The Truth About Battery Ready Solar Systems

Do you want your new solar system to be battery ready? First you have to know what that means...

Do you want your new solar system to be ‘battery ready’? First you have to know what that actually means…

Most homeowners I speak to are really keen to get batteries.

Just not right now.

They want to wait a few years until the price has come down substantially. And who can blame them when a typical battery system in 2016 has a payback of 20 years but only carries a 10 year warranty.

So clued up homeowners looking to buy solar are really keen to get a system that they can easily add batteries to in the future.

Solar companies have caught on to this and we are starting to see solar systems sold without batteries but advertised as ‘battery ready’.

Here at SolarQuotes we are getting more and more questions about what ‘battery ready’ actually means in these adverts, and the best way to buy a solar system that really is battery ready.

The truth is that ‘battery ready’ can mean lots of things. Many of the ads I have seen are technically correct – in that you can add batteries in the future – but probably not as easily, or with the functionality that many people will be expecting.

In my opinion if you are going to claim a specific solar system as battery ready then you must be more specific and explain the basis on which you are making the claim.

In this post I’ll go through 3 different interpretations of ‘battery ready’  – make sure you understand which one you are being offered.

Battery Ready #1: Ready for ‘Hybrid Without Backup’  by using AC coupling, a battery inverter and batteries.

First let me explain the technical terms in that heading because it was full of jargon (sorry!).

‘Hybrid’ – means a solar system that is both on the grid and has batteries. It is hybrid because it’s kinda half on-grid / half off-grid.

‘Hybrid Without Backup’ – means if the grid goes down, your entire hybrid system will switch off too, despite the batteries. You have no backup. If that sounds ridiculous, please read this.

‘AC Coupling’ – has nothing to do with amorous Air conditioners. It means that the wires connecting the battery part of the system to the solar part of the system are 240V AC (the alternative is DC coupling which means – well – you can work that out for yourself I reckon).

A ‘battery inverter’ is an extra inverter designed only for batteries. It must:

a) convert the batteries’ DC to a grid compatible 240V AC

b) convert the solar inverter’s 240 AC output to DC to charge the battery

c) control the charging so that the battery is not damaged

d) only charge the battery with solar electricity (or possibly off-peak grid electricity)

e) only discharge the battery when the household needs it, don’t discharge it back into the grid – as that would be pointless (unless you have Reposit – but we’ll get to that later).

As you can see the battery inverter must be quite smart. It can only do (d) and (e) if it knows what the house power consumption is and what the solar power output is at any given time.  So the battery inverter will need to talk to a power-monitor (or current transformers which are really cheap meters) that attach to the wires coming both out of the solar inverter and into the house

So to add batteries to any solar system in this fashion – you simply take the battery inverter, and connect it into the home 240V circuit. Connect the power-monitors onto the solar output and household input wires. Then you plug your battery into the battery inverter and Bob’s your mother’s brother: you have a Hybrid System without backup. Twenty four hour solar power – as long as the grid is working.

It looks like this (metering not shown for clarity):

a block diagram showing battery inverter and solar inverter ac coupled

Almost every grid connect solar system ever installed in Australia is ‘battery ready’ if you want to upgrade like this.

The critical thing to note here is that compatibility between the solar inverter and the battery inverter is not necessary. That is – they do not need to talk to each other, because the output of the solar panels does not need throttling.  Because this is a grid connected system the solar panels can just run at full whack all day with excess energy simply flowing into the grid.

This means that your existing system can have the most obscure solar inverter on the planet and you could still call it ‘battery ready’. It also means that if you have decided that you don’t want backup functionality and don’t mind shelling out for a battery inverter with your batteries, you can buy any solar system – safe in the knowledge that this type of upgrade is possible.

Battery inverters that will allow this kind of upgrade include the super exciting SMA Sunny Boy Storage, and the micro inverters in the Enphase AC Batteries.


SMA sunny boy storage and Enphase AC battery

The SMA Sunny Boy Storage and Enphase AC Batteries can be added to any solar system – as long as you don’t need backup.

But what if you want backup?

Battery Ready #2: Ready for ‘Hybrid With Backup’  using AC coupling, a compatible battery inverter and batteries.

