SonnenFlat Review: Is It A Good Deal?

sonnenFlat review

SonnenFlat — good if you already have a Sonnen battery but maybe not good enough to buy a Sonnen battery.

I think the German home energy storage company, Sonnen, must be having an exciting time in Australia at the moment:

  • Their latest generation of home battery systems is now available.
  • South Australia has introduced a hefty battery subsidy that may be exclusive to Sonnen systems for its first two months.
  • Sonnen is creating a battery assembly plant in Adelaide that will apparently employ over 400 people.  I’ve been told it will start cranking out battery systems in November.

While LG Chem is the largest supplier of battery systems in Australia at this time, Sonnen should start to gain some ground, especially since the Powerwall 2 is hard to get and very recently increased in price (again).

With all the activity at Sonnen Australia, now is a good time to tell you the details of a deal available to homes that have a large enough solar power system and a large enough Sonnen battery.  It’s called sonnenFlat and supplies you with all the electricity you need — up to a limit — for one flat fee per month.

If you have a suitable Sonnen battery and solar power system sonnenFlat is likely to save you money, but it will depend on your location and circumstances.  By joining sonnenFlat your battery becomes part of a Virtual Power Plant and Sonnen will take control of it and use it to make money trading electricity and supporting the grid.  Unfortunately, while it can pay for itself if you already have a Sonnen battery, the benefits of sonnenFlat may not be enough to make you go out and buy a brand new Sonnen battery.


SonnenFlat is only available in the National Electricity Market, which counts out Western Australia and the Northern Territory.  It’s also not available in Victoria.  I don’t know why Victoria is left out.  I guess they’re just not not cool enough.  But Sonnen says it will be available soon.  I’ve also been told it’s not available in the Ergon network area, which covers regional Queensland.

The areas where sonnenFlat is available are:

  • South Australia
  • NSW
  • Tasmania
  • South East Queensland
  • ACT

When I went to this Sonnen page and tried to bring up information on the Essential network area that covers regional NSW the link didn’t work.  But I have been assured this is an error and sonnenFlat is available there.

Requirements — Solar, Sonnen Battery, & Smart Meter

There are three requirements to get sonnenFlat:

  • A solar power system with at least 5 kilowatts of capacity.
  • A Sonnen battery with at least 8 kilowatt-hours of capacity.
  • A smart meter.

The size of the solar power system and Sonnen battery determines which of the three sonnenFlat plans you can get.  If you don’t have a smart meter then Sonnen will charge $207.90 to install one.

“I’m glad you’re excited about sonnenFlat, Stanley, but are you sure you should be saluting like that? It’s not very sensitive.”

SonnenFlat Plans — Economy, Family, & Autonomy

There are three different sonnenFlat plans based on how much electricity your home can consume per year before you start being charged extra.  The plans and their Annual Usage Allowances are:

  • Economy: 7,500 kilowatt-hours
  • Family: 10,000 kilowatt-hours
  • Autonomy: 12,500 kilowatt-hours

The usage allowance covers all the electricity consumed by your home.  It doesn’t matter if it comes direct from your solar power system, from solar energy stored in your battery, or from the grid — it all counts towards the Annual Usage Allowance.

Most Australian households use under 7,500 kilowatt-hours a year.  This suggests the Economy plan should be enough for most homes, but only those with high electricity consumption are likely to install a Sonnen battery, so it will come down to individual circumstances.

The good news is the Annual Usage Allowance does cover a full year and is not divided up into months or quarters.  This means if you use a lot of electricity in winter but not so much in summer, or vice versa, it will average out over a year.

You Must Prove Yourself To Sonnen

Sonnen requires proof your electricity use over the past year did not exceed the Usage Allowance for your chosen plan.  I’m surprised this is necessary given they charge for excess usage.

Also you must grant them access to your battery, both physically and cybernetically:

sonnenBatterie access

But there’s nothing there about letting them come between the emotional bond you share with your battery.

The SonnenFlat Rate Is Very Flat

Sonnenflat lives up to its name as the rate you are charged is very flat and the same across Australia for the three plans.  The monthly charge, no matter where you are, is:

  • Economy: $46.20
  • Family: $57.20
  • Autonomy: $68.20

Sonnen has the power to change what they charge, either up or down, with notice.

