The cheap & cheerful way to add a Tesla Powerwall to your exisiting solar system?

a powerwall on a wall

What might be the cheapest way to install a Tesla Powerwall + solar?

[Note: This blog post is an educated guess on how the Tesla Powerwall might work based on stuff Elon Musk has publicly said and the press section of their website. So bear that in mind if making any decisions based on this post. Tesla have released sketchy and often contradictory technical details about the Powerwall, so it is impossible to be sure how the thing will work or integrate into an existing system. I’ve tried asking Tesla US and Tesla Australia for clarification, but with no luck.]

Since Tesla announced their low cost home battery, many solar installers have been looking for the cheapest way to integrate one into your home’s electrical system.

This has led to the emergence of a whole new category of residential solar system, that had never really been considered here in Australia before the Tesla announcement – the not-very-catchy “Hybrid Solar Without Backup” system.

This is a system which, to save thousands of dollars on the install, removes the ability for the batteries to power your home if the grid goes down. The batteries are only used to store solar energy. They cannot be used in an outage.

Why remove the backup function?

Because (at the moment) it is very expensive to implement this functionality. You need a special battery inverter that can work in ‘island mode’. Such inverters start at about $3,000. On top of that, you may need to modify the house wiring so that high powered appliances like ovens and air conditioners can’t be used in ‘island mode’ (as they could either overload the battery or empty it very quickly).  Things can start to get very expensive.

So if you are happy to forgo protection from grid-outages, just how cheaply could you add a battery such as the Powerwall to your existing solar system?

Adding a Tesla Powerwall: The cheap & cheerful way

Requirements: Your inverter is compatible with the Tesla Battery Management System (BMS) and has a spare DC input.

If you have a spare input on your inverter then Tesla have said that you should be able to plug the Powerwall’s output directly into your inverter. No extra inverter needed.

Update: there are mutterings that the Tesla DC-DC converter is so smart that it does not need a spare input and it can go between the panels and the inverter. I don’t understand how this would work without messing up the MPPT optimisation on the panels. I asked Elon Musk to clarify how this would work via Twitter, but he did not reply 🙁

Further, Tesla have said that you will be able to charge the Powerwall directly from your solar panels (i.e. there will be a built in DC charger). To share the DC output of the panels like this would require the Powerwall’s Battery Management System (BMS) to talk to the existing inverter.

If you have a big brand inverter (SMA, ABB, Fronius etc.) then the chances are that the Tesla will be able to talk to it (this is an assumption by me as Tesla have released very few technical details).

I’ve seen comments online saying that the Powerwall will work with any existing inverter. Not so. Tesla have explicitly said:

“Powerwall consists of Tesla’s lithium-ion battery pack, liquid thermal control system and software that receives dispatch commands from a solar inverter. ”

and

“Inverter: Pairs with growing list of inverters”

So you will need a compatible inverter.

The Powerwall will also need to know if there is surplus solar available to charge the battery. You don’t want to charge the battery with solar energy that is needed by the house. The inverter doesn’t have this information. It only knows about the output of your panels, not the power needs of your house.

Tesla haven’t talked about this aspect of the Powerwall control system. But I can’t for the life of me see how the Powerwall will know when to charge or how much to discharge, unless it knows how much energy your home is using. I’m happy to be wrong on this – please leave a comment if you think I’ve missed something bleeding obvious here!

So I think the Powerwall will also have to talk to your import/export meter. Unfortunately this is nigh on impossible with the majority of dumb Australian meters. But the installer can add a cheap, $20 current-transformer (CT) on the output of your meter which will provide the required power measurement (note to Elon Musk: please add a port in the Powerwall for a CT – it will only cost a few cents!).

The CT will tell the Powerwall whether the home is importing or exporting energy, and whether it should either, charge, discharge or do nothing.

The nature of the conversation will be this:

Powerwall (californian accent): Hi Inverter buddy! What’s those solar panels outputting dude?

 

Inverter (German accent): Ve have 2.65 kW!

 

Powerwall: And Mr CT, what’s the house drawing from the grid?

 

CT (Chinese accent): The house is exporting 1.2kW to the glid.