If you think you might want backup when you get round to adding batteries, then the system you will need looks very similar to system #1 but it is different in 2 important ways. Let’s play ‘spot-the difference’.

ac coupling block diagram with communicating inverters

In Battery Ready scenario #2 you can operate off grid. But this means your battery inverter and solar inverter need to talk to each other.

Did you spot them?

The obvious difference is the off grid switch. This allows you to keep the power on when the grid goes down and not put energy into the grid. Because this could kill the linesmen trying to fix it.

The other, more subtle difference is that the battery inverter needs to talk to the solar inverter. The battery inverter needs a way ask the solar inverter to throttle its output. It needs to do this because when it is operating without the grid, and the battery is full, excess solar power has nowhere to go, except up in smoke in a toxic pile of melted wires.

This has big implications when you are buying your ‘battery ready’ solar system. You need to make sure that your solar inverter can talk to battery inverters.  There a bazillion ways that electronic devices can talk to each other, but the standard that most reputable inverter manufacturers seem to have adopted is called MODBUS. If your inverter can talk MODBUS, then you should be good.

You should also be aware that battery inverters that can go off grid are much more expensive than ones that shut down with the grid. They need to disconnect from the grid and control electricity supply and demand. They also usually need some rewiring in your home to put your essential loads on a separate circuit to the non-essential stuff.

Good off-grid capable battery inverters that talk MODBUS include the all-aussie Selectronic SP PRO and the old faithful SMA Sunny Island

SMA Sunny Island and Selectronic SP pro

SMA Sunny Island and Selectronic SP pro are battery inverters that talk MODBUS and can provide backup

Now I’ve must confess. I haven’t been 100% truthful. You can actually operate the system in the diagram above without a comms link between the inverters. Instead of nicely asking the solar inverter to throttle its output you can just go BANG! And cut the power to the solar inverter. Then you can switch the solar inverter back on when you are ready for more solar power.

You can disable the solar inverter off by cutting its power with a switch controlled by the battery inverter or you can get the battery inverter to shift the frequency of the AC so that the solar inverter craps itself on a low or high frequency trip. Back in my control engineering days we used to call this “Bang Bang” control. Because you bang it on and then bang it off again.

It is a horrible way to control anything – and it will seriously reduce the life of your solar inverter. It may also play havoc with many of your 240V appliances. It will actually invalidate many solar inverters’ warranties too. So I am not going to recommend this style of control. It is just shithouse engineering in my unprofessional opinion.


Battery Ready #3: For real. You just want to plug batteries in and it will work. No additional ‘battery inverter’ thank you very much. 

I suspect that this is what most people assume ‘battery ready’ means. They assume that when the time comes that batteries are affordable, they will be able to buy the battery box and get the sparky to quickly wire it into a couple of terminals on their solar system’s inverter. Voila – a fully functioning hybrid system.

If you want a system that is this level of ‘battery ready’ then you need to get a system with a hybrid solar inverter that is compatible with your favourite battery brand.The hybrid solar inverter converts both the solar panels’ and the batteries’ DC power to AC, and handles all the battery control and switching functions. As you might imagine – these hybrid inverters are much more advanced than a standard solar inverter, and so they are more expensive. Expect to pay $1,500 – $2,500 more for a hybrid inverter over a solar inverter.

clock diagram of hybrid inverter solar system

A true ‘battery ready’ system in many peoples’ expectations means that you simply add batteries when the time is right. That requires a hybrid inverter.

Hybrid inverters include the well received Sungrow SH5K and the Redback Smart Hybrid System which comes out of Brisbane and can do just about anything you can think of (including trading electricity with your neighbour – even though that is technically illegal under the Australian Electricity Rules).

The Sungrow SH5K and the Redback Smart Hybrid System

The Sungrow SH5K and the Redback are both hybrid inverters that you can simply plug batteries into.

If you do specify a hybrid inverter, watch out for these gotchas (make sure they are in the contract you sign so there are no surprises when the day comes to add batteries):

1. Specify if you need backup functionality or not. Not all hybrid inverters allow you to run when the grid is down.

2. Specific if you want ‘true UPS’. UPS means Uninterruptible Power Supply. Many hybrid inverters will power the 240V AC down for 10-30 seconds when they switch from Grid Connect to stand alone. If seamless switching is important to you – then ask for ‘True UPS’.