Excess Usage Rate

If you exceed your Annual Usage Allowance you will be charged for every extra kilowatt-hour you consume.  The Excess Usage Rate varies according to location and can be higher or lower than what’s available from the grid.  Below I’ve compared the Excess Usage Rate to the lowest cost grid electricity available:

sonnenflat excess electricity usage vs market

  • South Australia:  38 cents vs. 33.5 cents — grid is 12% less
  • Sydney, Endeavour Network:  23.5 cents vs 22.1 — grid is 6% less
  • Tasmania  26.4 cents vs 26.4 cents
  • South East Queensland 24.6 cents vs. 25 cents — grid is 3% more expensive
  • ACT  21.3 cents vs. 23.1 cents — grid is 8% more expensive

I consider it appropriate to use the lowest cost electricity available as a comparison because, as I’ll soon explain, you don’t receive a lot of feed-in tariff with sonnenFlat.

Because Sonnen has said you won’t pay more than the cost of grid electricity for excess electricity use, it may be possible to convince them to lower the Excess Usage Rate it if you can show grid electricity is available for less.

Solar System Size And Minimum Annual Generation

The three sonnenFlat plans all have a minimum solar system size that is the same no matter the location:

  • Economy: 5 kilowatts
  • Family: 7.5 kilowatts
  • Autonomy: 10 kilowatts

There is also a Minimum Annual Generation amount.  If your solar power system doesn’t produce at least that many kilowatt-hours you won’t qualify.  These are dependent on location and for South Australia they are:

  • Economy: 6,515 kilowatt-hours
  • Family: 9,198 kilowatt-hours
  • Autonomy: 12,264 kilowatt-hours

In South Australia a minimum sized system should be sufficient whether the solar panels face east, west, or north as long as there is no significant shading.  But in Tasmania only minimum sized systems that face north and are unshaded are likely to exceed the Minimum Annual Generation amount.

You Must Exceed Annual Export Threshold To Receive Feed-In Tariff

When you have sonnenFlat you won’t receive any solar feed-in tariff for solar electricity sent into the grid until it exceeds the Annual Export Threshold.  This varies according to location and plan and for South Australia the amounts are:

  • Economy: 1,305 kilowatt-hours
  • Family: 1,840 kilowatt-hours
  • Autonomy: 2,453 kilowatt-hours

A north facing 5 kilowatt solar system in Adelaide can easily produce 7,300 kilowatt-hours a year.  A typical household will export about 75% of that, which comes to 5,475 kilowatt-hours.  This makes it look easy to exceed the Annual Export Threshold, but having a battery changes this.  If the household uses 2,000 kilowatt-hours of battery storage a year with a round trip efficiency of 86%1 then it will have to be charged with 2,326 kilowatt-hours of solar electricity.  This will reduce the amount of solar electricity exported down to 3,149 which is only 1,844 kilowatt-hours above the Annual Export Threshold for the Economy sonnenFlat in Adelaide.

SonnenFlat Feed-In Tariffs Are Low

The solar feed-in tariff Sonnen will pay you for exported solar electricity over the Annual Export Threshold is not generous  Below I’ve compared the sonnenFlat feed-in tariff in different locations to AGL’s feed-in-tariff.  AGL is a large electricity retailer and where they are available I have simply put down the feed-in tariff they offer.  Rather than use AGL’s 20 cent feed-in tariff, which hopefully is still available, I’ve used the highest feed-in tariff they actively advertise online:

Sonnen feed-in-tariffs vs market

  • South Australia:  7 cents vs. 16.3 cents — AGL’s feed-in tariff is 47% higher
  • NSW:  8 cents vs. 11.1 cents — AGL’s feed-in tariff is 39% higher
  • Tasmania  8 cents vs. 8.5 — Aurora’s feed-in tariff is 7% higher
  • South East Queensland 9 cents vs. 10.6 cents — AGL’s feed-in tariff is 18% higher
  • ACT  7.5 cents vs 11 cents — AGL’s feed-in tariff is 47% higher

South Australia SonnenFlat Info

South Australia is where the most Sonnen battery systems are likely to be sold initially, so I’ve excised the following section from a sonnenFlat energy price fact sheet to have the information all in one location:

sonnenflat electricity pricing information

Sonnen Becomes Your Electricity Retailer And Takes Control Of Your Battery

When you join sonnenFlat, Sonnen becomes your electricity retailer.  They also take control of your battery and use it as part of a Virtual Power Plant, which lets Sonnen make money from buying and selling wholesale electricity and providing services that help stabilise the grid.  In return for being able to use your battery in this way Sonnen offers you a flat rate that, hopefully, will save you money:

Sonnen battery virtual power plant

Sonnen promises if they charge the battery with grid power as part of operating their Virtual Power Plant it won’t be added to your Annual Usage Allowance.  You also won’t be charged for it since you’re paying a flat rate.  However, this will increase the amount of wear and tear on your battery.  The good news is the Sonnen battery system is warranted for the first of 10 years or 10,000 cycles and the ability to handle a high number of cycles should mean being part of a Virtual Power Plant won’t result in a significant deterioration in capacity.  The bad news is that Sonnen has never — as far as I am aware — provided clear information in Australia on how much we can expect their batteries to deteriorate.