 

Powerwall: Awesome dude, I’m not fully charged so I’ll start charging with that 1.2kW instead so we are exporting zero to that lame assed grid!

Hopefully you get the picture.

Basically the battery will charge if it is not full and there is spare solar. It will discharge if the house would otherwise be importing expensive electricity from the grid.

[Note: That is a huge simplification! The Tesla software will make much more sophisticated decisions. Further, if it has got Reposit Power’s grid trading software on it then it will crunch a heap more inputs such as weather forecasts, wholesale market pricing and what time you usually use your appliances to decide whether to charge, discharge or do nothing. But this post is about hardware so forgive the simplification.]

With this design (assuming a Tesla compatible inverter and a spare inverter input) the only extra hardware required is a $20 Current Transformer (assuming it can plug straight into the Powerwall) and some wire.

If the power wall is mounted next to the meter, the cable runs will be very short, and the most difficult part will be mounting the 100kg on the wall.

So you could be looking at installation for around $400.

Add the Powerwall, installer markup, shipping and GST, and you should be able to get 7kWh of daily cycled batteries added to your system for around $6,000 AUD.

[Update: 11 Jun 2015. Elon Musk has just said that he expects the 7kWh Powerwall can be installed at a final, total cost to US consumers of $4,000 USD. ]

Here’s what the ‘cheap and cheerful’ install might look like:

block diagram

How the Powerwall might be installed without backup

If you are looking to buy solar, and want your system to be ready to accept a Powerwall as easily and cheaply as possible then here are the things you need to do to ensure your system is “Powerwall Ready”.

About Finn Peacock

I'm a Chartered Electrical Engineer, Solar and Energy Efficiency nut, dad, and founder of SolarQuotes.com.au. My last "real job" was working for the CSIRO in their renewable energy division.

Comments

  1. You are assuming single phase power. In case of three phase some more complications will arrive.

    It will be better if you connect inverter output to POWERWALL with a cut off switch sensing direction of powerflow.

  2. Ted Green says:

    Hi Finn, assuming you’re correct on all that, we’d need to stump up $6K to add this to our existing system? Mmm…I think the resulting savings in evening peak power would take a long while to add up to that. If, for example, we saved $150/quarter – $600/year (over and above what we’re currently saving with our panels), it would still be ten years before we’d covered the outlay, not to mention the opportunity cost vs. paying the same $6K off a mortgage, for example.

  3. David Brien says:

    And then there are the households with Micro-inverters. Left in the lurch with this methinks. Still waiting for news on the enphase battery solution. I have heard nothing as to how that is proposed to work

    • Hi David,

      I can tell you exactly how the Enphase solution will work.

      1) They come in 1.2kWh packages.

      2) The first one is installed by an electrician. Should take about an hour. It talks with your existing Enphase comms hub (if you don’t have Enphase micro inverters already you’ll need to install a comms hub too).

      3) Subsequent 1.2kWh batteries can be self installed because they plug into the first.

      4) They should be in Australia soon – as Enphase are launching them in Australia first – due to the low FiTs.

      This makes a lot of sense (if the price is right) – because the first kWh of storage will always give you the best payback compared to subsequent kWhs.

      So I think Enphase micro inverter owners are actually best placed to invest in storage gradually with the best payback – assuming the “AC battery” is not too expensive.

      Finn

  4. Dave Cadwell says:

    Finn,
    What brand of inverter works best with a powerwall? Thanks.

    • Hi Dave,

      So far Tesla have announced that they will work with Fronius’ Symo range of inverters and SolarEdge inverters.

      But I’ll eat my hat if it is not compatible with ABB and SMA inverters at the very least by the time it is release as these have a large market share in the USA.

      BUt Fronius Symo is the safest bet right now.

      Finn

  5. Michael Cashmore says:

    Hi Finn,One question only,l have 30 Ulica Solar panels connected with Enphase Micro Inverters.Will the power wall battery backup system work with Micro Inverters? Regards Mike.

    • Finn Admin says:

      Hi Mike,

      Yes you can use the Powerwall with microinverters using a technique called AC Coupling. But you will have to buy a separate inverter to achieve this which is compatible with the Powerwall.