3. Make sure the consumption monitoring is connected at the initial install. In order to control the battery charging the hybrid inverter needs to know your house consumption. It does that with its own meter. Even before you have batteries, this provides really useful information for you so you can find opportunities to move your loads to daytime and make energy efficiency improvements that will jump out at you when you see exactly how much energy you use and when. You need to specify this as many installers will assume you don;t want monitoring until you have batteries.

4. Check the max amount of kWh that the hybrid inverter will handle, and which batteries it can talk to. If you have your heart set on a Powerwall in the future then make sure the hybrid inverter can talk to it (and then sit down and decide if you *really* want a Powerwall when there are better batteries out there!).

5. If you want  to participate in the wholesale market and potentially get a much better price for exported electricity at peak times, then find out if the hybrid inverter is compatible with Reposit Power. I would contact Reposit and ask. If the inverter is compatible then expect to pay an extra $800 for the Reposit controller and software when you buy the batteries.

6. Check if the inverter has energy management relays. These are switches that you can use to switch loads, like hot water heating, on when you have excess solar and off when the solar power drops. If deployed intelligently, this ‘thermal storage control’ can reduce the amount of energy exported to the grid. And it can mean that you need to buy fewer batteries down the line.

7. Check if you need to pay for an extra ‘battery inverter’ card when you get batteries. Some hybrid inverters don’t actually have a battery inverter in them – just space to slide a battery inverter module in – which you have to buy.

8. Be aware that choosing a hybrid inverter now, locks you into using a battery that is compatible with that inverter in the future. The battery’s Battery Management System (BMS) has to be able to talk to the hybrid inverter. So you are limiting your future choice of batteries.

So which type of ‘battery ready’ system should you get?

If I was buying a solar system today I’d be happy with option #1. I’m really not worried about grid backup. My local grid goes down for about 2 hours every 2 years. I can live with that. And I’m a picky bugger. I’d enjoy the freedom of choosing any solar inverter I liked.

If I wanted backup, I’d go for option #2. I’d buy a Fronius, SMA or ABB inverter, because I know they talk MODBUS (the language a good battery inverter talks).

What I would not do is Option #3. I think it is unwise to buy a hybrid inverter with the hope of buying batteries in a few years. I don’t think batteries will make financial sense for 3-5 years, and in that time the hybrid inverter you chose could be obsolete, or worse, the manufacturer could be out of business. In 3-5 years time battery inverters will be so cheap that it will almost certainly be cheaper to buy a battery inverter and batteries, and couple them into your non-hybrid inverter than buy an expensive hybrid inverter now.


About Finn Peacock

I'm a Chartered Electrical Engineer, Solar and Energy Efficiency nut, dad, and the founder and CEO of 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. john nielsen says

