and fully discharge it EVERY night we ‘save’ from the grid 10585 kWHr’s. As we have paid $11,900 for our system this works out to a price of $1.10/KwHr. (and this is after I ignored install costs which will bump it up another 15%)
    But we really want this system to work so let’s take some risks and run it out to 6000 cycles, way past the 5 years warranty to 15 years. Crunching through the same figures we get 34c/Kwhr. Now added to this (in both cases) we need to add in the cost of our ‘free’ solar energy, which based on a 5kW system at $8000 depreciated over 10 years gives us about 11c/kWhr. (for Melbourne)
    Which all means that
    • If we can nurse our Samsung along to 15 years life
    • Fully charge and fully discharge our battery every day (no mean feat)
    • And we get the system installed for free
    Then we get power at 45c/kWHr.
    Just got an updated tariff letter from my retailer. Now 22c/kWhr. Down 7.5% from last year and now down nearly 27% in 3 years. (rural Vic). And If I didn’t have solar it would be down at 18c/kWhr.
    I honestly don’t know why we are even having the discussion . My own number crunching for my solar install is in shreds on the floor. Payback now out to 10+ years. I used CPI for price of power in my calc sheet. The 27% drop in prices has stuffed everything. Even worse as I pay 4 cents more for imported power as I have solar. Just on claws back the FIT they give me.

    • Finn Peacock says

      I agree totally that no-one would buy a battery right now for financial reasons. The battery is unlikely to last long enough to pay for itself. In 5 years when battery prices have halved and their lifespan has doubled it will be a different story. In the meantime there are a good number of people that will buy batteries for other reasons, who will help drive the cost curve down for the rest of us.

    • Richard Oswald says

      If you look at the situation from another perspective, the ‘consumer value’ of a Powerwall is approximately 5 units of grid-power avoided…. in your case 5 x 22c or $1.10 per day. Assuming a purchase price ‘installed’ of say $7500 (+ 15%) or $ 8,625.00, then this gives you a ‘value’ payback of 7,840 days or 21.5 years.

    • I cannot understand how did you calculate 45 c per KW? If you use 11 c during the day and 34 c per KW at night, you should have the average which makes it 22 c per KW…Just my 2 c…

      • I think he means that the battery component costs him 34c/kwh and the solar panels cost 11c/kwh, adding to 45c/kwh.

    • Erik Christiansen says

      athomas > Erik, Redflow now have the ‘Zcell’ $17,500 to $19,500. You may be pleased to know, that includes the inverter…
      The battery has its good points, but I think the price is way too high.<

      Thankyou, that price killed it for me when their email arrived yesterday.
      I didn't read that it included the inverter. (And presumably MPPT charger too, then.) OK, that's not quite so bad if it's a whole system minus panels.

      I'm leaning toward lead-acid now; perhaps keeping the small 24v system for lighting, PC, and soldering iron, then adding a 48v lead-acid bank and SMA island inverter (I like Finn's find) for the rest. Wouldn't run the washing machine off battery, but the water pump (tank water) and the lathe or milling machine could be used on a gloomy day, and at night.

      The biggest challenge now is diverting excess power from an oversized PV array to the boost element of the on-roof solar water header – only when the battery bank is charged. The 133% rule doesn't appear to apply to off-grid & battery inverters, so I plan to go with at least a 200% array, for winter power without spending a fortune on batteries.

  4. john nielsen says

    Hi Finn,
    In year 2014 I was told that the AC-Battery would be available early 2015. Now is it is early 2016 and they are still testing it. I am glad for my lead gel batteries and don’t believe Tesla or Enphase has got it right yet. When a grid tie inverter sees 240Vac it will output power and that can simply be done with a few second hand lead acid batteries. Of course you will not have much capacity after the sun sets unless you have a small household, one fridge, led lights etc., run all heavy loads during the day, washing machine, air con, etc., I have 12 off 12V 250 Ah lead gel batteries, and run all off grid from my 6 kW micro inverter system, and an 8 kW low frequency inverter.
    I run 2 air con during the day plus washing machine, oven, etc., Night time air con is from the grid on separate circuits as we have 4 split systems, 6 person household.
    My batteries are now one year old. I don’t know how long they will last. Max charge 12.90 volt, min charge 12.00 volt controlled by Morningstar relay driver. Voltage varies several times during the day, according to sun, clouds, loads, etc. So this might be several cycles during the day. Finn, please tell me what this does to my bats when they vary up and down 10 or more times a day between these two limits.

    John Nielsen, Silkwood.

    • Finn Peacock says

      Short answer: I don’t know.

      Any battery experts out there who can answer John’s question?

