How To Save Money With The South Australian Home Battery Scheme

South Australian Home Battery scheme

Wahey! In SA it is possible to save money with batteries – thanks to the battery rebate.

For a long time now Australia has been drawing closer to the day when home batteries will pay for themselves.  With the announcement of  South Australia’s battery subsidy scheme we know that day will soon come — unless the scheme’s lack of competition prevents battery prices from falling enough.

First I want to give warning to everyone who might be in a rush to buy solar batteries.  Even with the large subsidy from the South Australian government it is still possible to lose money buying home batteries.  You are only likely to come out ahead if:

  • You can buy a battery system at a low cost per kilowatt-hour of storage: typically by buying 10 kilowatt-hours or more.
  • You have a solar power system with at least 5 kilowatts of panels and usually produce enough surplus solar electricity to fully charge the battery on winter days.  If you don’t have a solar power system I recommend getting 6.5 kilowatts or more.
  • Your average overnight electricity consumption each month is greater than the usable storage capacity of the battery.
  • The battery system lasts well beyond its warranty.

Meet these criteria, take a gamble on the system significantly outlasting its warranty, and a subsidised home battery system might save you money.  Just be aware that the return won’t be as good as rooftop solar without batteries, the average return from the Australian share market, or many home energy efficiency improvements.

Because battery sales may be about to take off I’m going to cover eleven things you need to consider before buying one.  Then I’ll examine how good an investment they are.

Major Details Of The Battery Subsidy

While we still don’t have a full picture of South Australia’s battery subsidy, the main points are:

  • The subsidy will be $500 per kilowatt-hour of storage or $600 per kilowatt-hour for pensioners and people receiving most Centrelink payments.
  • The maximum subsidy for any household is $6,000 and the minimum size is 2.5 kilowatt-hours.
  • It starts in October[1. Update 9:57 am 21st Sept: The SA battery subsidy scheme apparently starts on the 18th of October.] but battery choice will be very limited until December.
  • Only ‘CEC Approved Retailers‘ are allowed to sell batteries under the scheme. This limits it to 4 South Australian solar companies or about 20 national retailers.

I don’t know why only ‘CEC Approved Retailers’ will be able to install subsidised batteries.  This excludes some of the best solar installation companies in Adelaide. The State Government says it’s in favour of small business.  Perhaps what they actually meant was they are in favour of a small number of businesses.  Whatever the reason, as a result there is a lack of competition so we can’t be certain we will see prices low enough for solar batteries to pay for themselves soon.  But we will get there in time as battery prices are falling rapidly.

Home Batteries = More Complicated Than Solar

The basics of rooftop solar aren’t too complex.  Solar panels turn sunlight into electrical energy and your solar inverter turns it into the type of current homes use.  But when you start to get into the details it is more complex and I’ve seen plenty of people get confused or tied up in knots through no fault of their own.

But batteries — they are a whole new level of complex.  I’m here to help you out, but even I’m finding it difficult to keep everything straight.  Working out whether or not a solar battery is likely to save you money is no easy task.  Here are 11 factors to be considered:

  1. The total installed cost of the battery after subsidy.
  2. The battery’s usable capacity, which will be less than its total capacity — with one or two exceptions.
  3. The total number of kilowatt-hours of storage you are likely to get out of it over its lifetime before it dies.
  4. The capacity factor you will use it at.  If you use most of its usable storage capacity each day it may pay for itself.  If you only use it at a capacity factor of 50% then even at subsidised prices it’s unlikely to pay for itself.
  5. Does your solar power system regularly produce enough surplus electricity to fully charge the battery?  If you can’t fully charge it in winter or on cloudy days it will reduce its capacity factor.
  6. What type of electricity tariff are you on and how much do you pay per kilowatt-hour after discounts?
  7. What is your solar feed-in tariff?  The higher the feed-in tariff the lower the return on your solar system.  This does not mean that if your feed-in tariff is low you should get a battery.  It means you should get a higher feed-in tariff.
  8. The battery system’s round trip efficiency.  The less energy the battery system wastes the better the return.
  9. Do you need (or want) to pay extra for backup capability so the battery will work during a blackout?
  10. Is your battery virtual power plant compatible?
  11. Do you have better options when it comes to investing your money?