    Hi Finn,
    What a fantastic piece of information. You have done a lot of homework and I think your 3 to 5 year battery scenario is correct. What do people do in the meantime when kWh is increasing and grid connections are limited by the utility companies, say useless 3 kW.
    It is 20:33 right now as I am writing this in Silkwood and the grid, Ergon is off. I am not the only one here on the block with solar, but all the others are sitting in the dark as they don’t have batteries. I have a stand alone system, but can draw from the grid on separate circuits for my air cons. No air con tonight though as my 36 kWh bat bank don’t supply enough power for night time use.
    Over the past 12 months, I have had the stand alone system running fine. More than 12 different wiring diagrams and as many changes on my garage system circuitry board.
    Perhaps an easy and cheap way to go off grid with your intended PV system is as follows: may I say you can expand on this system as bats becomes cheaper and your need increases.
    This system will work for both string system as well as micro inverter systems.
    With string systems you may have two strings feeding a string inverter. With micro inverter systems you can have as many strings as you have micro inverters, but maybe realistically just two.
    So you will need 2 off 40 Amp SSR, 1 DP DT micro relay with a 12 Volt coil. A relay driver with perhaps 4 ports. I use the Morningstar relay driver. You will need a charger, 240 Vac input, 48 Vdc output, 120 Amp. You can use an 8 kW inverter/charger, but disconnect the inverter side. If you can find anything that can charge like that without the inverter, please let me know. You need a battery bank, technically you can use 4 ex rubbish dump car batteries but you would be sitting in the dark most of the night. You will need an inverter with no charging. You can get a cheap 3.5 kW one for say about $350.
    I haven’t summed it up but I guess the charger would be $600 and the inverter as said about $350 the relay driver about $250 and the other bits about $50
    The idea is to have as much PV power as you like without exporting to the grid.
    The micro switch is either on the grid or on stand alone. The two SSR are for the grid and the stand alone. The charger feeds the batteries, either from the grid or from the inverter, but NEVER from both. If you use a large inverter which also has a charge function, then use power diodes to eliminate feed back to the batteries. The inverter feeds the load and also feeds back to the charger. With the relay driver you have 4 ports: one will go to the micro relay and drive either of the SSR depending on the voltage you have programmed into the relay driver. The other ports can be used to switch off one of your strings when the voltage creeps too high. It can also be used to divert power to a water heater or an air con. I use it to switch on two air cons during the day when I have plenty of power. Large inverter/chargers in my opinion are not suitable as they fluctuate the load to the batteries and I don’t think this is healthy.
    I see all these systems with massive communications from inverter to load and I am bewildered when things can be so simple and cheap. It is 21:11 and I have to finish up, the others are still in the dark.
    Great work though Finn,
    John Nielsen, Silkwood.

  2. Finn
    Thanks for another great article. What about car (EV) batteries for battery storage for a residential PV system? I understand that Tesla and Solar City are working on or have created a system to allow your EV car battery to store energy from your solar system. Do any of the EV charging systems allow one to take power out of an attached EV battery for use in the house when needed?

  3. Finn, thanks very much for this. Two years ago I bought a standard 3KW system with a Growatt inverter and twelve panels for a nett cost after whatever rebates I earned of $2,999. Satisfied with it, and I’ve calculated it’s saving me $600-$800 a year, depending on what happens to my bills as the cost of electricity rises. From what you write, can I assume I currently have option 1, “Battery Ready #1: Ready for ‘Hybrid Without Backup’ by using AC coupling, a battery inverter and batteries”? So in future if I decide to buy a battery, I’ll need that and a battery inverter? Also, you mention the SMA Sunny Boy Storage. I went the link and there’s no mention of price. Can you say roughly what this (as an example) AND a battery inverter would cost? I assume I’d also need to add installation cost to hook everything up to my existing system.


    • Finn Peacock says

      Yes – the Sunny Boy storage is due for release in a few weeks – I have no idea what the cost will be – but my guess is around $1000 (warning: total guess). We’ll find out very soon.

      Installation should be a couple of hours. Unless you are getting a Powerwall which takes an inordinate amount of time to install mechanically and electrically by all accounts.

  4. Phil Greaves says

    Hi Finn,

    I have gone down a different path with solar. With the arrival of LED lighting I have wired my house for 12V DC using 3, 5 and 10watt LED downlights. I can run the whole house lighting on the 250W solar and 100Ah battery installation. It has been running now for over two years without loss of lighting even during a grid failure on my incoming mains and a number of blackouts.
    It is a great feeling when the rest of the neighborhood is in darkness I have safe and efficient light!

  5. Does this mean that portable/semi portable solar of a serious capability is becoming a closer reality for rental people? It’d be nice to have a personal useable stand alone system for use wherever I rented, with the security of mains to cover the shortfalls.

  6. john nielsen says

    Hi Finn,
    Again, what a great forum. I have learned a lot from this forum and I am sure a lot of other people also have. I would like to comment on your scenario #2. Where will the power go? In my opinion not up is smoke as you said. All the inverters I know off have a min and max voltage threshold and simply switch off when power is outside this threshold.
    I have read many blogs from the US on this problem “where does the power go” bang. I am confused about this. I don’t believe I am right but I know of some string inverters and they also have a threshold and switch off outside this limit, this is why I am raising this question for your wisdom.
    My Enphase micro inverters don’t go up in smoke when the power they produce exceeds the load demand they simply switch off. This is why I have 3 strings so that they don’t all go off and leave me without power. Each string is governed by the relay driver which turns off and on according to my programmed threshold voltage.
    The micro specs are as follows: over voltage limit 270 V, under voltage limit 200 V
    Over frequency limit 54.0 Hz, under frequency limit 46.0 Hz. Trip time (voltage and Hz) 1 second, reconnect 60 seconds.
    Finn, I will be grateful for an explanation and your comment to the “up in smoke”.
    John Nielsen, Silkwood