      • Peter Wise says

        Need a lot more info for this one to even make a half educated guess. Key ones are
        What % load (or charge) are batteries delivering (recieving) when the voltage measurements are made
        Need to record the above hourly over a long a period of the day as possible. (Don’t worry about overnight – a reading before nodding off and again when awaking will be enough to do some extrapolation for overnight.
        This is just a starting point.
        But based on info supplied I would say your batts are taking a bit of a hammering. Really, resting voltage (nothing in nothing out) should be above 12.3V which is about 50% charge. This is worst case. Ideally we would like to keep these batteries above 75% charge meaning a steady state of 12.6V.
        There are lots of factors other than this that wil limpact life. Next biggie heat. Keep them cool. They hate heat.
        Something wrong though if your are never seeing higher than 12.9V. Are you measuring at battery posts?. Is meter accurate?. Ideally your batts should be getting fully charged most days which is 13.5V at the posts when batt is being floated (trickle charged). (dependant on temp a bit)

        • Erik Christiansen says

          Have to agree fully with Peter, and the comments below don’t do much more than add some background and a hint or two. My batteries are resting now at 13.0v – pretty much fully charged, I think. If John’s 12.0v is not measured on significant load, then they’re very deeply discharged. A top of 12.9v seems OK only if you’re measuring resting batteries. It is real trouble if that includes charging. In my (limited) experience, a decent charger will put out voltages in the general vicinity of:

          Lead Acid Battery Charging:
          * Sealed Batt. – Absorption (13.8V to 14.1V); Float (13.2V to 13.5V)
          * Wet Batt. – Absorption (14.1V to 14.4V); Float (13.4V to 13.7V)
          Typically, it will only be 90% charged when it first reaches
          14.2V, with I tapering off as it subsequently charges at that voltage.
          The float voltage, e.g. 13.8V maintains a full charge.

          The absorption phase pushes in charge at a useful rate, and will be ended when the charging current drops to a low value. The float phase is just to maintain the battery’s state of charge, as I understand it.

          I have sealed batteries, which need lower charging voltages than the vented ones, yet are pushed to 14.2v on absorption, and 13.5 on float. If John doesn’t see his batteries cycle up to similar figures, then I’d measure the voltage across the battery terminals while charging with the array in full sun, as Peter suggests, and then across the charger terminals/output leads. Perhaps there’s a high resistance connection in the charger path?

          The absorption phase would occur from first sun, for the day, on the array.
          If there’s a sufficiently high resistance in the charger path, the charger would promptly drop to float, and adequate charging would not occur.

          As for counting battery cycles, these are discharge cycles, with Depth of Discharge dictating whether you count it as a full or partial cycle. Take for example, the LiFePO4 battery installation I cited from Renew, issue 128.
          The installer increased the battery bank capacity, so DoD could be reduced to 45% (on average), to bring the expected lifetime cycle count up to 4000.
          He must have had his hands on a graph from the battery datasheet, to quantify that trade-off.

          My expectation, not supported by documentary evidence to hand, is that several shallow discharge/charge cycles are better for it than one very deep one. (I need to get my hands on one of those graphs.)

  5. Something to watch for is different companies define “cycles” differently which makes comparing warranties and expected lifetimes difficult. Some view a cycle as going from charging to draining then back others view a cycle as drawing the rated available capacity of the battery once.

    An example:
    Day one cloudy and battery only charged 50% which is then used that evening.
    Day two same cloudy and battery only charged 50% which is then used that evening.
    Some view that as 2 cycles of use and draining and others as 1 cycle of 100% capacity. A few days with intermittent cloud and this could rack up some big differences.

    • Finn Peacock says

      Great point Mark

    • Peter Wise says

      Absolute minefield here. Erik earlier on talked about Redflow which seem to be doing the warrenty on cumulative output delivered by battery. This seems to be a more absolute system and a fairer to all

  6. Tesla’s website talks about charging from the grid at the cheaper time of use rate over night. Does anyone know if this an either or choice (solar or grid) or if it can be used with both sources?

    Can they be set to discharge at a set time to prevent wasting stored power at shoulder rates of 17c/kwh when the peak rate is 50c/kwh?

    By my Calcs 6.4kwh x $0.5 =$3.2/day

    $7500/3.2/365 = 6.4 years to pay off

    • Finn Peacock says

      The Powerwall is controlled by the inverter. At at this stage that’s either a Fronius Symo Hybrid (3 phase installations only) or a Solar Edge + Stor Edge interface. I’m not sure if these can be programmed to only use the battery at certain times. But it is the inverter functionality you need to check, not the Powerwall.

      I know the Alpha ESS unit (which is a very viable Powerwall alternative and by all accounts a great product) can definitely do this:


  7. john nielsen says

    Hi Finn,
    Thanks for searching answers to my question about battery cycles, and thank to Peter Wise and Erik Christiansen for your comments, much appreciated.
    My inverter requires 48 volt input, thus I have 4 off 12 volt bats in series, by 3 banks.
    I don’t remember seeing the voltage below 12.3 volt. If higher than 13 volt the inverter spits the dummy, thus I limit the input voltage to 12.9 measured at the posts.
    As I said, my system is off grid and thus my inverter is UPS bi-directional. I don’t see how I can keep a steady voltage. I have 4 ports on my relay driver which I have set to different max and min voltage and thus cut in and out 3 lots of panel voltage inputs. Mostly the voltage is around 12.70
    Can anyone who runs stand alone systems with micros please comment !?
    As for a hybrid system, I believe you can charge your bats during the day (one way power) and then switch over at night to drain, thus have one cycle in 24 hr.
    As far as I can see, you will not be able to get off the grid with the PW but I can see that the Enphase AC-Battery will help doing that but how many units? And at what cost? So, I think hybrid power will be here for a long time to come.
    I can understand that many people are anxiously waiting to get off the grid as export in many cases are limited to 2 and 3 kW installations and a FIT at about 6 cents.
    Yes, I might be giving my bats a bashing, but I don’t see how to avoid it…. during the day there are many variables, sun, clouds, large loads, small loads, all fluctuating with bi-directional battery power. From the micros the power goes to the load and excess to the bats, when the micros don’t supply enough power, then there is a drain on the bats. If you attempted to get off the grid with the PW would that not be the same problem? Many cycles during the day?
    John Nielsen, Silkwood.

    • Erik Christiansen says

      John, you might like to grab a copy of this month’s “Silicon Chip” magazine at the newsagent. Their article on a (small just-for-emergency-lighting) MPPT solar battery charger provides a brief overview of battery charging requirements, complete with some simple graphs. It may be more conversational than stuff on the internet.

      Whatever we read, the answer is the same, we need around about 13.8 to 14.4v for the bulk/absorption phase of lead-acid battery charging, depending on whether sealed or vented type. If you don’t push, the charge won’t go in. The batteries will not charge at a viable rate.

      OK, it’s a dysfunctionally sensitive inverter which is enforcing the battery mistreatment. You speak of switching panels with relays to regulate charging, so I figure there’s no MPPT box in between. (They are just a DC-DC converter, operating in buck mode, to put out more current, at a lower voltage, than the panels generate. I.e. match arbitrary panels to the battery bank, and improve efficiency.)

      To keep going with the cranky inverter, I’d insert two (nice fat 50Amp) silicon power diodes in series between battery and inverter. That’ll drop about 1.4 to 2 volts, depending on inverter current, and allow the panels to charge the batteries to at least 12.9 + 1.4 = 14.3v with the inverter operating. At night, the batteries will drop to 13v or so on no load, and less on inverter load. I’d use a relay to short out the two diodes then, to avoid the power loss.
      (Mount the diodes on a pair of substantial aluminium headsinks, even bits of flat plate, screwed down on insulated mounts. If you use less than 1 kW, then with 48v batteries, 25 Amp diodes would do if the inverter efficiency is 80% or better. A finned heatsink can be made by nesting a couple of Al sheets folded into square channel of increasing width, if you don’t have heatsinks in the junkbox.)

      It’s not as efficient as without the diodes, but it allows the batteries to take more charge, providing more power at night – and the batteries won’t kark it quite so soon.

      As mentioned before, clouds going over are not battery cycles. Drawing a percent or two of capacity during a cloud shadow is just an interruption in charging, and is not deep enough to trouble the battery, as I understand it. Automotive batteries can be killed by just a few deep discharges, yet last for thousands and thousands of shallow discharges – at hundreds of amps while starting. A small wibble is not a charge/discharge cycle – to be counted it must be deep. Try this, perhaps: By all means count the wibbles, but multiply each by the DoD it reaches, then divide by the DoD used for the nominated battery life, e.g. (3×5% + 1×10%)/80% = 31.25% of a cycle – not 4 cycles! (And batteries have longer lifetimes for shallow cycles, so three days of 33% of nominal, eat less battery life than a full cycle. Going down to 20% SoC is a much bigger killer.)

      As for the attraction of going off-grid, my latest electricity bill shows 33c per kWh for peak, but the GST is added on after they calculate that, so I’m paying 36c/kWh. And that’s not counting the service charge, which doubtless takes it toward 50c.

      • Erik Christiansen says

        Finn: “headsinks” should read “heatsinks” in the prior post

      • Erik – I like your lateral thinking with this technical solution. The 10% drop in efficiency is a bit of ouch though. But would cheaper than trying to recover the cost of a thumping big DC DC convertor (which will have some losses anyway).
        Yes – fully agree with all your other points. I get a bit frustrated at problems like this. Want to chuck the gear in the car and go and see what’s really happening!
        I was thinking about Johns problem during the week from a design angle. Seems we have 36 KwHrs of battery running a 6KW invertor and another 8KW ‘low frequency invertor. (not sure what this animal is). Has some air cond load and other appliances.
        You asked about DOD curves earlier on. Below in table form is DOD for a Giant 230AH AGM battery. I have also included price/KwHr based on retail price in Aus. ($471)
        DOD Cycles KwHrs $/KwHr
        25 2000 1500 0.314
        28 1800 1512 0.311507937
        31 1600 1488 0.316532258
        36 1400 1512 0.311507937
        38 1200 1368 0.344298246
        42 1000 1260 0.373809524
        46 800 1104 0.426630435
        58 600 1044 0.451149425
        As can be seen we are OK to 36% DOD (a fair bit higher than I expected). So basically John has 13KwHrs to play with at any time (assuming we solve the charging problem). For an On Grid system I am OK with this design, although experienced designers may want to validate this conclusion.
        The Air Cons maybe the killer. No advice on size, but a typical mid size 7Kw cooling / 8 kw heating air cond would pull about 2.5Kw. A few hours of running something like this and our 12KwHrs is soon exhausted, especially when we have other appliances in the mix like washing machines/dishwashers etc.
        Based on the above storage costs I am still having trouble seeing where it stacks up financially for on grid system.
        Erik – what state are you in?. I suspect Qld with prices like that. In Vic there is no need to be paying any more than 22c/KwHr and 11c/Kwhr off peak ‘Time Of Use’. All plus GST. Many are though, simply because they don’t shop around. Heaps still on the old pre privatisation tariff of 30c plus. The retailers are happy to leave them there of course!. Several months ago got my daughter 16.5c/KwHr + GST. (peak only)