      But if you have Enphase micro inverters your best bet may be the Enphase AC battery which can be installed very simply and is controlled via your existing Enphase comms hub. It should be in Australia in 2016 – although the price has not been announced.

  6. Kaz Nagy says:

    Hi Finn,

    Thank you for making some sense of the technology to an enthusiastic dinosaur like me.

    We recently installed a 21 panel ( 5.25kW ) rooftop system.

    The Inverter is a JFY SUNTWINS 5000TL.

    As Murphy’s Law dictates, The Tesla Powerwall was unveiled a matter of mere months after we went on line, and the F.I.T. in regional QLD is a measly 6.53 cents/kW, and then from this July it seems they will not accept more than 5kW/day to feed into the grid from any single installation.
    (I may have this part wrong, but on the surface, it seems unfair, as any EXCESS excess we might produce would simply be wasted.
    Hence my immediate interest in the Powerwall, and indirectly in the Hybrid concept.

    Could you possibly advise whether or not my Inverter is compatible to be adapted for overflow battery storage, to Powerwall or any other alternative? As a pensioner, I have limited funds, but would rather spend on clever longer term infrastructure than be a slave to exorbitant gouging on my power usage.

    Watching for your wisdom.
    Cheers, Kaz.

    • Hi Kaz,

      Short answer is I don’t know.

      But although there are a lot of JFY’s in Australia – they are not particularly common worldwide, so my best guess would be that they are unlikely to be on the priority list for Tesla to integrate with. So it is unlikely to be compatible.

      You may need to change your inverter, or get a 2nd inverter for the battery.

      Finn

  7. Chris Smith says:

    Hi,

    So in theory once the sun goes down the battery would kick in and keep powering your house until it runs out of juice? Then you would change back over to the grid? Is that correct?

    Regards

  8. Hi Finn, then what about us poor old early adopters of going solar…yes us off-gridders! Where do we stand in the powerwall equation?

  9. I live completely off grid already and am interested in battery technology as it is the only means I have to provide 24/7 240V to the house. These batteries can be coupled to increase the storage capacity and but their stated output capacity per battery is quite small. How many cycles will they tolerate in their 10 year warranty though? My current inverter is a Selectronic and it can grid connect with a battery backup but it is not Lithium-ion compatible. Software upgrade would fix that. It reads the amount of power the house is using and the amount of power coming in from the sun then takes from the batteries as needed. We can use most appliances during the day when it is sunny but as mentioned you have to be careful at night or on cloudy days. To say this system would end your power bills may be stretching it unless significant lifestyle changes were made. You have to know exactly how much power your appliances use and be prepared to live without some of them after dark (when peak usage occurs). Much will depend on the output capacity of the batteries but at 2Kw you could run a microwave but not a hairdryer, dishwasher or electric oven. Most people will still want their conveniences so the system will have to be able to draw from the grid when demand outstrips supply and you will have to pay for that unless your credits cover it. We use an auto start generator for the rare times we have high night time power drain which our inverter is programed to start. We also need a charge controller which directs the DC from the solar panels to the batteries as required through the charging cycles. Some inverters have a charge controller included but I am not sure if this system requires one. Most importantly is the cost versus return but another consideration is if you live in an area prone to power outage for lengthy periods and you need to keep the power running. As with all new technology the cost will come down in time and systems will improve their capacity so it may pay to wait a few more years until things become more competitive.

  10. Hi Finn, great site! I am just starting out with solar and based on my bills and your very useful guides am looking at at 2-3 kW system. What is the best way to future proof for battery backup – Tesla or competitors? The Fronius Symo seems to be the best bet as you say above, but seems much more expensive compared to others. We are about to I’ve from the ACT TO Sth NSW so about to pay double per kWh! Appreciate your thoughts.

    • Here’s what you need to consider if you want your system to be “battery ready”:

      https://www.solarquotes.com.au/fb-hybrid.html

      Any big brand inverter should be compatible with the Powerwall by the time it comes out, I’m thinking SMA, ABB, Fronius, SolarEdge are all safe bets.

    • Hi there. If you compare Symo to any inverter of comparable quality and then add the cost of monitoring and 10 years warranty to the other inverter, I think you’ll find it should be cheaper. I’m moving my premium offer to Fronius ‘cos I reckon on a value for money basis, it is awesome.