    • Finn Peacock says

      HI John,

      Yes – ‘up in smoke’ is an exaggeration. They should trip on voltage first. But my point is that – although you can make it work by tripping the solar inverters on and off, it is not a good way to do it. Controlling anything by relying on the safety trips is bad practice, often dangerous – and will invalidate the warranty on many inverters, produce a really variable voltage for your appliances, kill the whole array earn you only want a little less power. It is a bang-bang solution instead of a variable-output solution. It is less of an issue with micro inverters, but still not ideal.



  7. Option 2 vs option 3 seems to me to be just a difference in the number of boxes. I’d think how “integrated” they are functionally has more to do with the engineering, modbus could allow two separate inverters to be well integrated, right?

  8. Dennis Stanford says

    We added a 3.5 kw system to our original 1.5 kw system a couple of years ago (Cairns) in preference to installing a solar hot water system,2 systems – 2 SMA inverters.(Total outlay $7000) With two adults in the house and two adults living in a bungalow meaning all the white goods have been duplicated. Pre heating a 250 liter hot water tank with a low wattage element and timer in the middle of the day feeding into a second 250 liter tank topped up on tariff 33 (being changed to tariff 31) if required – the hot water portion of the power bill is down to $60 a quarter with an overall quarterly bill of around $350 – until recently that is when aircons have worked hard with the ongoing average temperatures well above average for months on end.My question is as we have reached Ergons limit is is it viable use what we have adding more panels with battery storage on a stand alone system running some items directly connected, which could be taken with us if we were to move on ? We also have large generator backup enabling us to switch off from the grid.

  9. stu currie says

    thanks 4 the imfo I will certainly be following it over my purchase

  10. Tom Martin says

    I recently spoke with a company designs microgrids. There are a lot of island type communities in my area where the power comes in from one source then is distributed through out the community. If each of the homes had a smart meter that could track electricity flow in either direction, how feasible would it be to have a central battery storage facility serve the community. Many of the homes have solar but some are only occupied part time and end up giving away too much electricity to the utility company. The central battery bank would be a way to store the surplus electricity and share it in the community. We would also hope to obtain TOU rates so that excess energy produced during the day could be stored locally and used during prime time without paying prime time utility rates.

  11. Hi Finn, what I am curious about is what is an inherent technology which limits normal string inverter to work as a hybrid?
    Can’t we have a cheap $200 box to attach to string inverter which can also accommodate batteries?
    Why can’t we even do that with Offgrid inverter even? I understand it requires some sort of AC sine wave to immitate? but why should it be so expensive?

    P.S. – I don’t have any electrical background, I just have sketchy electrical knowledge. But from my understandings, all it requires is some capacitors, and transistors to do these jobs.

    • Ronald Brakels says

      Hi Ronak, Ronald here. Batteries require a way to be charged with low voltage DC and they need a way to get the low voltage DC they provide converted into higher voltage DC an inverter can handle or inverted directly into AC and conventional inverters can’t handle this task.

      There are boxes that can be used that will allow a home with a conventional inverter retrofit batteries. One method is to use a battery-inverter that takes AC power from the inverter, converts into DC power for the batteries and when the batteries discharge convert it into AC. An example is the Sunny Boy Storage:

      Another method is to use a DC converter which takes higher voltage DC power from the panels, supplies lower voltage DC to the batteries and then converts lower voltage DC from the batteries to higher voltage DC for the inverter. One example is the Goodwe GW2500-BP:

      Unfortunately both these methods cost a lot more than a couple of hundred dollars, but I do expect prices to fall and for people to have more options in the near future.

  12. The only real argument against waiting is the loss of opportunity. That calculation can be as complex or as simple as you like because one size does not fit all.

