Are micro inverters & battery backup compatible?

3 microinverters and 2 batteries

Can you combine micro inverters with batteries? Yes you can!

Can you use a micro inverter off grid? Or even for grid connect with batteries?

With the growth in the use of micro inverters, I’m starting to get more and more emails asking: can micro inverters be used in off grid (or hybrid) systems?

The short answer is yes they can! In fact a number of micro inverter battery backup systems are already operating here and abroad.

The longer answer gets a bit technical – but I’ll try to keep it as simple as I can! 

Let’s get back to absolute basics for a minute and focus on off grid systems: The fundamental difference between Off Grid and Grid Connected systems is that Off Grid systems need to store the energy in batteries. The energy produced by your solar panels (or wind turbine, hydro or generator) needs to captured and stored so it’s available when you need it.

Charging batteries with DC

Charging batteries from an energy source needs some sort of control to prevent them being overcharged. Historically, this has predominantly been managed with a regulator, which accepts DC power from your energy source, monitors how the battery is reacting and adjusts it accordingly:

block diagram of a regulator based system

The simplest battery storage system. A panel a regulator and a battery Everything is DC.

If you only ever use DC power, this can work very well. In fact this is the system I use when I go camping to keep my Coopers Pale Ale cold in my DC fridge. Trust me – it works!

Charging batteries with 240V AC

If you have a 240V AC energy source (e.g. a petrol generator or 240V wind turbine) then a battery charger is typically used, which performs the same function, just in a slightly different way:

block diagram of a charger based system

To charge batteries with AC you use a Charger.

In both cases when the batteries are full the regulator or charger will switch the energy input off.

Once it is stored in your batteries you can use the energy as Direct Current (DC) if you have DC appliances, but in most cases an inverter converts the DC power to 240V AC, just like mains power.

So if you have an off grid system that can be charged from either solar or a backup gen-set, then putting everything together, you have a system that looks like this:

off grid block diagram

Putting everything together for a proper off grid system.

So, we have our energy stored, a regulator and charger and an inverter to power the loads.

Years ago, inverter manufacturers realised that combining these into a single unit made sense and invented the inverter/charger (also known as an off grid inverter). These were typically designed to accept a variety of inputs and outputs AC input from generators or mains power, DC input from batteries and so on. Typically, solar panels and other devices continued to be connected via a regulator to the batteries.

modern off grid block diagram using inverter/charger

Modern off grid systems use a single inverter/charger (AKA off grid inverter).

Although it took a while, around ten years ago some manufacturers realised that they could already convert the solar (or other source) input to AC with Grid Tied inverters, so to simplify things a bit more AC Coupling was born, and it looks like this:

ac coupled system block diagram

An AC coupled system using a grid tie inverter and an inverter/charger (also known as a battery based inverter).

By coupling everything except the battery on an AC Bus, management, monitoring and efficiency was optimised. Although it cost a little more in electronics, transmission losses were minimised, cable sizes were reduced and flexibility improved. One of the major benefits is that if the AC generation source matches the load size, it effectively powers it directly, bypassing the batteries. Another is that if more power is needed, the AC generation source and the inverter charger (drawing power off the batteries) can parallel together, increasing short term capacity. AC coupling can be done at a household level or in much larger micro grids and they can operate with or without a mains connection.

For completeness here is what an AC coupled system looks like with a grid connection (can you tell how much I love block diagrams yet?) :

grid tie battery backup ac coupled block diagram

A grid tied with battery back up AC coupled system.

So, now, we have our energy stored, but energy sources and outputs are all 240V on the input and load sides.

The sophistication of the control and regulation systems required to make this all work smoothly cannot be underestimated; there can be a hell of a lot going at any point in time especially if you have multiple energy sources which can vary their output, and loads which obviously vary a lot too as we go about our daily life.

Luckily, the world has some very clever people and a number of inverter manufacturers highlight their ability to handle AC coupling from solar including Selectronics, SMA, Schneider, Outback Power, Magnum and Midnite Solar to name a few.

Although I don’t want to underestimate the sophistication required to make a great inverter/charger there are two major tricks in getting AC Coupling to work successfully.