Battery System Cost

There are a range of different home battery systems on the market.  Fortunately we have a Battery Comparison Table with the details of every home battery system on the market in Australia we’re aware of.  If you look at the prices you’ll see they range from $1,700 for the 2 kilowatt-hour Soltaro 2 up to $26,000 for the 11.6 kilowatt-hour Sunverge SIS.  Most of these prices are estimates of the uninstalled retail price based on information manufacturers and importers have given us, but the Sunverge SIS has one of the few where we’ve been given the fully installed price.

Installed Cost

It would be nice if I could give you a single estimate of how much solar batteries cost to install so you could simply add that to the uninstalled price.  Unfortunately, batteries are far too complex for that.  There are several different ways to install them and plenty of variation in just how difficult they are to get working.  When you are buying a battery what you’ll need to look at is the total installed price after subsidy.  Later I will give an estimated installed price for a system based on prices I have seen advertised.

Usable Storage Capacity

Nearly all solar batteries suffer damage if they are completely drained.  Because of this their usable capacity is less than their nominal capacity.  The exception is the Redflow ZCELL, which has no problem at all being sucked dry since it is a zinc bromide battery and not lithium.  Unfortunately for lithium batteries both their nominal and usable capacities will decline with use and over time.   Most batteries warrant they will maintain at least 70% of their original capacity but some only promise 60%.  So after 9 years a battery that started with 10 kilowatt-hours of usable storage might only have 6.1 usable kilowatt-hours and still be within the terms of its warranty.

Battery Cycles

For our purposes a battery cycle occurs when a battery system is fully charged and then its usable capacity is then fully discharged.  On average home batteries are cycled less than once per day.  If your solar battery becomes part of a virtual power plant this may raise the number of cycles above one.  Most battery warranties are for 10 years but some cover a set number of cycles instead.  For example Sonnen’s warrants its battery systems for 10 years or 10,000 cycles — which. ever comes first.  This means you could average 2.7 cycles a day and still not hit 10,000 before the warranty runs out.  This is far more than you actually need as most homes use less than one cycle a day.

Cost Per Warranted Kilowatt-Hour With One Cycle Per Day

Differences in nominal storage capacity, usable storage capacity, and warranties make it difficult to compare battery systems on price.  To simplify things we have included the cost per warranted kilowatt-hour when cycled once per day on our battery comparison table.  The result for every battery system on the table is on the graph below.  It starts at 20 cents per kilowatt-hour for the DCS PV 13.5 and goes all the way to $1.07 for the Hansol AIO 7.2:

Solar battery costs per warranted kilowatt-hour

Three Ways To Install Battery Systems

The three different colours in the chart above represent three types of battery systems:

  • Orange – Batteries Only:  These systems are batteries in a box — or sometimes not even a box1.  If you already have a solar power system it is unlikely that your inverter is a hybrid.  The exception to this is Huawei whose standard solar inverter is a hybrid.
  • Dark Red – Batteries & Battery Inverter:  These systems have a built-in battery inverter.  They work independently of your solar power system so there are no compatibility issues with your current solar inverter.  This makes them very convenient for people who already have solar panels.  The disadvantage of these systems is that homes in South Australia with single phase power, which is most of them, are now limited to a total of 5 kilowatts of inverter capacity including solar and battery inverters.  It can be possible to get around this with export limiting but this is an extra cost.  The most famous — and lowest cost per kilowatt-hour — example this type of system is the Tesla Powerwall 2.  Unfortunately, according to Tesla, no more Powerwalls will be available until time early 2019.
  • Blue – All-In-One:  These battery systems are all-in-one units that come with a hybrid inverter that you can directly plug solar panels into. No separate solar inverter required.  The lowest priced example of this is the Opal Storage 13 kilowatt-hour system which has a 4.6 kilowatt SolaX inverter that can accept up to 6.13 kilowatts of solar panels.