  11. Hi Finn,

    What about Growatt? We have a 3KW system purchased from and installed by True Value Solar in April 2014. Is Growatt likely to be compatible, do you think? Obviously, I understand it’s only your best guess.

    Thanks,

    Ted Green.

    • Growatts are relatively popular in Australia,, less so overseas. My guess is that they will become Powerwall compatible eventually, but will not be a priority for Tesla.

  12. So with the quick and dirty implementation solution of using a spare DC input on the inverter, how do we stop the grid tie inverter from exporting the entire contents of your battery bank to the grid? At 6 cents in Vic.

    At a discharge rate of 5kW, a 5kW inverter would be able to manage that in 7/5 = 1.4 hours…..

    • Tony,

      Have a look at the block diagram (particularly the grren line to the CT) . Then have a read of the entire article again 🙂

      “The CT will tell the Powerwall whether the home is importing or exporting energy, and whether it should either, charge, discharge or do nothing.”

      and that is the reason the inverter needs to be “Powerwall compatible”. i.e. it needs to be able to send dispatch and charge commands to the Tesla BMS.

      Finn

  13. Hi Finn

    Our solar system is facing North and West hence using both DC inputs of our ABB 5kW Inverter.

    Do you think we will still be able to connect the PowerWall to this setup, or will we be forced to upgrade the inverter or have a second inverter?

    Thanks,
    Diego

    • Hi Diego,

      As more details leak out – the latest word is that the DC-DC converter software may be able to allow the battery to share inverter inputs with panels arrays.

      I’ll post more as I learn more (and I’ve contacted Tesla to ask for clarity)

      Finn

  14. karene thomson says:

    Hi Finn

    Is Tesla system actually available in Australia ? The last aI heard they were struggling to supply the orders they had taken already in the States and there was a 10 month waiting list. Apparently their shares are going through the roof!

    • Telsa have promised Australian availability in early 2016. My guess is that they will five some priority to Australia for the 7kWh daily cycling mode, because Australia’s low feed in tariffs make for a much better payback that in the US where most folks have a 1-1 FiT.

  15. john neeting says:

    I have a Fronius IG60HG [5.5KW] with spare DC input terminals and plan to invrease the solar panels from 22 to 44 until I discovered Tesla. I am assuming my inverter is perfect for the power wall so – which would give me the best advantage. More panels plus a power wall or just stick in a power wall ?. we have a large property with a 2 pump pool plus the usual stuff [ dishwasher – front loader, clothes washer – frontloader, 2 fridges and 4 freezers, 4 window aircons and 2 split system air cons. Our bill used to be nearly $900 a quarter, it was only cut in half with the 5.5KW system.

    • Hi John,

      To answer your question properly you would need to log and then analyse your hourly energy usage. That would show you how much energy you used when the sun is down, and how much a battery would save you compared to more panels.

      Having said that, you almost certainly use more than 7kWh at night, so adding a single powerwall is likely to chop around $1.50-$2 per day from your bill (depending on what you currently pay for electricity compared to your Feed in Tariff). I’d be tempted to start off by adding a single powerwall.

  16. Can’t see the trees for the forests??
    From the “block diagram (particularly the grren line to the CT)” all I see is a lot of complicated (therefore more likely to fail) and hugely expensive technology.

    eg Instead of the panels directly feeding the “charger input”, why wouldn’t you you get the desired effect of feeding your battery-bank by simply pugging it into a household power-point via a simple ( and relatively cheap) three or four stage battery-charger (about $100 at Repco or Supercheap).

    Then connect a cable from the battery via a suitable but uncomplicated inverter ($500 tops)~ AND ANOTHER FROM THE GRID INPUT after the meter ~ via a simple and conveniently-positioned heavy-duty 2-way (manual) rocker switch which lets YOU choose whether to use power from the grid OR the battery bank.
    The outlet from the switch connects directly to the main circuit-feed (fuse-board? whatever) of the house circuits. (Wish I could draw diagrams!)