From a safety perspective, Grid Connect inverters are always searching for a 240V AC reference source and are designed to shut down if it isn’t present simulating a line worker switching off the mains for maintenance, for example. So, in a non-mains connected AC coupled system, the Grid Connect inverters need to be tricked into believing its present by creating a reference point. The inverter charger can do this by creating 240V AC from the battery or if other sources are present (such as a generator) they can too. Either way, as you can imagine, this involves some pretty sophisticated controls to be done safely and reliably.

The second issue is how to regulate the AC energy source to prevent the batteries being overcharged and, to prevent the voltage and frequency from being distorted. If the grid tied inverter and the inverter charger can communicate with each other then the inverter charger can ask the grid tied inverter to gently throttle the power flow up and down as needed.  If these 2 boxes don’t talk to each other then the inverter charger needs to either use a more brute force way to get the grid tie inverter to reduce the flow of power from the solar panels. Different inverter chargers and control systems use different techniques to do this; SMA uses a process of shifting the frequency. Most of the others “throttle” the power in a more traditional way but both systems reduce the energy input into the battery.

Ultimately, there also comes a point where there may be no load, the batteries are full and lots of energy being generated. Some systems can cope with this by throttling, others require control relays to divert energy and in some cases this needs to be “dumped” into a load such as a hot water system or water pump.

A crucial point here is that you can’t just cobble a system like this together. The systems all need to communicate very rapidly, highly reliably and the obviously need to be matched in terms of their ability to handle the power flows. It’s not simple but it absolutely does work and has been gaining favour as a solution for many years.

So, logically micro inverters which present solar as an AC source can indeed be coupled into these types of systems. In the last 2 block diagrams above you simply swap out the Solar Panel and grid tie inverter for all your AC solar panels. And a quick call to a couple of suppliers revealed that there are already a number of such systems operating in Australia. By co-incidence, I also spoke to a local chap the other day who regularly conducts training in off- grid systems and lives off grid in Victoria. While I was on the phone, he literally plugged a micro inverter equipped solar panel into his off grid system and it simply connected and started generating; doing exactly what it was supposed to.

As is the case with non-micro inverter based systems, everything needs to be matched and configured properly but as one example, Enphase have a specific white paper on the issue.

Interestingly, Enphase highlight that one advantage of using micro inverters instead of string inverters is it allows each panel to be individually “throttled” (assuming a compatible inverter-charger), maximising flexibility and efficiency. Recently however, Enphase announced a new generation of smarter micro inverters, which will effectively allow them to make many of the traditional components of an AC coupled system redundant by embedding more of the control and power conversion functions of the inverter/charger into the micro inverter itself.

So if you buy a microinverter based system you won’t be left high and dry if you want to add batteries in the future, you’ll simply need an AC coupled system.

In fact the way technology is progressing it would not surprise me if batteries will soon come with “micro inverter/chargers”. So you may be able to have AC solar panels plugged directly into AC batteries. You read it here first folks! (And I must confess that I actually read it here first last week).

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. John Salisbury says:

    Thanks for this article Finn.Two months ago I installed an Enphase micro inverter system.On your recommendation.Yesterday It generated 22kwh.I am quite happy.Interesting that my system has future potential to store some of the power the solar panels generated.

  2. Remember when the space-shuttle (actually several of them) blew up or crashed? (one only last week.)
    Remember the teaspoon-sized metal link the failure of which destroyed more F-111’s than enemy activity ever did?….and before that the super-sophisticated F-104 Starfighter?
    Remember when a simple, non-computerised, vehicle outlasted outlasted three generations of owners? (I believe they deliberately built rust into Holdens because otherwise they’d have lasted forever!)

    Wonderful thing, technology; but the KISS principle reigns supreme: the more parts there are the more likely it is that something (even quite minor) will bring the whole thing down ~ and ALWAYS at the worst possible time. 😉

    Some time ago I asked whether some whiz-kid could find a way to circumvent/eliminate the “240V AC reference source” in ‘ordinary’ string inverters, so’s they could be used in a stand-alone situation, and got no response. Any ideas? ~ That’d still be my favourite option; easy to install and protect from the weather, convenient to inspect/maintain and cheap to replace if necessary. I’m too old to be dancing around on the roof in a storm, torch in hand, because the bloody lights have gone out.