    I have mentioned this before and that is the outcome for many aged persons who pensions are going to be affected by a change to the way assets are assessed.

    Solar is not only about how much money you can save but how much in the case of age pensioners of a particular asset class can loose. We are not talking chicken shit amounts.

    Think about it escalating cost of energy add to with a decrease in pension entitlements.

  13. well its been 14 months from this post and electricity prices have gone up 45% due to the generators shutting down, and you can pay back a battery in 6 – 8 years.
    Id say the people that went hybrid are pretty happy

  14. Robert Beaton says

    Re your article “The Truth About Battery Ready Solar System”
    I have recently added a new Enphase solar system with micro inverters but I have an existing older system on another roof. This older system uses an older (Sunny Roo) inverter. I have been considering adding a battery – and am currently leaning towards the AC version of the Telsa Powerwall 2 which I believe has an inbuilt DC-AC inverter. I assume that it would be simple to add this to the Enphase system – but I am wondering how such a battery integrates with the older system which has the Sunny Roo inverter. How does this fit in with your three approaches in your article “The Truth About Battery Ready Solar System – or is it a different solution? Can I just patch this into the 240v main circuit? – what communications if any do I need between the Enphase system and the Sunny Roo Inverter and the battery?

    • Finn Peacock says

      You’ll need a qualified installer to check all the details of your installation and cabling, and local DNSP (your local elec network) rules – but an AC coupled battery can connect into any grid connect solar system at the switchboard – including an Enphase system.

      So DNSP permitting you should be able to add a PW2 no problem. The disadvantage of AC coupling is that any backup function will be limited. i.e. once the grid disconnects, you won’t be able to recharge your battery with solar until the grid is restored.

  15. At last, someone has explained this battery ready battery compatible fluff ball. Thankyou. All inverters are ready, they just need a battery inverter when you buy your batteries in 2020. Its that simple…..isnt it ?????? Ive been getting quotes from salemen and wow, its a minefield trying to decide who will sell you anything and who is just trying to give you what you need. Im going with 12 sunpower 327 and a fronius primo 3kw .$5900 My transformer is only 10 kva so im limited to a 3kva sysytem. Thanks Finn for the clarity.

  16. John Fox says

    Hi Finn
    Have read the info above and wondering if you have come across the Hybrid Home from hybrid energy website ( Perhaps you could do some research and comment on the suitability for this system either as hybrid or stand alone. It is reasonably priced and batteries are recommended for up to 60% dod. They also have a modbus card and the system appears to cover most aspects that people might require of a hybrid system with reasonable storage which can be expanded if required.

    • Ronald Brakels says

      Hello John, Ronald here. I had a brief chat with Hybrid a couple of weeks ago. They have done some large installations. I’m afraid I don’t have the numbers I’d need on cost and reliability to do a write up. But now you have mentioned it, it is possible I will get around to getting them at some point.

  17. I know this is an old article – as usual the technical info is spot on. However, I don’t agree with Finn’s conclusion on hybrid inverters for three reasons.
    1. A hybrid inverter won’t become magically obsolete. Power electronics and conversion is well understood. They already get up to 97% efficiency. They won’t get so much better that it makes the current one’s obsolete. If its legal to connect to the grid today that’s unlikely to change.
    2. For exactly the same reasons a quality, high power battery inverter/charger is unlikely to suddenly become loads cheaper. For a start the majority of systems won’t go down this route. They’ll be hybrid inverters or AC coupled because its easier, cheaper and simpler. Of course China may prove me wrong but I just don’t think they will be a high demand item. In fact my bold prediction – more expensive- a specialty item.
    3. I agree buying from a quality manufacturer doesn’t guarantee they’ll still be around in 3-5 years and in any case most warranties max out at 5 years. It’s a valid point but shouldn’t be a deal breaker. Here’s why buying a hybrid inverter now allows you to install a battery the moment they make sense to you without additional install costs apart from commissioning. It will cost more than an inverter but not as much as an inverter for the panels and compatible battery inverter / charger. That is actually a more expensive option and not as flexible.