        • Erik Christiansen says

          Oh-Oh, I didn’t realise that John was pulling _that_ much power from the batteries. He’ll need an additional 25 amps rating on the diodes for each kW drawn at 48v, even if the inverter efficiency is good. And, then, _big_ heatsinks, as e.g. at 6 kW, 2v*150A = 300W needs to be lost to ambient. OK, now the current is that high, the diode drop will move a bit further up the V/I curve, maybe toward 3v, so 450W of heat. Diodes are difficult to reliably parallel, so go for some really big ones, so that’s not needed.

          Thinking of a DC-DC converter between batteries and 240v inverter is tempting for high power levels, but if that achieves e.g. 93% efficiency, it’s even stevens, for a heap more money.

          Better to spend a few hundred on an MPPT charger between array and batteries, to gain around 20% in efficiency, more than making up for the diode losses, I figure. (And then get rid of the troublesome inverter when it eventually turns its toes up.)

          Thank you Peter, for the DoD vs lifetime table. It should help John, and it’s an eye-opener for me. I didn’t realise that “Deep Cycle” lead-acid batteries went downhill quite that fast at half-deep DoD. Even LiFePO4 is a step forward, then.

          As for me paying the high grid tariff, I’m in Victoria, but haven’t changed supplier in 26 years. It sounds like I need to find out who offers that 22c figure, even though the 8 – 10 tonnes of firewood going through the wood heater each year seriously hits the electricity bill on the head.

          I plan to move out to the farm in a year or two. There’s no mains out there, so grid rates will become irrelevant. Straw bale walls, and double glazing will greatly reduce the air-con requirements. (I have a builder who’s keen to do the straw bale. That’s not often the case.)

          • Erik – your lifestyle mimics mine. I am rural with a huge wood heater that I feed with trees from my block. Whoops – hope no offence with my comments re not shopping around (wipes sweat of forehead).
            Yes – will be big diodes. But they are out there. I think a call to inverter manufacturer may not be wasted either. Falling over at 13V???.
            Re tariffs. I am Power Direct in Powercor distribution zone. TOU with solar. 20c/KwHr peak, 10c/KwHr OP and 6c FIT from them. My daughter AGL in SP Ausnet area. 16.5c/KwHr all power. All above + GST. These were kind of special deals. The AGL one was a deal through work. Interestingly the Power Direct one was through Electricty Wizard and Power Direct refused to match it when I went direct to them. Go figure. This is not uncommon it seems.
            Simply Energy RACV deal is usually pretty good and Alinta looked OK last time I checked. But you need to shop. Same retailer will have heaps of different tariffs out there. The Gov’t web sites that allow you to compare always have the highest. Avoid iSelect site. Not impressed. Won’t say anymore here in case Finn gets sued.
            Cannot speak too highly of Power Direct and AGL. I have 5 acounts with AGL at work for gas and power. Never an issue with billing and the like. It just works. Power Direct owned by AGL but they seem to operate independantly and will actually undercut each other.
            Where you have a high cost to extend the grid then renewables are a no brainer. I was off grid for a couple of years out here until there were enough houses to reduce the cost. Was a lot tougher then. Big LPG absorbtion fridges, generators running out of fuel in middle of evening and so on. Yep – was fun.
            Always been impressed with straw bale construction. If you are rural the BAL fire constraints in Vic may dictate thicker glass. Which when you double glaze makes for some very heavy sliding doors and windows. Done it here with an extension I put on. Nearly killed us lifting into position. But it can be done, (Boral didn’t bat an eyelid at the order)

  8. Jim Mathieson says

    The ABC Catalyst episode “Battery Powered Homes” on 2 February addressed many of these issues, showed several real families’ experience and foreshadowed some changes in battery technology and implementation at a system level.
    Well worth 30 min of your viewing time if you are considering the broad picture.