    Apart from huge potential cost-savings it’s a system that’s so simple you’re in control of it and can trouble-shoot any problem that may arise (but probably won’t, since any decent inverter ~ or several of them feeding different circuits ~ will protect the system by shutting down; at most you’re looking at an in-line fuse replacement.)

    Needless to say such a system can be modified in any number of ways; eg adding a demand-start generator input to avoid using a battery to run microwaves/washing-machines, timing switches/sensors ($10 each from Bunnings) to connect the grid automatically if the sun goes out, etc. etc.
    (I reckon there’s huge scope for an ambitious young engineer to make a motza!)

    But the wisdom of the years urges me to remind you all about Murphy’s Law.
    ……..and, personally, I reckon Murphy was an optimist!

    • Chris Thaler says:

      Jason, the battery pack usually runs at a similar D.C. voltage level as your solar panels, e.g. 270-450 VDC. The battery chargers you mention are generally 12 – 24 VDC only therefore of no value.

      Tesla batteries (and similar) will be using Lithium technology and require very specific charging regimes to prevent individual cell damage. I expect the charging equipment will be built into the package to meet these specific requirements.

  17. Scott Elford says:

    Hi Finn,

    Great website, thanks. I live in WA, I have 5KW system on my current house, I have no plans to add batteries to this system, but I have a second off grid system that I’m buildiing up that has started life and I’m planning to upgrade the whole lot as I can.

    House system is a 5kw growatt which is only 18 mths old and I have already had the inverter replace due to what I will call a bad setup. It was programmed to have input voltage of 230 V, the replacement unit has had some software update & the voltage has been shanged to 240V. My elec bill has the total power I use & then a total power that I have produced, so it makes no difference when & what power I use the solar produced gets taken off the total power used. I’m only getting 8 cent per KW and I pay 24 cent per KW. But thats ok as i cover 80% of my power bill.

    The more interesting system is my off grid unit which is powering my computers and HA system and later on my marine fish tank. Current setup is 12 x 2V 300Amp hr batteries connected up as a 12 volt system, being chargered by a battery charger currently, solar panels are comming.. 1.5 kw inverter. When I get the whole lot running I’m looking at areound 2 kw/hr.
    Will be changing over to a 24 V system as cash allows.. LOL. I like the idea of the powerwall but I will require atleast a couple of them. I’m still looking for more info on the powerwall as it’s a little on the lacking info side at the moment. I’m looking at least a 5 kw solar array to feed this system.

    Any comments, I’m open to ideas.

    PS. I’m an electrician.

  18. john nielsen says:

    Hi Finn,
    I have a 6 kw system, 5 kw with micro inverters, 1 kw with 2 x 48 volt to my battery bank. I cannot see how you can use the PowerWall with micro inverters!
    I read the spec on PW and the input voltage is in the order of 300 vdc and my micros output 230 vac. I think these new battery gadgets have a long way to go, and I chose to stay away from them,,, I didn’t have a choice, they are not available…yet, and when? So for $5,700 I bought 12 off 12 volt 250 ah batteries which runs between 13 – 11.8 volt the whole house, day and night through my $440 8000 watt inverter,,, all stand alone. When summer comes, I will need the help of the grid for my 4 aircons but they are on a totally different supply line.
    So PW has an input of say 300 vdc and only 2 kw 230 vac output,, what a useless idea in my opinion, and that other brand of new gadget seems no better. I am glad that I stuck with the old gel bats Perhaps Enphase will change my mind,,, but when and how much?
    Great forum you have here Finn.
    Thanks
    john

  19. Correct me if I am wrong, but a CT clamp setup can measure amps , but they can’t tell if it is being imported or exported with you be the required for BMS to do the right thing?? So I would assume we are going to need something better than a CT clamp??

    Am I missing something?

  20. david leitch says:

    This is a great article thanks Finn.

    Among the host of issues three burning ones from me: (i) Why are “Island” mode inverters so much more expensive?
    (ii) I now believe the vast majority of Australian rooftop PV installations don’t have “island mode” inverters so that the situation with a battery is no different to the current situation, no grid, no power. Is that correct? (iii) What do we know about the new meters being installed by retailers, eg AGL in SA, NSW and QLD, I am assuming they have quite a lot of communication software, but are they being designed to commuicate on the “residential bus” = PV + battery + intelligent devices or just communicate back to dad at grid central?