    The cheapest option is likely to be AC coupled as these present the simplest install and the battery comes with an inbuilt inverter/charger. For example the PW2 can be retrofitted to most existing systems. If you think that is the way to go then a hybrid inverter is not the right choice as the PW2 will make no use of the DC charger and secondary inverter.

    If you’re thinking DIY or a DC battery (that’s a weird thing to say – blame Enphase and Tesla) then a hybrid inverter makes the most sense.

    • About the PW2 with AC-coupling. That’s a #1 system with no chance of being PV powered when the grid is off, right?

      I’m looking at a PV system with micro-inverters which “directly” outputs AC, and adding a PW2 (or whatever) later. However, I want to build the PV system right to be a #2-type system (hybrid grid-tie with battery back-up when battery is installed).

      So I should make sure there’s an:
      * additional inverter that talks to grid (and disconnects from it on grid-down) and battery aside from PV inverter?
      * consumption monitor
      * MODBUS
      * won’t void PV warranty
      * etc?

      • “About the PW2 with AC-coupling. That’s a #1 system with no chance of being PV powered when the grid is off, right?”

        I don’t think that is correct – the PW2 has an automatic transfer switch available (which I think would be used in most installs) so when the grid goes down it continues to provide power and continues to recharge via the PV system. I think this might not have been available at first release.

        “So I should make sure there’s an:
        * additional inverter that talks to grid (and disconnects from it on grid-down) and battery aside from PV inverter?
        * consumption monitor
        * MODBUS
        * won’t void PV warranty
        * etc?”

        If you want to use micro inverters then you may be better off going with the system recommendation – this is because microinverters would not typically talk MODBUS – instead they are fed to a controller box which (hopefully) does. Enphase is the best known and they have their own battery system (which I don’t particularly like, but then I’m not an installer) – I’m not sure how well integrated they’d be with other solutions, or how many hoops you’d have to jump through.

        The other solution to consider for shading issues is power optimisers like SolarEdge. They feed DC to a traditional but proprietary inverter/controller.

        • Thanks for the reply. So PW2 may have, or include, a grid-disconnect switch-over. That would be reassuring, if Tesla or some installer would just say that. Seems almost nobody knows exactly what’s going on for this detail.

          I don’t get much choice about the micro-inverters with a SP Equinox system. I’m getting the system at a great price / watt and longevity (better than “SC”) which is why I’m going that route against most general advice about SP.

          So then I should get the installer to include some sort of controller-box as the go-between as part of a initial install, so that a later PW2 add-on for grid-blackout backup would almost be drop-in? Something like a Sunny Boy Island?

          • Finn Peacock says

            The PW2 comes with full off grid functionality as standard. This can be retro-fitted to any grid connect system using the ‘Tesla Gateway’ box. I’ve seen it work – it works well.

            More info here:


          • Thanks Finn!

            With your feedback, and I’d poked my installer a bit harder, and it looks like PW2 will be self-contained to manage on-grid, and off-grid charging — whenever I make the leap, to PW2 or any other AC-coupled battery system.

            I am worried that without end-to-end “modbus”-type communications, the AC frequency banging would damage something eventually.

            Meantime, Tesla really should stop beta-testing on their customers.

  18. Finn,

    A couple fo questions if I could. I have learnt a lot from reading your articles but fully realise that there is a lot I don’t know.

    I currently do not have any solar but am now considering a 5kW system. Some articles suggest that the smart choice is to install a hybrid inverter instead of a simple one to enable an easier battery connection should they become attractive. An offer has been to upgrade from a Growatt 3Ph 5kW unit to a Fronius Hybrid for an additional $1500. The question regarding this is that as it appears that the Powerwall 2 carries the necessary smarts and does not require a hybrid inverter that this is “old” news?
    The second question relates to our power supply. We have three phase connected to the electrical switchboard (for future air conditioning unit) but only use a single phase. Are there advantages of installing a three phase inverter rather than a single phase?
    Thanks for any advice.

    • Ronald Brakels says

      Hello Peter, Ronald here.

      Home battery storage is a rapidly changing field and personally I don’t recommend doing anything now to prepare for getting batteries later, as the preparations could end up being a waste. The Powerwall 2 is designed to be AC coupled which means it doesn’t need a hybrid inverter and can operate no matter what kind of solar inverter the house has. Other battery systems can also be AC coupled.