  9. john nielsen says

    Thanks Erik and Peter, very useful information. My other inverter/charger is similar to Giant Power’s 5 kW high freq hybrid MPPT inverter. I will not mention the brand as it is presently for repair. It was never overloaded as I have power on several circuits for different purposes, i.e. 3 phase from the grid for my elevator, single phase from the grid for my night time air cons. My 8 kW low frequency inverter will run the usual household appliances during the day as well as a couple or small split system air cons. If the load is too great, the relay driver switches off some loads. If the voltage goes too high it switches off one or two panel sections. When the panel micros go off, they stay off until the voltage again drops to my determined level, when they again switches on. The micros produce power 60 seconds after they see 230 Vac.
    When the Enphase AC-battery will be on the market, I intend to use one for solving my battery problem. My information is that I can be grid connected with my micros without exporting, zero export limited can be set on the AC-bat. As I have said, I am not allowed to export to the grid. I will hope to use the AC-bat as a zero export device, and draw as little as possible from the grid. My 6 kW system produces enough power for all my day loads and any excess I will put into the battery bank using, as Erik suggest diodes in series, I don’t know how many but the 50 A ones are about $20,,, it doesn’t matter how many, .7 volt loss per diode is not a problem. Anyway I first heard about the AC bat over 2 years ago and I am told that they are now testing it!!!!! I might never see one as I will be 80 next year. Meanwhile if anyone has a good idea of how to get AC one way, I will be please to know. I was considering one line from the grid feeding perhaps 3 or more lines with CTs to switch on or off according to flow. There are some zero export devices on the market, but they are expensive, and complex in that they distribute the excess power from my PV. I don’t need that as I am already doing that myself. Thanks for the tip about Silicon Chip, I sometimes buy the magazine, just to know what’s new if anything. My last quarterly bill was $84 compared to same period last year just over $800. Just one last ting, my bats are sealed lead gel. I have known automobile bats to last more than 5 years. The voltage around 12.5 V with the old type voltage regulators which fluctuated quite a bit depending on the lead in my right foot.
    John Nielsen, Silkwood.

    • Erik Christiansen says

      John, 50A diodes will do for 2 kW, but cook if you go much over that. It is very hard to achieve current sharing when paralleling, so for 4 kW you’d need a 100A diode, e.g. a 1N3288A (Digikey have them for about US$53 plus a fair bit of postage to here. You might find a better price.) That diode will drop about 1.5v at 100A, so you’d probably only need one, except that the drop will be significantly lower at e.g. 25A. Since you already have a smart switcher, you might be able to switch in none, one, or two, depending on battery voltage. (A form of stepped series regulator. 😉 Some hysteresis in the switching thresholds would be good.

      If building up a large finned heatsink, the (large) contact area between layers will need to be very flat, for good contact, and a smear of thermally conductive paste in between would be good. A big fat finned commercial extruded heatsink would be better.

      It’s fortunate that the batteries are sealed – the 0.3v (approx) lower charging voltage is probably all that has allowed the use of the sensitive inverter, yet still put some charge into the batteries.

      In 37 years of motoring, I’ve not had a battery last less than 7 years – but then I do quite a few country runs of 300 km, and never less than 10 km.

    • John I actually reread your post last week and picked up you had Gel and not AGM as I did my DOD table on. Researched Gel and they are a cut above AGM. You can get close to 50% DOD before chewing into batt life. BUT – they don’t like the higher voltage that chargers usually give AGM batteries at the end of absorbtion to ‘clean the plates’. Hopefully your charger is suited to Gel, and from what you have descibed seems little chance of this happening with your modus operandi.
      See your are from Silkwood. That explains the air cond load. Spent a few weeks last year at King Fisher caravan park at Kurramine. Still drinking my way through some of the wines from that fruit winery across the highway from you..
      In my sixties and I am in absolute awe of the system you have put together. So hope I am as active as you are technically in another 15 years. Bloody great effort John. Must be the warm humid weather up there!
      Anyway Erik is way more current and competent than me on sorting out your technical issues so good luck with it.

  10. Nick Pfitzner says

    A technical point: Powerwall is 2kW standard draw and 3.3kW peak, according to my manual. The 5kW peak of the LG would definitely be handy, but you’d want to fully expand it first or you’d be out of juice in an hour on the standard battery!

    The Powerwall is also rated to 5000 cycles as reported by some sources (my installer said 6000); not sure why they’d offer a 3000 cycle system (8.2 years) with a 10-year warranty (which isn’t pro-rated), NOT an expected lifetime of 10 years – though the BOL/EOL stats are stated quite clearly in the manual, too 🙂

    I think people are underestimating some of the people that work at Tesla…

    Ultimately, the Powerwall is not designed to be an off-grid product. Nor is it going to run your toaster, kettle, iron and dryer simultaneously. If that’s what you’re looking for, then I agree other options are probably where you need to look. And you’ve probably misunderstood what Powerwall and similar products (including the LG, which is remarkably similar when I look at both data sheets) are designed to do.

    For me its definitely not all about the financial side. If the system as a whole helps me wipe out 80% of my power usage, then ROI is almost 10 years on the dot (a bit over 8 years at 100% which admittedly would be a stretch). But I’m getting solar panels paid off in that time as well, so pretty much a bargain.