    The main problem with a KISS-based system seems to be they leave little scope for drawing boxes….with cute little arrows sticking out of them.

    ps. Just for interest’s sake:- How does one connect multiple AC sources? Simple series/parallel setup as in DC? Isn’t frequency-matching essential?

  3. ps…Are you sure you’re an engineer? You strike me as being far too chirpy and enthusiastic to be one of those.

  4. Rowan Reynolds says:

    Thanks for the article Finn. The Enphase storage idea is you link to near the end of the article is particularly interesting. How do you suppose the connectivity to the panels would work, given the microinverter is at the battery not at the panel. Obviously it would be a lot of wiring if each panel were connected to a storage module, so do they run say a 24VDC buss to the modules?

    • I assume that they all hook up to the 240V house circuit like any other appliance, and use power line comms (or even wireless) to determine which batteries charge/discharge at any point in time. I don’t think you need to directly connect each micro inverter to a corresponding AC battery individually.

  5. YesMinister says:

    I’m EXTREMELY interested in this stuff however I’ve been advised on numerous occasions by supposedly knowledgeable folk that using grid inverters off-grid is completely impossible. Seems to me it should be a relatively simple task to ‘fool’ a grid inverter into believing its actually` on grid when it isn’t.. Will certainly read up on Enphase shortly and would greatly appreciate any comments others can provide rev this issue.

    • I’ve been trying to achieve that for a couple of years ~ and have asked here several times. (it should be a relatively simple task to ‘fool’ a grid inverter into believing its actually` on grid when it isn’t.. ).
      With no real response from anyone who might know.

      Still waiting.

    • Hi Guys – sorry for the delayed response. Please see the second to last block diagram. It is absolutely possible to use a grid tied inverter in an off grid system – you need to use a battery based inverter (AKA inverter charger) which produces a sine wave good enough to fool the GC inverter that it is the grid.

      From the article:

      “From a safety perspective, Grid Connect inverters are always searching for a 240V AC reference source and are designed to shut down if it isn’t present simulating a line worker switching off the mains for maintenance, for example. So, in a non-mains connected AC coupled system, the Grid Connect inverters need to be tricked into believing its present by creating a reference point. The inverter charger can do this by creating 240V AC from the battery or if other sources are present (such as a generator) they can too. Either way, as you can imagine, this involves some pretty sophisticated controls to be done safely and reliably.”

      Selectronic, outback power, midnite power can all supply battery based inverters to do this among many others.

      Hope That Helps,

      Finn

      • Good news, if a long time coming.
        I did get a response to my question some time ago from someone who sounded as though he knew what he was talking about, and the suggestion was that there was a real risk of the (coverted) grid-connect inverter would ‘overpower’ the inverter providing the ‘reference point’ and destroy it…and likely the battery-bank to which it was connected. (Synchronisation was part of the problem, as I recall…..as was conjecture about how much ‘power’ may need to be available in the ‘reference point’.)

        If what you say is accurate, and it’s a not-overly-difficult job, is it possible for you to perhaps list a few sparkies/engineers in each state that would do the job for an appropriate fee? I’ve asked a few and found no interest in ‘doing things differently’.
        I’m currently using several ‘MeanWell’ inverters for various things, and they do a good, no-problems- job (sound 3-year warranty as well) and wonder whether one of them (200w -700 watts) would do the job.
        Cheers
        Jason

  6. Hi Finn,
    I would also have thought it could have been done such: use a change/over switch. switched one way it draws power from the grid to a battery charger 240vac to say 24vdc which in turn power a 24vcd to a 240vac pure sine wave inverter (no batteries). dump the excess power into a water storage using one of the gadgets you mentioned in a blog. No grid application, no export for a pittance to the grid, design your system so that you purchase the minimum from the grid and sell nothing to the grid. A large battery bank for 5kw system costs about $7,000 and might last diminishing 7 years, and that will add $250 to your quarter bill plus the lack of interest on your money.
    I would like a comment.
    I am about to install a 5kw system using Enphase inverters, but cannot get a detailed quote from any installer, i.e. cost of: racking, panels, inverters, cabling, monitoring, installation, deducting my STC credit separately.
    All the electricians here want $3,000 for two hours work.
    I will do it myself and sacrifice the STC credit rather than give it to some rip off merchant.
    john Nielsen, Silkwood, Qld.