You Are Not Locked In

A good thing about sonnenFlat is you are not locked in. You can leave whenever you like and there are no exit fees.  However, it can take up to three months to change retailers2.  This is a problem that is part of the Australian electricity retail market and not something specific to Sonnen.  Note that if you join for three months and burn through your entire annual usage in that time and then leave, I imagine Sonnen would have something to say about that.  I suspect you’d only get 3 months worth of Annual Usage and would be charged the excess rate for the rest.

In the future you may have the option of choosing between between different Virtual Power Plants from lots of electricity retailers and battery manufacturers, so not being locked in is a good thing.

It Is Possible To Save Money With SonnenFlat

It is difficult to determine how much a household can potentially save or possibly lose with sonnenFlat, but a reasonable estimate can be made.  If an Adelaide home has the following characteristics:

  • A 5 kilowatt solar system that generates 7,500 kilowatt-hours a year.
  • A solar electricity self consumption rate of 25% before battery storage is used.
  • A 10 kilowatt-hour Sonnen battery system with a round trip efficiency of 86%.
  • 7.2 kilowatt-hours of battery storage is used on an average day.
  • 7,000 kilowatt-hours of annual electricity consumption.
  • The household uses the first AGL retail electricity plan our comparison tool displays, which has a yearly supply charge of $341, a per kilowatt-hour charge of 32.5 cents, and a feed-in tariff of 16.3 cents.

In this case the household’s annual electricity bill would come to $734.  If they had instead been on the Economy sonnenFlat plan they would have been charged $46.20 a month, which would come to $554 a year.  In addition the household would receive $88 of solar feed-in tariff, reducing their yearly electricity bill down to $466; making them $268 a year better off.

If we lower the household’s electricity consumption down to 6,000 kilowatt-hours a year while keeping the other assumptions the same, the benefit of using sonnenFlat is reduced but it still comes out ahead.  If we instead increase the size of the solar system to 6.6 kilowatts of north facing solar, then it’s almost a tie.  If the solar power system performs well then the household would be worse off with sonnenFlat.

Getting A Sonnen Battery May Not Be Worthwhile

It certainly looks possible for households that meet the requirements to save money by getting sonnenFlat.  But this doesn’t mean it is worthwhile to buy a Sonnen battery just so you can join sonnenFlat.  As I point out towards the end of this article, even with South Australia’s large battery subsidy a Sonnen system is unlikely to pay for itself.  So while sonnenFlat can be worthwhile if you already have a Sonnen battery, its advantages may not be enough to make it worthwhile to invest in a battery, especially after the costs of installation are considered.  The game may not be worth the candle.

game not worth cable meaning


  1. This is the upper limit of my estimate of Sonnen battery system’s average round trip efficiency.
  2. Which is bizarre since it should take no time at all as you’ll have a smart meter.
About Ronald Brakels

Joining SolarQuotes in 2015, Ronald has a knack for reading those tediously long documents put out by solar manufacturers and translating their contents into something consumers might find interesting. Master of heavily researched deep-dive blog posts, his relentless consumer advocacy has ruffled more than a few manufacturer's feathers over the years. Read Ronald's full bio.


  1. Hi Ronald,
    It appears that you have forgotten to mention the fact that the SonnenFlatrates are ONLY for the supply of electricity per month. On top of that charge the customer still has to pay for the solar PV Array + the Sonnen Battery cost, per month. Based on those calculations, it is a very expensive option. This is the info passed onto myself from my FLEX (apparently the ONLY distributor of Sonnen in Australia) rep. If thinks have changed since the info I received, then, I take it all back. But………….