    That figure doesn’t include the value of any FIT or Reposit Power generation that will be engaged once I switch over to Diamond Energy.

    There were other battery systems out there with pros and cons, but Powerwall was right for me. Other systems will suit other circumstances.

    It hangs on my house wall with little fuss, doesn’t take up space in my garage, or require a steel box for something as simple as weatherproofing. It keeps the house ticking away when the sun goes down, running through until the morning with about 25-30% left on a good day. Last week I went through four days without using any power off the grid at all which wasn’t bad for summer.

    Yes, on a bad day, where the air conditioner needs to crank, its not so great. However I’d say that is more about the ducted air con that came with the house than the shortcomings of the battery. I’d get rid of that damn thing if I could… That is one mitigation measure that just isn’t on the table.

    Finn’s point about price moving down is probably the big takeaway here. I’m not going to pretend I did it for the noble cause of helping everyone else out. I did it for the reasons I stated in the press: a financial underpinning against my running costs (and future price rises!) , supporting my want of renewable energy and fulfilling my tech curiosity.

    The people so vehemently criticising the Powerwall and Elon Musk in general need to sit back for a moment and ponder: if the other options are so much better, why has the storage market moved at such a glacial rate until very recently?

    This is a consumer arena, not a science fair, like it or not.

    I look at Redflow in particular with excitement, but it also has shortcomings that need consideration.

    Overall, rather than criticising this system or that, everyone here should be happy that things are opening up, and everyone can benefit.

    I’ve certainly learned a lot in the last few weeks, and once the Reposit kicks off there is another curve to ride. These are exciting times.

    • Finn Peacock says

      Hi Nick,

      Thanks for sharing your experience. Most armchair critics (including me) don’t actually have a Powerwall on their house, so it’s good to get it from the horse’s mouth.

      Please let us know how you get on in the medium to long term. Especially when you get Reposit going.



      • Nick Pfitzner says

        No problem Finn. I’ll be dropping info into my blog for anyone interested (just click on my name to view the site) and building it up as time goes on.

        Keep up the good work everyone. We can be constructively critical about anything, as long as we are moving in the right direction.

    • Erik Christiansen says

      Nick Pfitzner’s comment about an old airconditioner hammering the batteries on a hot day is thought provoking. It’s a great idea, I think, to overdimension the PV array, so that more battery charge is available on an overcast day – and that overcapacity ought to drive the airconditioner on a very sunny day, without any discharge of the battery. Rather, it should still be charging.

      So, for a 5 kW system, 8 kW of panels can still give a 2 kW charge rate on an overcast day, and 3 kW charge rate plus 5 kW of airconditioning on a stinker.

      Ah … yes, it was issue 128 of ReNew which had an article on “Oversizing PV arrays”, seems to conclude that a 50% oversize is optimal.

      I’m busy sketching floorplans to straw bale dimensions at the moment. The roof will also be highly insulated, to reduce the aircon requirement, and double glazing will do its bit. I have to talk to the truss people, because I want enough area of 30 degree northern roof slope for a swathe of panels.
      It is unambiguously off-grid, and I don’t want to fire up the generator too often, so I’ll oversize the array for running full rated load plus charging the battery, in good sun. (Lathe or milling machine, plus aircon, etc., and the domestic stuff.)

      Why a 30 degree roof, when we’re at 38 deg S here? Well, 38 deg is scary steep, and anyway, counting on my fingers, I get:

      Melbourne is at: 37.78° S. Earth’s axial tilt is 23.44°.
      Panels tilted up 37.78° would be: 100% optimal at an equinox
      cos(23.44) = 92% optimal at either solstice.

      Panels tilted up 30° would be:
      cos((37.78 – 30)) = 99% optimal at an equinox
      cos((37.78 – 30 + 23.44)) = 86% optimal at the winter solstice
      cos((37.78 – 30 – 23.44)) = 96% optimal at the summer solstice.

      (I hope I was holding my mouth right while doing that, as it’s a first attempt)

      A bit more oomph for summer aircon is worth a smidge less in the depths of winter, as it’ll be the woodheater, not reverse-cycle aircon that’ll be heating.
      Not that there’s much in it. I begin to see what installers already know – there’s a fair bit of latitude in the latitude correction.

      And Solarhart mandate “earthquake zone” hold-downs on their roof mounted water heaters if you go above 30 deg. It seems to be the sweet spot on a number of counts.

      It is only good, I think, that the Tesla Powerwall launch is surrounded by hype. We need the mass energy storage market to burgeon rapidly, and pioneers take big risks. It is the technology which follows which will later dominate the market, but it is early adopters and producers who create the market.

      • Nick Pfitzner says

        That is my dream – to be able to build a house to MY specifications and make it as efficient as possible. Earth-sheltered is one of my criteria. As it is, I’m stuck in suburbia for the next ten years or so to capitalise on my solar hybrid investment 😉 so maybe I’ll save that one for my fifties!

        My air conditioner isn’t that old (house was 2012 build), but it was probably the cheapest one the original owner would fork out for. I never really thought about it much until I got solar, but now I watch every Wh like a hawk!