  7. Hi Finn, I like your boxes and lines but I think there is an arrow missing from the off grid inverter to the grid inverter. As you described about the grid tie inverter needs both DC and AC voltage to operate. About the Enphase inverters:
    They have voltage limits: over voltage limit 270 V, under voltage limit 200 V
    Frequency limits: over frequency 54 Hz, under frequency 46 Hz.
    Trip time, voltage or Hz = 1 second, reconnect = 60 seconds.
    I now have my 5 kw system sitting nicely on my roof, and no Utility grid approval yet, and maybe never. Not to worry. My plan is different from my last blog.
    I will use: 30 Amp isolator, change over switch, Morningstar relay driver to measure the battery voltage, it will drive 2 relays (the 5 pin auto type) which again will control two solid state relays, one for the grid and one for off grid.
    If you take a couple of take offs from the panels via Y connectors then you have both 230 Vac nominal plus 24 – 32 Vcd. Connect this DC to the hybrid inverter which again will charge the batteries. The battery voltage will determine whether you are on the Utility grid or you are stand alone. The system will wake up in the morning automatically and shut down sometime in the evening depending on your battery bank size. Don’t buy to many of these as so much new development is happening in the battery world including prices. The utility Grid people are doing all they can to stop us using solar, but they will loose out in the end. I can see the day coming when I will tell them to come an pick up their meters and wires and that will be two accounts lost.
    john Nielsen, Silkwood, Qld.

  8. Question: I am about to install a 5kw system on my colobond roof that has a 30 degree pitch and is facing north. It’s it possible to be totally independent of the grid without going totally off grid? I.e can I still be connected to the grid without using the power from the grid with a battery storage say, tesla power wall? I’m thinking of the obvious service charges etc that can be forced on me for totally going off grid so if I still have the grid connected as a final back up, is this possible?

  9. Caius St.George says:

    Hi Finn. Thanks for the very interesting article. I am in the Bahamas and have as you call it, a Hybrid Solar system which includes 9,240 watts solar panels feeding an SB8000 coupled to x2 SI4548 and 856 ah batteries.
    Yes I use the grid as backup but here we are not allowed to feed in! Bahamas should be Bananas as in Republic!
    I called SMA and they said that it is not possible to connect micro-inverters to my home grid as it would upset the Sunny Islands and there could be a possibility of overcharging the batteries or doing some electronic damage.
    I would like a little more power using micros but who do I believe? You make sense but SMA is the manufacturer. Many thanks for your time…. Caius

  10. hello,

    Somebody can recommend some brands of hybrid inverters ?

    Regards

    Peter

  11. john nielsen says:

    Hi Finn,
    Finally we got a little info about the Enphase AC battery. Costs: each unit $1,150 kWh, capacity of each unit 1.2 kwh, thus cost per unit $1,380
    I will need 20 kWh to run my 4 split system aircon overnight. Thus I will need 17 units which will cost me $23,460. The units can DOD to 95%, and 7300 cycles or 10 years whichever comes first. I have a 6 kW enphase micro inverter system, but will not be able to consider the AC battery at this cost. I have 36 kWh lead gel batteries which runs my house 24/7 completely isolated from the utility grid. During aircon season I will have to purchase from the grid (isolated from my PV system on separate lines) as my bats and 2 x 8 kW inverters will not cope with this extra load. As I see it, it isn’t possible to go completely off grid unless you have a small household load. Without aircon I am fine, but I don’t think anyone using between 2 to 4 aircon can do so without a huge expense whether AC battery or PowerWall. One day we will have a Flow battery and this will be a game changer. At present a 2.5 kW Flow battery costs around $13,000 but I see the day when you can purchase the kit on Ebay (bar the tanks which you can make) and put it together yourself. I would like to hear from anyone who has done it.
    John Nielsen, Silkwood

    • Mick Tilley says:

      Hi John
      I know its sometime since you wrote this article so I hope u are still around to give a reply!

      I am considering the pro’s and con’s of a central or micro inverta system and have been quoted on an Enphase miscro inverta system which I am learning toward. I too would like to add AC lithium ion batteries but the cost is still prohibitive.