      A single phase inverter will do, but three phase has some advantages, especially if you decide you want a larger solar system. Finn wrote about some advantages of 3 phase here:

      • Ronald,

        Thanks so much for your response, I can now put that concern to bed.

        In regards to single phase or three phase I had been concerned that if I had a single phase inverter and then installed a three phase air conditioning unit that at times when producing excess energy and using the air conditioner that I may get charged for the two phases effectively drawing from the grid. I have been told that this is not an issue with a smart meter as it should balance up the total of power being drawn across all three phases against the single phase output of the inverter.

        I’m hoping that I have been reliably informed.

        • Ronald Brakels says

          Hello Peter

          If you have three phase power and solar you may end up exporting solar on one phase while importing it on two others, but as far as your electricity meter is concerned, it will only look at the net amount being used or exported. So if your solar system is exporting 2 kilowatts on one phase while your air conditioner is importing one kilowatt on each of the other two phases, as far as your electricity meter is concerned your house is using no electricity.

          A smart meter isn’t required to do this. A normal, digital, 3 phase import/export meter will do the same.

  19. Hi Finn/Ronald,

    Regarding the comment in this blog: “If I wanted backup, I’d go for option #2. I’d buy a Fronius, SMA or ABB inverter, because I know they talk MODBUS (the language a good battery inverter talks).“

    I’m about to have a system installed with a Frontius Primo 5.01 which has MODBUS. Therefore does this fit the bill as an inverter that can be connected to a battery system and work off grid during a power shutdown? I can’t find this explicitly outlined on Frontius website or specs.

    Recognising this article is now 2 years old and things may have changed.

    Thanks, Paul

    • Ronald Brakels says

      If you are getting a Fronius Primo installed, since it’s not a hybrid (also known as multimode) inverter you will need to install AC coupled batteries. These operate independently from your inverter and so compatibility is not a problem. The most well known AC coupled battery system is the Powerwall 2. It can provide backup power during a grid failure and charge off your solar panels during the day during a blackout. They’re not available at the moment, but if you are not in a hurry then hopefully you’ll be able to pick one up next year if you are interested.

      However, if you are not too interested in having a battery and are just concerned about having backup power you may be better off economically with a generator.

  20. Need to add batteries for off grid power to a existing Enphase micro system which is 10.84 kw, i also have a 9000 watt portable generator i would like to be able to use if necessary. I understand ac coupling is the only way to go at this point. I want to spend the least amount t possible to basically just keep my solar up in running during grid failure. Can run genrator at night just want use of power from solar not to be lost when grid goes down. Any help would be greatly appreciated

  21. Hi

    I’m a newbie to all this, I’m confused when it comes to battery compatibility to inverter. What do we mean by compatibility when it comes to installing solar lithium ion batteries to an existing grid tied system.

    Please help


    • Ronald Brakels says


      Generally speaking, batteries by themselves will need to be paired with a compatible hybrid inverter in order to work. If you already have a solar system with a normal solar inverter it won’t be capable of accepting batteries. If you want batteries you can either get a hybrid solar inverter and compatible batteries or you can get a battery system that will work with any solar inverter. The Tesla Powerwall 2 is an example. Note if you are looking to save money batteries are unlikely to do this for normal families but they can provide convenient backup power.

  22. Hi, since this article is a good few years old now, is it still relevant? (I noticed it was linked to in the HYBRID INVERTER COMPARISON TABLE page)

    I’m considering going for something like the Fronius Gen24 but unsure whether the advice still stands, i e. go for option #2.


  23. Aaron Turner says

    I would like to use my batteries as if im ‘off grid’. NOT JUST AS BACKUP POWER!
    I’d like to have my solar/inverter runing my house, charging my batteries and exporting any excess during the day but then use the battery power I have stored for overnight or low solar output days.
    Finn, when you said the average household uses 20% of their produced power im blown away.
    Surely their is a common/safe way to use your battery power overnight and charge batteries during the day instead of exporting so much power.
    If their is a setup to do this or a switch available to customise this type of setup, could you let us all know about it. 20% usage is a horrific figure.


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