An Engineer's Case Study: Panels Are A Must, But Forget The Battery

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Updated to 31 March 2020




I am a retired electrical engineer and have compiled a spreadsheet based on 6.36 years of solar power generation in the Adelaide area.  We have 5.2 kW of panels on the roof, consisting of 20 REC 260 PE panels facing north at an angle of 27.5° above the horizontal, and with no shading. 


For the period from 17/11/2013 to 21/05/2017 we had just the panels with an SMA 5000TL-21 inverter.  On 26/05/17 we joined the AGL VPP and installed a Sunverge DC coupled battery system.  On 01/11/18 the Sunverge system was replaced by a Fronius Primo 5.0 kW inverter and a Tesla 13.5 kWh Powerwall 2 battery.  Since 1st January 2019 I have downloaded detailed data from AGL Solar Command, Tesla, Fronius and AGL’s metering data.


A detailed analysis of this data showed that the annual savings from the panels without a battery was $2,063.96, and the annual savings from panels with a battery was $2,408.68.  All pricing includes GST. 


Thus the annual savings resulting from adding the battery was therefore the difference, $344.73.


If you are intending to install a solar system with a battery, make sure to take into account the purchase price and installation costs of the battery itself to see if it is worth installing.


The table below shows how long it would take to pay off a battery based on various battery costs and annual savings of $344.73.  You may also need to replace the battery after 10 years or so.





The annual savings of $2,063.96 from the addition of solar panels only is detailed in The Significant Cost Benefits of Solar Panels, Table 8.


The annual cost of having neither panels nor battery has been derived from the data below from 1st January 2019 onwards, as has the annual savings for having both panels and a battery.


Table 1.jpg

Table 1 data was obtained from AGL Solar Command, showing household consumption and total solar production.


Table 2.jpg

Table 2 data was obtained by downloading the weekly data from the Windows app, Powerwall Companion.


Column 15, battery output divided by battery input, is the efficiency of the battery storage process.  The losses associated with this process would, without a battery, be otherwise available for in-house use or for sale as feed-in.


Table 3.jpg

Table 3 data was obtained from Fronius solar.web Premium, and AGL metering data was obtained from our bills.


Table 4.jpg

Table 4 is a summary of the data from Tables 1, 2 and 3, allowing easy recognition of the consistency of the data.  The AGL metering data is revenue grade, or better than 1%, and Fronius data is also supposed to be revenue grade.  Tesla data is supposed to be accurate to about 2%.  Given these limitations, the data appears to be reasonably consistent.


Table 5.jpg

In Table 5, the monthly data in Columns 1 to 4 were derived from the averages of the three respective columns for each variable in Table 4.


Column 5 was derived by multiplying the current tariff by the home consumption data, given that, without solar panels, we would have to import all of the data used in-home.


Column 6 was derived by multiplying the current peak import tariff by the grid import data in Column 1.


Column 7 was derived by multiplying the current feed-in tariff by the grid export data in Column 2.


Column 8 is the sum total of the imports and exports for solar panels and a battery.


Columns 9, 10 and 11 summarise the savings.  The tariffs used to derive the cost totals in Columns 6 and 7 are the current tariffs for our AGL Solar Savers plan, highlighted in Row 4 of Table 5 above.


The panels and battery savings in Column 9 are derived by subtracting Column 5 from Column 8, the difference between what we are paying now with panels and a battery, and what we would have paid without panels and a battery.


The annual result in Column 10 is derived from Table 8 in The Significant Cost Benefits of Solar Panels.  Note also the strong correlation between the estimated annual home consumption in The Significant Cost Benefits of Solar Panels, (Table 5, Column 11) 3,934.1 kWh, and the annual average home consumption in Column 3 in Table 5 above 3,750.40 kWh.


The savings in Column 11 are those due to the addition of the battery.  They are the difference between the savings with panels and a battery, and the savings with just panels only.