      Questions: Have you had any problems with the Enphase Micro inverta e.g heat related?
      Have your lead gel batteries performed to your expectation? i.e. the AC coupling issues ever been a problem?

      Thanks for your time
      Mick Tilley
      Mt Tamborine QLD
      0411 423 579

      • john nielsen says:

        Hi Mick,
        Yes Mick, I am 80 this year and still above ground. No, I have never had any problems with my Enphase Micrio inverters, they have worked fine from day one, and as you may know I am in Silkwood, N.Qld. never any heat problem,,, but my panels are at 15 degree angle supported on a structure placed on a flat concrete roof, thus a lot of ventilation,,, how they would perform sitting 100 mm from a tin roof might be another matter.
        No, I am not happy with my gel batteries. They will be 3 years old next January.
        They are still performing but I am not sure how to desuf… them… I would have chosen lead acid instead….
        I don’t know what ac coupling means. My batteries supply power to the load and thus to the micros as they need to see about 230 vac to wake up,,, then the load is connected to my inverter which runs bi-directional from/to the bats.
        The system runs fully automatically, governed by the battery voltage which is read by a Morningstar relay driver with 4 ports, each with different thresholds and delays, thus I can turn off or on: one 2.5 kW or two 2.5 kW or just a 1 kW, I can send power to my HW tank with the 1.8 kW element depanding on the power available.
        Summing up: I am happy with the micros, but should have chosen lead acid instead of lead gel….I am saving about $3,200 yearly in power in this household of 6.
        Instead of trying to get rid of the utility grid, I shall later try to get on the grid with my system. I was denied grid access because I applied for 5 kW so I instead of installed 6 kW outside the grid. I have 3 phase connection for my 3 phase elevator motor,,, and I now believe that it is possible to have 5 kW on each phase…. I have had 3 phase power here for 27 years, and I can only assume that the grid people are all against solar power,,, but lately maybe some change of mind,,, if you can’t beat them —- join them… maybe.
        Cheers
        john

        • Mick Tilley says:

          Thats great info John. I will go for the AC microinverter system grid connect but leave the batteries for a while.
          Thanks for your time.
          Mick

  12. With the new generation batteries with higher efficiency, is there still an advantage to running a grid tie inverter, battery inverter and AC bus, rather than than using solar regulators and an inverter/charger?

    Also has there been any updates on the easiest way to wire micro inverters to the battery and the AC switchboard?

    Finally, if its hard to find people with the knowhow, could solutions in your other article be used, for example, using DC optimisers, solar controller/s, traditional string hybrid/inverter charger?

  13. “By coupling everything except the battery on an AC Bus, management, monitoring and efficiency was optimised”

    What if there needs be multiple autonomous solar systems onsite, e.g. for different buildings/families who need to be responsible for their own power bill? In this situation we can’t simply couple everything on an AC bus on the output of a battery inverter. With a battery inverter where the PV power is fed into the battery with a solar controller, and the external AC input is controlled separately, each household can be programmed on what conditions it is allowed to interact with the other households. We don’t want one power hungry household dragging the whole system down and not able to learn from the consequences of pushing limits and eventually having their solar system scale back its power.

    Can micro inverters be integrated to achieve connected yet autonomously programmed households in this way?

  14. Hello Finn
    Need your advice. I have 4000 watts home inverter / battery power back up powered from the grid. In Nigeria we need it because of frequent power loss from the grid for hours. Can I now use an the power from an array of PV + MICRO INVERTERS (4000 watts) as substitute to the grid in the afternoons (my total power usage is 2700watts hr at any given time and my home back up is needed for alittle while in the night when the grid fails.
    Thanks
    Adam

  15. And also we would not be surprised if the microinverter becomes integrated into the module, with true AC panels plugged into a common rail AC cable. A perfect solution for partly shaded panels and just a matter of time …

  16. Hi Finn,

    Excellent info on the website which has been very helpful.
    Could someone please clarify:
    1. What is the difference between Hybrid Inverter(s) and Micro Inverters?
    2. What does it mean to be battery ready? No more to pay or will there be some expense when we eventually add the battery to the solar system?
    3. Brands of good Hybrid inverters please?

    Many thanks for your advice.
    Regards,
    Ashwin

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