    There is so much to learn about this topic

    • i) 2 reasons – a) higher margins because they have always been a niche product b) they have more power and control electronics in them to keep everything safe and reliable. But my guess is that this would only warrant a couple hundred dollars premium over standard inverters if they were mass produced.

      ii) yep – no grid, no power for 99.9% of Aussie installs

      iii) The smart meters are pretty dumb. I don’t know of any that can talk to a battery BMS. They simply send the metering usage info over a network (usually GSM) back to base, and have the ability to do ‘Time of Use’ tariffs.

  21. Hi Finn,

    I can’t see why a CT couldn’t be used in the inverter output to as well as the mains in. This way the Powerwall would not need to communicate with the inverter at all and can simply calculate and adjust it’s charge rate without requiring inverter upgrades

    • I think that is possible – but it depends on how the powerwall is designed, it may rely on the inverter’s power electronics to throttle its output. If the battery can throttle its own output then it may not need to talk to the inverter at all.

  22. john nielsen says:

    Hi Finn,
    I would advise anyone contemplating waiting for the pie in the sky, Powerwall or Enphase AC Battery to forget about it, but instead: purchase 4 off lead-gel 12 volt 250 ah batteries and thus have 12 kwh of power storage for the next 10 years if you don’t dod the bats to more than 70 percent. The cost of the batteries would be around $2,000
    If your system is a new installation I would recommend to run 6 mm2 tps cables from the paired panels to a solar charge controller. 4 panels (1 kw) two strings of 48 volt (one string for every two panels). You can double up as many times as you like but will need a controller for every 1 kw of panels. The cost of each controller about $300
    What else you need is an inverter, I purchased two 8 kw inverters on the net, each for $438. I use only one, the other is a spare. Then you will need a relay driver like the Morningstar relay driver, a few micro relays, and a couple of contactors and a bit of wire like 6 mm2 tps by 100 m cost $300 from Masters. Now the highest dc voltage you are using is nominal 48 volt so quite safe. Your output from the inverter will of course be 230 nominal ac for your loads.
    So for a 5 kw non exporting system the cost would be $2,000 + 5 * 300 + 300 + driver and relays say $400 = $4,200 plus panels and mounting. My utility grid company (government) wouldn’t allow me to have a 5 kw system, but wrote to me that I could have as much pv power as I like as long as I didn’t export it to the grid. Fine, so I installed my 6 kw system. The one kw system is running to the suggestion above, but the 5 kw are with micro inverters. The whole lot could have been as described above.
    My system is isolated from the grid and you can do that with gear mentioned above.
    I didn’t throw away my off peak electricity water heater when I installed a gas water heater, and have now reinstalled my electricity heater where I dump the excess power I have. All is fully automatic controlled by the Morningstar relay driver via the battery voltage. I dare say that you could make any existing grid – string inverter system stand-alone in a similar way, also as I have done with a micro inverter system. Basically when the battery voltage drops low to whatever threshold you have chosen, the system automatically switches over to the grid until the voltage again rises above the threshold you have set. You can also have a manual change over switch like the one used for plug in generators, just to please the grid company, then use you own automatic relay switching system. But make sure that there is no way you can send power to the grid under any circumstances. There are a number of zero export devices on the market, but they are expensive and perhaps not very reliable. Many people where I live have a generator which they plug into a socket in their meter box and then turn a witch which will isolate the generator from the grid in order not to send power down the line when linesmen are working on the power lines. Your system must be similar with a change over switch installed and approved by the grid.
    Why wait for the Powerwall or AC Battery, when you can have your system up and running now for perhaps less than the promised pie in the sky.
    John Nielsen, Silkwood.