Given that 6.6 kW panels and an inverter can be purchased and installed for under $4,000, there is absolutely no reason not to do so in my opinion.  It is quite likely to be paid off in under 2 years, as the data above shows.


Solar batteries, based on our installation, would require a lot longer to pay off, and do not appear to be worth while at this stage.  Remember that the process of charging and discharging a battery involves losses of at least 15%, so by not having a battery, you have at least 15% more energy to use in-house or sell as feed-in.  You may also have to replace the battery in 10 years or so.


It is also surprising at just how quickly a battery discharges when powering the house at night, or during a blackout.  If you are intending to install a battery, I suggest you aim for the largest capacity you can, commensurate with having sufficient solar panels to keep such a battery fully charged.


2 Replies 14530 Views

Thanks for that article -  I was in 2 minds whether to get a powerwall to go with my 10 kw panels (limited to 5 kw  Feed in )   after  reading that it would save $320  a year I was convinced   - no battery  !  I will keep article for  further reading   -  a lot of work  well done   - i'm in  Adelaide

2 Replies 14155 Views



Confused about your statement "my 10 kw panels (limited to 5 kw Feed in ) ".


Does this mean you can only put a maximum of 5kw into the grid?


So you have 10KW of solar panels and a single 5KW inverter?


Why are you wasting this opportunity?




0 Replies 14150 Views

I was going to  get  3kw   more panels  than the  5 kw I already had  - but the  installer ended up giving me 6,6 kw  system  plus a free 5 kw  inverter  for the original  quote  price  - I  would have been stupid not to take that deal  even though I can only export  5 kw  -  i can still run everything  during the day   --   cheers      Finnegan

1 Reply 14142 Views



in SA, SA Power Networks limits feed-in to 5 kW, but allows up to 10 kW of inverter capacity.

You are also allowed to have 1.33  more panel kWs than the rated capacity of the inverter.  That is how you can have 6.66 kW and a 5 kW inverter.

I am installing another 5 kW next Tuesday, along with another Fronius 4 kW inverter, to go with my existing 5 kW Fronius inverter.  The panels are facing east and west, so their output will be 16% to 18% below their rated power.  So, even though I'm installing a total of 5.28 kW of extra panels, it is better to use a 4 kW inverter running nearer to its capacity, and therefore more eficiently.





1 Reply 14139 Views

hi  Richard      - so are u going to get a battery  ?   I see your point about 4kw  inverter  - but I got  my 5kw  Fronius   brand new  for free   -  lol   can't get much more efficient than that   -  cheers  Graham    (  Finnegan  is my  dog )

1 Reply 14137 Views

Hi Graham,


I've been in the AGL VPP since 26/05/17, and have a Tesla Powerwall 2 battery.

Have a look at some of my other postings, and you can see a detailed analysis of the cost/benefit of panels and batteries.  In particular, check out The Significant Cost Benefits of Solar Panels and An Engineer's Case Study: Panels Are A Must, But Forget The Battery





1 Reply 14135 Views
hi  Richard I did read your previous posts  - u said  you only save $320   a year with battery  -  so  it’s  going to take 12 years to  get back the $4,500   it costs  --      as long as the panels  pay for my elect  Plus the gas I’m happy  -   which they do   -  I was with Energy aust I looked up a winter bill from  last year June, July , August  it was $78      that was before I got the extra panels    --   & I have the air cond  heater going every night    -- besides I have been told I will have to upgrade switchboard  to get  VPP       -  $1800        lot of money for a few circuit breakers
1 Reply 14125 Views

Hi Graham,


I also have a spreadsheet of 30+ SA electricity plans and 19 SA gas plans.

If you live in SA and would like me to tell you accurately and definitively which are the best plans for you, I can do so if you send me your bills.

One year's worth will give a very good result, although it can be done from one bill, but two years would be slightly better as it would average over repeated seasons.


My email address is for easier communication.





0 Replies 14120 Views

hi  Richard   - I don't think  u  are getting my emails at that address