Renewables and Energy Technology

Wind turbines, solar panels, home battery storage - if it's discussion about renewable energy you're after, you'll find it here.

Wind turbines, solar panels, home battery storage - if it's discussion about renewable energy you're after, you'll find it here.

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


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.




hi  Richard   -  did  u get any of my emails    on your  gmail    ----  i've had this trouble before  -- I send the emails ok but people don't get them   -  only happens with gmail   -----     have to take dog for exercise be back later   - looks like i'll have to go through this program all the time  - have u got any other email accounts   cheers  Graham