If you already have a solar power system, then getting one of the battery systems in dark red on the chart is the simplest option as they retrofit to any existing solar system and you won’t need to get rid of your existing solar inverter.  But this doesn’t mean you shouldn’t consider other options.  For example, you may consider it worthwhile to replace an old inverter with a new hybrid inverter with a fresh warranty.

If you don’t have a solar power system2 then you have nothing to lose by considering all battery system types when working out which is best for you.

Installed Cost Per Kilowatt-Hour

It is possible to get battery systems installed for around $1,000 per kilowatt-hour.  While not all the installers making these kinds of offers are what I would call reputable, reputable ones are definitely among them.

But looking at wholesale pricing, I don’t see how it’s possible to make a sustainable margin installing solar batteries at that price and I don’t think anyone is.  To me it looks like installers are simply positioning themselves in the battery market, discouraging competition, and building up experience and goodwill ready for when batteries take off — such as when South Australia introduces a massive battery subsidy.  I feel bad for those that spent money preparing to become battery installers and are now effectively locked out of the market because they aren’t one of the few businesses permitted to install subsidised systems.

I’m sure the approved companies are carefully considering what kind of deals they will offer over the next few months.  I hope that we’ll see similar low prices for systems that attract the subsidy.  But if we don’t see them in the next few months presumably all we’ll have to do is wait, as solar batteries will continue to fall in price.

Be Wary When It Comes To Solar & Battery Packages

While the details aren’t yet released, it will be possible to get a low-interest loan for the balance of installed battery system price.  If you get solar panels installed at the same time it will can be covered by the same loan.  For this reason, if you need solar too, it makes sense to get a package deal.  But be wary of companies who offer a great deal on a battery as cover for charging you too much for a solar system.  Make sure it’s not possible to get a good quality solar and battery for a better price by getting them separately.  Also watch out for anyone who makes the return from batteries sound better than it is by blending the payback of solar and batteries together.

Battery Size And Subsidy

The battery subsidy is $500 per kilowatt-hour of storage or $600 for pensioners and most people who receive Centrelink payments.  While it hasn’t been make clear yet, I am confident this will apply to the nominal capacity and not the usable capacity of battery systems.  A battery has to have at least 2.5 kilowatt-hours of capacity to receive the subsidy and the maximum amount of subsidy received for a system is $6,000.  So it tops out at 12 kilowatt-hours (or 10 kilowatt-hours for pensioners and Centrelinkers).

Despite this it can make sense to get an even bigger battery — provided your electricity consumption is high enough.

Backup Capability

Not all battery systems work during a blackout.  And yes, this does sound dumb.  If you want backup capability — and most people do — you usually have to pay extra for it.  I think many people overrate how useful backup capability is.  Rather than explain why here I’ll just note that when it comes to working out how good an investment subsidised batteries are I’m not going to bother to place a value on it.  But please feel free to add in any value you think is right.  Just write it on your screen with a marker pen.  Don’t worry —  you can always liquid paper it out later.

Capacity Factor

If you buy a large battery with 13.5 kilowatt-hours of usable storage and then only use 4.5 kilowatt-hours per night, you only use one-third of the battery’s capacity. Its capacity factor is one-third or 33%.  The higher the capacity factor the faster it will pay for itself, but smaller batteries cost more per kilowatt-hour.  So it can be more cost-effective to use a larger battery at a capacity factor of 80% and not a smaller battery at a capacity factor of 95%.

Two things that determine a battery’s capacity factor are:

  • If the solar system regularly produces enough surplus solar electricity to fully charge the battery.
  • The household’s overnight electricity consumption.

Electricity Tariffs And Batteries

As far as I am aware, South Australia does not have the year round time-of-use tariffs, common in other states, that can improve the economics of batteries.