    • While reading it sermed implucit to me
      #PV array is up and paid for
      #Batter exists and paid for (except when it is mentioned that (para) “…would this be enough to make you go buy a sonnen batt … no”

      So i think here we are really dealing with the guts of
      sonnenFlat Vs dealing with the leccy supplier on your own terms (with whatever config you have)

      I agree with you that a start point view (no pv or batt) woukd sllow decision making frim the outset – but i also feel like that question is answered in the negative even if you already have the array paid for (as you clearly point out yourself)

      Not seeking to argue – just a POV – inteterested in your thiughts if i got your point wrong (please forgive type-o’s damn apple auto correct coupked with iOS inability to correct in this page leaves me at your mercy – and im an iOS BB eng so double apology)

  2. Hi Ronald,
    When I looked into this last year Essential Energy was initially uninterested but they came to the party by putting a higher monthly charge on Sonnen Flat than the other NSW distributors.

    • Yes – Essential Energy I believe added $15/month to the SonnenFlat charges last time I looked…

      In my opinion, If the battery system is designed properly – they should have almost no bill anyway and be drawing next to nothing from the grid…

      A battery system should really have enough storage to cover evening use, and have enough solar to charge battery, cover day time use + feed in enough to cover their “service availability charge” to result in a Zero bill.

      The Sonnen Battery system is great, I’m not really convinced SonnenFlat is a great offer (if you battery system is designed appropriately)

      • Lawrence Coomber says

        Yes well said Brendan.

        Can you expand on what you meant by “(if you battery system is designed appropriately)”? please. The key technical aspects of your battery design ideas would be appreciated Brendan.

        Lawrence Coomber

  3. Hey Ron,

    Why do you think we’ve seen a trend towards more and more middle-manning? Isn’t this essentially counter to the mood driving people to install batteries (i.e. the ideal of independence from the grid, be that real or perceived, economics be damned)?

    • Ronald Brakels says

      Lots of people were motivated to go off-grid but it doesn’t make economic sense. However, it looks like home batteries are reaching a point where they can pay for themselves if in addition to providing a household with electricity they also provide ancillary services to the grid to help keep it stable. At the moment individuals can’t receive payment for providing ancillary services. The only way they can do it is join a company’s Virtual Power Plant. So it looks like being part of a Virtual Power Plant is what will be pushed for now.

      However, some of economic benefits of providing ancillary services can disappear pretty quickly so Virtual Power Plants may not be as cost effective as they appear at first. For example, Tesla’s big battery in South Australia appears to have dropped the cost of providing frequency stabilization to a fraction of what it was.

      At current prices going off-grid won’t pay for itself and even if cost of the required batteries and hardware fell low enough to make it worthwhile

  4. Last year, I nearly took up the sonnenBatterie system with the sonnenFlat plan when the a solar company advised that there was extensive delays with the Tesla PW2 (my original order). I was offered the 10kWh sonnen system. So, there was a 4kWh hit to battery capacity. But what got me then was the fine print in the plan, you had to make sure that your generation exceeded your usage. This was a bit of a grey area for me and declined the offer to take the sonnenBatterie in lieu of the Tesla PW2 and just waited out (it was only another 2 months). So, it was sneaky that sonnen put their plan in fine print, which may have made it not worthwhile.

    Not to be nitpicky… sonnen is spelt with a lower case “s”, not capitalised. In German spelling, it’s a big difference saying Sonnen or sonnen. “sonnen” means to bathe in sun, so it’s a string of words in action….. Whereas Sonnen can be a surname, which are capitalised. Captitalising the word changes the context of the word. sonnen the company always starts with a lower case, even if it’s the first word of the sentence, because you are dealing with a public company’s name as it’s spelt and written.

    Look at the sonnen website, every word sonnen is lowercase. You have to think in the context of the German language, not English……. as it is for other languages. It’s a bit of arrogance to think in English phonetics when writing/pronouncing other words in other languages.

    Another Pet Hate I Have Is Websites That Capitlise Every Word In A Title (New York Times Is Bad At This Practice Too). It Can Change The Context Of The Meaning Sometimes. It’s Bad Form Of Writing. Trust Me, My Mother Was A School Teacher And I Was Corrected A Lot Of Times Not Write Like This. Looks Very Bad And Is Poor Form Of Communicating.

    If you want a title to stand out, bold it or underline. Not cap every word.

    Otherwise, great website! 🙂

    • Ronald Brakels says

      I was wondering why they never put a capital at the front of Sonnen… I mean, sonnen. I assumed it would be easier for Australians to read if I capitalized it, but that is just an assumption on my part.

  5. over and over and over and……. The moment you try to accommodate two people NEITHER of them get their best interests served. Then multiply that principle by dozens or thousands. (as depicted in the failure of ‘democrcy’.