        One other thing I’m going to put on my design wishlist is external window coverings – stop the heat even getting near the windows. Whether this takes the form of an adjustable shutter system or soft awnings will come down to budget. The former offers some advantages in terms of automation.

        Best of luck with the build.

        • Finn Peacock says

          Hi Nick,

          I would argue that a piece of wood designed properly is all the awning technology you need to allow the winter sun in and keep the summer sun out:

          As implemented on my house:

          • Nick Pfitzner says

            It is certainly a better choice in terms of renewables, if sourced from the right place.

            My only concern about timber is longevity, depending on what the climate is like. Most Australian hardwoods will take the punishment if finished correctly.

            Unfortunately, my west-facing windows are massive (one is 2700mm wide and the other 1800mm, both 1800mm tall) so a near-vertical system would be required.

            And I’d have to convince the wife who is only partly on board with this whole thing 🙂 I installed a fabric awning on our north-facing garage window to limit the heat in there so we’ll see how that goes.

          • Erik Christiansen says

            Nick, it might be easier to convince the wife if the horizontal version of the slatted sunscreen were used. (As Finn has depicted, but not implemented.) Tell her it’s a pergola, perhaps. If the (wide) slats were pivoting, then a connecting rod fitted to pins on one corner could tilt them all in synchronism. That might rattle a little in gusty winds, though, unless well fitted.

            Relying on a grapevine on the pergola for summer shade is problematic if you have possums. They finally killed the vine on ours, by continuously eating all the leaves. Now they’ve ripped the stainless steel mesh inlet strainer on the laundry tank, and thieved the one on the kitchen tank. I have to try to outsmart the varmints, but my attention is divided.

            That pergola is Yellowbox, milled from a fallen tree on the farm. Despite some borer holes, it has only greyed over the last 20+ years, and looks like lasting another 30 without trouble, so it’ll see me out, I figure.

            Made good progress last night, googling thermosiphoning solar + woodheater merged water heating. It’s all simpler than I thought, so long as the storage tank is mounted high. Will need glycol in the panel, as we have frosts in Gippsland. Is creosote condensation really an issue in a water-jacket in-flue heat exchanger? I’m considering a loosely-coupled unit, i.e. a copper pipe wound tightly around a stainless steel flue section. The limited heat transfer ought to virtually eliminate creosote condensation, and reduce boiling off excessive amounts of tank water when the HWS is full of hot water. (There’ll only be one occupant, so limited output is OK.)

  11. john nielsen says

    Hi Finn,
    There was a time when all service stations serviced your cars. There were also a lot of automobile workshops. Where I live there are none left. Those that were here are still here, but they don’t repair or service cars, they sell tyres and repair lawnmowers. They cannot service cars. The new cars have to go to the dealer ship, no one else has the computer to analyse what needs to be done, nor the tools. The car makers have the business sown up.
    I see the solar industry heading in the same direction. No money in selling panels, inverters and batteries. As I see it, they want to sell a whole system with proprietary items which no one else can service. I am referring to Enphase, Tesla, and no doubt many others. I think they will fail because sooner or later, someone will come up with a huge li-ion battery with power in and power out, simply just an affordable battery. Then someone will come up with an off the shelf control box which will suit all sizes and systems and contain no proprietary items. Just plug in your panels, connect your inverter(s), connect your battery and the plug and play control box, and you are off or on the grid by your choice.
    By the way Erik, I didn’t need tensor calculus to verify that you calculations are correct and Peter, yes that tropical wine place is just down the road from Silkwood Castle, and Finn, I don’t use the method “up in smoke”, I use relays set at certain voltages threshold’s trip points, I do however have battery chargers and other gear which will automatically turn off when a certain limit is reached. Usually nothing up in smoke, but sometimes a prayer before pushing the ON switch..
    John Nielsen, Silkwood.

  12. There is a recent paper concerning home batteries, published by authors from Melbourne University and IBM Research. Not as complete as some, but it would be hard to argue for a significantly different outcome, I think.
    It’s easy just to post a link, so I worked through it. No major errors that I can see.
    2.5kW solar per house. Most installations are 3kW or less. To get household consumption (demand), 80 Melbourne houses with solar were monitored by smart-meters at 1/2 hourly intervals, and recorded. Cost of solar, and remaining grid costs are not evaluated. Just the cost and ‘bill reduction’ of an added battery and inverter. Results are individually calculated for each house, and presented as a distribution.

    Inverter+installation (flat cost,size independent) = $1500.
    Battery 5kwh @ $310/kwh = $1550. Total $3050

    5kWh, 80% DoD = initial usable capacity of 4kWh.
    -20% capacity loss over every 1500 cycles @ 100% DoD. Partial cycling loss proportional to DoD. Remaining usable capacity at 20 years, 1.1kWh

    Tariffs, 2014;
    Peak $0.36/kWh, Off Peak $0.22/kWh, FiT $0.08/kWh, ($0.062 after 2015)

    Solar generation;
    Degradation is 0.5% per annum. Solar data is derived from one installation. Not the best, but a reasonable assumption considering all houses are on the same estate. Projections are made for some missing months of solar data.
    Also not good, but it’s hard to see how anything could radically change the conclusions.