Demand tariffs are available and these may make sense for batteries.  Looking at AGL’s demand tariff plan, which is the one with the highest feed-in tariff of 16.3 cents, I see that after discounts they charge 31 cents per kilowatt-hour at all times. On top of that you have to pay a demand charge.  This varies depending on the peak power you hit but is a minimum of 31.57 cents a day.  It is theoretically possible for a battery system to keep this charge at the minimum by ‘lopping the peaks’.  But in practice homes with battery systems generally use very little grid electricity, so the low usage rates are likely to have little value.  Also, if the battery’s priority is to maximise self consumption, it may not have enough charge available to lop peaks consistently.  For these reasons I will use a standard tariff when considering potential savings from a solar battery system.

The large majority of South Australian homes have a standard tariff which charges a flat rate per kilowatt-hour of grid electricity used.  While the amount varies depending one which retail electricity plan a household had, after discounts, it’s typically around 38 cents.

Solar Feed-In Tariffs And Batteries

The higher your solar feed-in tariff the less value a home battery gives.  This is because using a kilowatt-hour of stored electricity at night may save you 38 cents, but storing that energy costs you the feed-in tariff that you would have otherwise gained.  So if your feed-in tariff was 16.3 cents then with a 100% efficient battery you would only save 21.7 cents per kilowatt-hour of stored electricity you use.  That’s the 38 cent cost of grid electricity minus the 16.3 cent feed-in tariff forgone.  Unfortunately, no battery is 100% efficient.

It is possible to get a feed-in tariff of 20 cents from AGL.  If that is not suitable or becomes unavailable AGL also offers 16.3 cents.  If your solar power system produces enough surplus electricity to regularly fully charge a battery then it likely makes sense for you to have a feed-in tariff of 16.3 cents or more.  If your feed-in tariff is low it does not mean it makes more economic sense for you to get a battery.  It most likely means you should get a higher feed-in tariff.

Virtual Power Plant Participation

To receive the subsidy a battery system must be capable of becoming part of a virtual power plant in the future.  It is not yet clear how much financial benefit this may give people.  It’s even possible unscrupulous operators will shorten the lives of batteries without providing adequate compensation.  Virtual power plants are currently only under trial and you can’t sign up an independent battery at the moment.  (Although you can join Reposit if you have a compatible battery.)  If virtual power plants make home batteries profitable in the future then the time to let that influence your decision on whether or not to get a solar battery would be in the future.

Size Your Battery Part #1: How Many kWh Do You Export?

If your solar system isn’t regularly producing enough surplus electricity to charge your battery during the cloudiest months of the year you won’t be able to use it at high capacity.  Not unless you charge it from the grid – and that doesn’t make economic sense in South Australia at the moment.

If you already have a solar power system it’s not difficult to estimate how much surplus electricity it generates during the lowest output months of the year: May, June, and July.  All you need to do is:

  1. Look through your electricity bills to find the three months in which you earned the least from exports.
  2. Add the number of kilowatt-hours that received a feed-in tariff in those three months and then divide by the total number of days in those months.
  3. This gives you the average daily surplus solar electricity available to charge your battery during the worst three months of the year.

If you have a solar system with 5 kilowatts of north facing panels and your electricity consumption is fairly typical, then there’s a good chance you’ll find it sent an average of 8.5 kilowatt-hours of electricity into the grid each day over May, June, and July.  You may think this means you can install a solar battery with 8.5 kilowatt-hours of usable storage capacity and use it at a high capacity factor over the whole year, but it’s not that simple.  Because of losses that result from charging and discharging it is necessary to put more energy into a battery than you can get out.  For this reason it is necessary to look at the round trip efficiency of a battery system.

Battery round trip efff

(Image: Lithium Ion Battery Test Centre)

The chart above shows round trip efficiencies for a variety of brand new battery systems determined by the Canberra Battery Test Centre.  The result for the SimpliPhi battery may be better than indicated as they found they didn’t install it correctly.  The average round trip efficiency of the lithium batteries is 92%.  The Powerwall 2 isn’t on that chart.  Tesla gives its round trip efficiency as 88% but I have access to figures that suggest its actual performance is worse.  However, I will wait and see if it improves after its master gets tired of trying to find ways of tricking it into making a mistake.