    Beyond that:- Everytime you appoint/accept an ‘organiser’ of ANYTHING it costs unnecessary ~ AND GROWING AMOUNTS OF ~ money. YOUR money: NOT theirs. It’s the SOLE reason ‘organisers’ organise. (Which is to say they produce nothing of a wealth-generating nature.

    …….if any of you need the cheapest quote for transporting home a large coat-hanger shaped bridge (which I can also sell/lease you at a GREAT rate) please read the attached brochure which will explain the fineprint.

  6. “But in Tasmania only minimum sized systems that face north and are unshaded are likely to exceed the Minimum Annual Generation amount.”

    Did you mean maximum rather than minimum? If minimum, what about an unshaded NE or NW facing maximum?

    • Ronald Brakels says

      To get the Sonnen Flat Economy plan in Tasmania a solar system must produce at least the Minimum Annual Generation amount of 6,050 kilowatt-hours. Looking at the PVWatts site:

      I see that 5 kilowatts of north facing solar in Hobart is expected to generate 6,205 kilowatt-hours a year. For east facing solar it is 5,578 kilowatt-hours and for west facing solar it is 4,967 kilowatt-hours. As the PVWatts figures tend to be a little optimistic 5 kilowatts of solar panels will be hard pressed to meet the minimum required amount unless they face north and perform well.

  7. Hi Ron,

    AGL in South East Queensland offers a 20 cent feed in tariff, energy Australia offers a 16.1 cent feed in.

    • Ronald Brakels says

      I see AGL is now actively making information on its 20 cent feed-in tariff plan available to comparison tools. They used to keep it hidden away and only mention it on the AGL site and this made me worry its availability would only be temporary. I’ll take note of this change.

  8. I wasn’t aware of that, they do offer a 2 year lock in on the feed in tariff though, instead of the usual one year by most retailers.

    • The two year lock in on the feed in tariff is a little ambiguous. If you look at the fine print it can be interpreted as locking in a premium to their standard feed in tariff for two years rather than the total feed in. When I pressed them on this via chat they said that the total feed in wasn’t in fact locked in for two years, although the whole thing was still rather confusing and I wasn’t really convinced that they knew the answer.

      I’ve ended up on a special one big switch plan with Origin for SA that gives a better usage rate and a 20c feed in for 12 months which is better for me than the AGL offer anyway, but I’d still love to know whether it is really guaranteed for 24 months or not.

      Actually a lot of retailers don’t offer a guaranteed FIT even for 12 months, a lot can change that rate at any time if they want to. AGL and Origin’s other plans work like that even when they lock in a discount for 12 months.

  9. Hi Ronald,

    I see that my comment, about the actual costs for sonnenFlat doesn’t deserve a reply, yet the issue revolving around the use of the capital ‘S’ in Germany does.

    Weird. Priorities I suppose.

    • Ronald Brakels says

      I think Sc covered the points I would have made. Possession of rooftop solar and a suitable Sonnen (sonnen) battery system are taken as given.

      • Ronald,

        SC’s reply was not published when I checked, prior to this last comment. However, this is not how sonnenFlat is being deceptively marketed. You do still require a sonnen battery and the minimum sized PV array to qualify and these 2 things alone put the solution out of reach of most people as they are trying to get away from $1200/Q bills to jump into bed with someone and start paying that monthly is ludicrous. Sorry not a fan of sonnen and how it is marketed.
        Thanks for the reply.

  10. Hi Ron,

    I believe that in metro NSW (and in Endeavor Energy network) the AGL’s 20 cent feed-in tariff is still available.

    Presuming that you have a spreadsheet to calculate this, but how does the calculation compare ‘AGL 20c feed in Versus sonnen flat’

    We probably fit the Family or Automonmy model with a family of 6 and a pool connected to ‘off peak controlled load 1.

    Keen to see whether the numbers stack!

    • Ronald Brakels says

      Hi GLO

      If you have a suitable Sonnen battery system then it’s definitely possible to save money with Sonnen Flat, but if you don’t have a Sonnen battery then it’s not likely to be worthwhile to get one. You could get a better return and help the environment by expanding your solar capacity or investing in energy efficiency.

      To qualify for Sonnen Flat your solar system will need to produce at least 7,500 kilowatt-hours a year. A north facing system of 5.5 kilowatts or more can produce this in Sydney but even systems of this size may not qualify if they have shading issues or slightly under perform.

      Sonnen will become your electricity retailer and I don’t believe they offer a controlled tariff so with Sonnen Flat you won’t be able to have your pool on a controlled load.

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