    Battery charge/discharge;
    Charge/Discharge power is 3.45kW. That’s a high power to capacity ratio (5kWh/3.45kW), generally not available from home storage batteries.
    Fig 6 is the ‘optimal’ profile, derived from the consumption and solar data. Charging occurs between 9am and 5pm. Discharge would be mostly after 5pm, but also a little in the morning, between 6am and 8am, which would allow better use of the battery than may be possible in practice, or considerations of battery warranty. Not likley to be significant.

    The authors use an algorithm that includes solar input, household demand, FiT, two tariffs, and capacity degradation, to calculate bill savings for one year (2012-2013) of smart-meter readings, projected to 20 years. Forecast tariff increases (but not decreases), are provided, but the model otherwise assumes the given 2014 tariffs

    The process is generalised in the text and formulae. It amounts to the same as often done by hand.
    n(t) is the resultant grid energy, which may be consumption or feed-in output, according to the sum of demand d(t), generation g(t) and +/-b(t) battery charge/discharge. Performed for each 1/2 hourly interval, rather than ‘daily averages’ as is done by hand.

    The value of N(t) is calculated according the the relevant tariff, either peak, off peak or FiT, for each 1/2 hourly interval, accumulated over time T, displayed as month, year or the full 20 years.

    The rest of the process is a fitting method, where the programme reads ahead ( the recorded demand data) so that the best strategy for charge/discharge periods can be found (fig 6). As noted, the result is optimal for the given data, so the offered savings are the best possible, but likely less in practice.
    ‘Smart’ controllers may help approach that ideal. Less of an issue where there are flat tariffs. Then, the optimal concerns the best strategy to sell via FiT.

    Degradation of the initial 4kWh (fig 9) shows 3.5kWh at 2 years (87%) and 2.25kWh at 10 years (56%). Not far off Tesla’s or other similar batteries.
    A simple model that reflects limited available battery data, where temperature and load effects are not included within the specification or warranty.

    For the purposes of comparison, calculate the first year as is often done. Capacity, 4kWh. Assuming the battery displaces peak tariff, daily savings
    would be 4kWh*$0.36*365 =$527. Text and fig 8 say $307. Since the algorithm optimises return, and capacity is retained, load/demand mismatch is the remaining reason for the difference. Charge/discharge is within the peak-period, so ruling out tariff differences.

    Simply the accumulated ‘bill savings’ until cost is regained. The payback chart (fig 13) applies NOT to the calculated trial, but when tariffs are increased
    to the highest level. Even then, most are >7years, and up to 19 years for houses on the same estate. Fig 11 and 12, where battery prices fall to $150/kWh, show little improvement, and larger batteries don’t offer much better. As Energia’s analysis shows, the most frequent savings are small, with the smaller battery more often again.

    Fig 8 shows accumulated savings for the trial. The $3050 cost would be recovered around year 11 or so, but battery aging reduces income with time.
    The text gives 20 years savings as $4323. Earnings during the period after payback are approx $1200. That’s the optimal result, it seems, but also
    ignores many parasitic losses and other battery limitations.

    That’s about it. With current battery prices, it would be a washout in this case. The low projected battery costs are very unlikely, and capacity degradation is still a problem. When technically better batteries arrive, they will be valuable, and not sold off cheaply. A long wait, I suspect.

  13. Perhaps for comparison purposes. I am not sure if these batteries are available in Australia.
    Sony have a 4.8kWh Fortelion Energy Storage System.
    Each of the 1.2kWh modules contains 128 cells. Sony’s cell datasheet supports 80% capacity at 5000 cycles (1.2kW output power)
    Capacity will be reduced to 1.1kWh at that output. Nevertheless, a good cell, with exceptional DoD capability. Good output power.

    The modules can be assembled into racks, along with a controller.
    Inverter is additional, but suits SMA Sunnyboy etc. A European distributor has a 4.8kWh unit, for example.
    4 modules + controller + rack
    6,451 EUR = AUD 9760 ( directly converted from exchange rate, tax excluded)

    Initial capacity retention is good, and nearly linear, resulting in a mid-point average capacity of 90% over 5000 cycles. Allowing for the reduced capacity at the full load of 4.8kW, aggregate storage over 5000 cycles is 4kWh x 5000 = 20MWh.
    At the converted price of AUD 9760, storage cost is $0.49/kWh.
    5000 cycles is 13.7 years. Warranty data is not specific.

    Sonnen use the same cell, but claim 10,000 cycles, which is beyond the claims made by Sony, either for the battery or the cell datasheet, if fully cycled at the same output power. Depends on how Sonnen specify their claim.

  14. Hi Finn,
    Be good to update this page with Tesla Powerwall 2 – it is significantly better than the original Tesla Powerwall , and compare it to the current alternatives. How does it compare these days?



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