Because the efficiencies of all solar batteries are likely to get worse over time, I expect lithium batteries will have overall losses of at least 10%. So if your solar power system exports an average of 8.5 kilowatt-hours a day in the lowest output months of the year then your battery’s usable capacity should be about 90% of that, which is 7.7 kilowatt-hours or less, if you wish to use your battery at high capacity from the start.  On the other hand, because the capacity of lithium and most other batteries will decay over time, you may wish to install a solar battery with larger usable capacity to allow for this.  I would expect lithium batteries to have lost at least 10% of capacity by the middle of their lifespan.

The Redflow battery on the chart above has the worst efficiency because it uses zinc bromide chemistry instead of lithium.  While its efficiency is also expected to decline over time it has the advantage that its storage capacity won’t.

Average Solar Exports By System Size

If you don’t have solar power yet and are planning to get batteries I suggest installing as much .capacity as you practically can.  If you have 3 phase power and a large roof you shouldn’t have a problem installing enough solar to fully charge a large battery.  If you have single phase power as most people do then you have two options:

  • Install a solar system as close to the normal single phase maximum of 6.66 kilowatts of solar panels as possible.
  • Export limit your solar power system and install over 6.66 kilowatts of panels, provided you have room on your roof.  If you are installing a battery with a battery inverter then you will almost certainly need to export-limit your solar system unless you are happy with both a small solar power system and a small battery3.  With export limiting South Australian homes with single phase power can install up to 10 kilowatts of solar plus inverter capacity.

Without existing solar you won’t have a record of how many kilowatt-hours of surplus solar electricity you export, but it can be estimated.  Over the three least sunny months of the year the average household with a 6.5 kilowatt solar system that uses electricity for heating is likely to export around 11 kilowatt-hours of surplus solar electricity a day that could instead be used to charge a battery.  The amount of surplus electricity will be less if:.

  • People are usually at home during the day.
  • Electricity consumption is above average.

The amount will tend to be more if:

  • No one is usually home during the day.
  • Electricity isn’t used for heating.
  • Electricity consumption is below average.

So the actual amount could range from over 14 kilowatt-hours to under 8 kilowatt-hours.

If an average home had only 5 kilowatts of solar panels instead of 6.5 then I would expect it to produce an average of around 7.5 kilowatt-hours of surplus electricity per day during the worst months of the year with a range from under 5.5 to over 9.5 kilowatt-hours depending on the household’s characteristics.

Size Your Battery Part #2: How Much Electricity Do You Use After Dark?

Assuming you already have solar, knowing how much surplus solar electricity you export determines what size battery you can fully charge most days.  The next step is to calculate your average overnight electricity consumption to see whether you will drain the battery most nights. To do this you should look at your average grid consumption during the months of low grid electricity use.

I recommend looking at your electricity bills for the past 12 months and finding the three months in which you had the lowest grid electricity consumption.  Add your grid electricity consumption for those three months and then divide by the total number of days in them.  This will give you your average daily grid electricity consumption for those months which will mostly be from a couple of hours before sunset when solar output starts to rapidly fall off to a couple of hours after sunrise when solar electricity output starts to become significant.  It will also include some grid electricity use during the day which will mostly occur on cloudy days.

If you don’t have solar then you can or read your meter at sundown, then again at sunrise. Subtract the two and you’ll have last night’s electricity usage.

A typical household with a 6.5 kilowatt solar system may use an average of 7 kilowatt-hours of grid electricity a day during the months of lowest grid electricity consumption.  This appears to suggest that if they install a battery system with 7 kilowatt-hours of usable storage they will be able to use it at high capacity.  But most household’s electricity use is quite variable from day-to-day.  It’s common for grid consumption on one day to be 50% higher than average and the next day to be 50% lower than average.  So for this reason, to use a solar battery at high capacity through the year its usable storage capacity should be about 75% of the average daily grid electricity consumption for the lowest 3 months.  So instead of 7 kilowatt-hours of usable battery storage they should get around 5.25 kilowatt-hours.