  23. john nielsen says:

    Hi, I am not always right, but I have my batteries in series, 3 banks of 4 x 12 volt, 250 ah gel bats. If I didn’t explain myself correctly, then I shall try to here. The strings from the panels each carry nom 48 volt for input to my charger-controller which feed the bats. The suggested 4 off 12 volt batteries are in series, thus 48 volt nominal. The bats again feeds the 48 volt input on my 8 kw inverter,,, which feeds the load and my micros which again feeds the load, all connected together on the 240 vac lines. So are 4 off 12 volt 250 ah bat not equal to 12 kwh ? My front shed has a 3 phase motor for my lift, thus I use the grid for this, but my house isn’t connected to the grid and hasn’t been for a long time. The micros on the 5 kw supply the power during the day, and the bats all night long, all lights, fans, micro oven, but no washing machine, dryer, electric oven, or air con at night. I have a gas stove. I don’t know how long my bats will last, but the voltage has never been below 12.20 volt and never been above 13 volt as I have threshold settings on the Morninstar relay driver.
    As your power bill increases and the grid continue to limit your installation and export to the grid as well as increasing the kw/h rate there will be an increasing number of people who want to get off the grid,,,, all I am saying is: That I will bet London to a brick that you will wait a long time for that Powerwall or AC battery pie in the sky.
    John Nielsen, Silkwood

  24. Hi there, does it make any sense at all if you have a small 1.5kw system and you are getting paid .50c on the feed in tariff to utilise a powerwall? Are you better off exporting as much as you can rather than charging the powerwall. I run my pool and charge my car on the cheaper off peak tariff (18 hours supply per day). Could you charge the powerwall using your off peak supply to maximise your feed in and utilise the battery at night?

  25. john nielsen says:

    Hi Finn,
    I didn’t see any boxes showing how you could use the Powerwall with a micro inverter system such as Emphase. I know you answered readers that it was possible, but you didn’t explain how. I am interested in trying this pie in the sky if ever they will be available and here is my proposed installation:

    I have just read the spec on Tesla’s Powerwall battery. The voltage on the battery is listed as 350 to 450 volt dc. I have a 6 kW micro inverter system and 36 kWh battery capacity which runs my 5 bedroom home, day and night not connected to the grid. I have 4 rooms with air con which I run from the grid as I can only run 2 air con off grid during the day with present capacity. I want to increase my installation with another 4 kW and I am interested in the Powerwall but the way I read it, I would have to have at least 3 off them to run my 4 air cons. I think the Powerwall is only compatible with proprietary system owned by or associated with Tesla.
    The way I might use the Powerwall: have 4 off 12 volt bats, 48 volt input to an ups inverter/charger to supply power to the micros and the load via a 40A ssr (ssr1) from a second string off the micros to a 100A 1000V rectifier and further to a step up transformer 240VDC to 350VDC for input to the Powerwall, then use a standard string inverter with a string input of say 350VDC to the load via a 40A ssr (ssr2), I would need a CT to monitor the Amps in order to govern which ssr is ON. All of this if the Powerwall is just a battery, power in, power out. Quite frankly, I think the Powerwall is just a pie in the sky, but I might try it on my proposed 4 kW micro inverter extension.
    John Nielsen, Silkwood.

  26. Ray Cooper says:

    Finn, the Fronius Symo which is compatible with Tesla is 3 phase only for the foreseeable future. Why would you want to pay the premium price for the only single phase alternative which is the Enphase maximiser solution if you have the perfect roof and premium panels? Take a look at high voltage DC bus technology.
    Ray Cooper B.Eng Elec (Hons)

  27. I have 10 batteries and I want to connect them to a home solar system, each battery is 12V 100A.
    How do I connect all 10 batteries that I’m just getting a 24V 500A?

    • Ronald Brakels says:

      Sorry to come across all heavy, but if you need to ask this question you shouldn’t be doing this yourself as you will be dealing with currents that can kill you and you need to get a feel for how things work at a non-lethal level before attempting this. You shouldn’t be trying anything that may kill you without proper training and qualifications.

      But assuming you are not the one doing the work and you just want to know how it is done, then connecting batteries in series will add the batteries’ voltage while leaving the amps the same and connecting in parallel will add the batteries’ amps while leaving the voltage the same. So the batteries would need to be put in pairs and connected in series to get 24 volts and then the 5 pairs would be connected in parallel to get 500 amps. (Or vice versa.)

      There are tutorials on the internet explaining how this works. If you search for, “connecting batteries in series” and “connecting batteries in parallel” you should be able to find some.

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