Example Solar Systems — LG Chem RESUs

I am going to pick a couple of example solar battery systems and estimate how much their installed cost will be.  I’m not going to put any effort into selecting a battery system.  I’m simply going with the most commonly installed one in Australia which is the LG Chem RESU.  I will consider the RESU6.5 with 5.9 kilowatt-hours of usable storage and the RESU10 with 8.8 kilowatt-hours of usable storage.  These will specifically be the lower voltage RESUs, but the results for the RESU10 should be similar to those for the higher voltage RESU10HV.

I am going to assume the pre-subsidy installed cost of the larger RESU will be around $1,000 per nominal kilowatt-hour and will cost $10,000.  And I’ll assume the RESU 6.5 will cost slightly more per nominal kilowatt-hour and will cost $8,000 installed.

Installed Cost After Subsidy

With a subsidy of $500 per nominal kilowatt-hour the RESU 6.5 will have an installed cost of $4,750.  For pensioners and Centrelinkers the subsidy will be $600 per kilowatt-hour and its installed price will be $4,100.

For most households the installed cost of the RESU10 will be $5,100 and for Centrelinkers it will be $4,120.

Lifetime Kilowatt-Hours Of Storage

LG Chem RESUs come with a warranty for 10 years or:

  • 20,000 kilowatt-hours throughput for the RESU6.5
  • 30,000 kilowatt-hours throughput for the RESU10

Because their usable capacity diminishes over time and because most households are likely to cycle them less than once per day the warranty should last for the full 10 years.  If they are cycled once per day at an average of 80% of its usable capacity when new, the amount of time required to reach the energy storage limits given in their warranty would be:

  • 11.6 years for the RESU6.5
  • 11.7 years for the RESU10

While the warranty is for a maximum of 10 years it is reasonable to expect the average battery system to last longer than this.  But solar batteries are not like refrigerators where they may have a warranty of 2 years but we expect them to last at least 10.  Batteries deteriorate with use and over time and survival beyond their warranty period will unfortunately be limited.  I’ll be fairly optimistic and assume the average unit will survive for 15 years, which is 50% longer than what its warranty covers.  I’m also going to optimistically assume the hybrid inverter (or battery inverter) will also last for 15 years even though it may only have a warranty of 5 years.  While the electronics in inverters don’t decay in the same certain way the chemicals in batteries do, making no allowance at all for inverter replacement for 15 years is still optimistic.

Investment Comparison

There are a number of accounting methods to find the rate of return on an investment.  While they can be useful in the right circumstances, they do rely on knowing what people’s cost of capital is, which can vary from person to person and can get very complex.  So I am just going to work out how much money in electricity bill savings you will have at the end of a battery system’s estimated 15 year life.  Then I’ll compare that to what you would have if you invested the money elsewhere.

Battery Savings Assumptions

I am using the following assumptions to find the return from our example battery systems.:

  • The household uses the South Australian average of around 5,000 kilowatt-hours of electricity a year.
  • It has a 6.5 kilowatt solar power system.
  • 2,000 kilowatt-hours of its electricity use comes from solar and the rest is from the grid.
  • It exports an average of 11 kilowatt-hours of solar electricity a day during the lowest output 3 months of the year.
  • It average daily electricity consumption during the 3 lowest output months is 9.5 kilowatt-hours.
  • The cost of grid electricity after discounts is 38 cents per kilowatt-hour
  • The solar feed-in tariff is 16.3 cents per kilowatt-hour.
  • Electricity prices and feed-in tariffs remain constant in real terms.  Results are given in today’s money.
  • The battery’s average usable capacity will be 85% of its usable capacity when new.
  • The battery system’s average efficiency is 90%.

RESU6.5 Savings

With the above assumptions the household should have no problem using a RESU6.5 at a high capacity-factor.  Because there will still be periods of particularly bad weather when solar electricity output is low and times when overnight electricity consumption is exceptionally low I will assume its capacity factor will be 90%.  This means the household will use an average of 4.5 kilowatt-hours of stored electricity per day.  With a feed-in tariff of 16.3 cents and 90% round trip efficiency they will have to forgo 18.1 cents in feed-in tariff for each kilowatt-hour of storage they use.  This means each kilowatt-hour of storage will save them 19.9 cents on their electricity bills for a daily average of 89.6 cents.  Over 15 years the savings will come to $4,910

RESU10 Savings

With an average daily electricity consumption of 11.5 kilowatt-hours during low output months the household would be able to use a RESU10 at high capacity.  However, the number of days when bad weather would prevent it from being fully charged would be increased.  So I will estimate its capacity factor will be 80%.  This means they will use an average of 6 kilowatt-hours of storage a day for a daily saving of $1.19.  Over 15 years this will save them a total of around $6,540.

Batteries Vs. Term Deposit

Now that I’ve worked out how much the two different sized systems will save on electricity bills I can compare the results to investing the money elsewhere.  Because there are 2 system sizes and two subsidy levels — $500 per kilowatt-hour and $600 per kilowatt-hour — there are a total of four prices:

  • RESU6.5 installed for $4,750
  • RESU6.5 installed for $4,100
  • RESU10 installed for $5,100
  • RESU10 installed for $4,120

I’m going to show on a graph how much these amounts would equal in today’s money if they were invested in a term deposit for 15 years4 and compare them to the savings on electricity bills the batteries give.  But to make the comparison fair I am going to assume all the battery savings were also put in a term deposit.

Solar battery savings after 15 years

Looking at the graph we can see the following:

  • A normal household receiving a $500 per kilowatt-hour subsidy is better off putting their money in a term deposit than getting a RESU6.5.
  • While household receiving a $600 per kilowatt-hour subsidy is better off with a RESU6.5 than putting the money in a term deposit, getting a RESU10 is clearly a better deal for both subsidy levels.

What Batteries Have To Beat

I’ve shown that, with fairly optimistic assumptions, it’s possible for a household that buys a battery to accumulate more money after 15 years than if they put the money into a term deposit.  But there are plenty of people in South Australia who have better things to do with their money than to put it into a term deposit.  The first thing I recommend is to get the larger battery.  Because of the way the subsidy works there is very little difference in price and it provides a better return:

  • Households that will receive the $500 per kilowatt-hour subsidy are better off buying a RESU10 if their best alternative is to invest the money at a real interest rate of 3.3% or less.  This means getting a solar battery is a better investment than making an extra home loan payment.  But if you have a business loan at a real interest rate of 4% it makes more sense to pay that off.
  • Those receiving the $600 per kilowatt-hour subsidy are better off buying a RESU10 if their best alternative is to invest at a real interest rate of 6.5% or less.  So if you had a car loan at 7.5% real interest you’d be better off paying that off.

One thing batteries can’t beat in South Australia is rooftop solar.  Even if you use no solar electricity yourself and send it all into the grid for a feed-in tariff you will still get a better return than from a battery at typical solar installation costs today.  So if you are ever considering installing solar and a battery always consider if you would be better off just installing more solar.

A Word Of Caution

I’ve said it’s possible for subsidised solar batteries to pay for themselves under a specific set of assumptions.  But there is no guarantee these assumptions are correct and I recommend being more pessimistic than me. I especially recommend pensioners, impressed by the extra subsidy they receive, carefully consider if they are likely to save money as their overnight electricity consumption is likely lower than average making it difficult for any battery to pay for itself.

If you delay you might lose a little money by not starting to save sooner.  But if you dive in quick you might lose a lot.  Of course, if you just want a solar battery and don’t care about the cost, please dive right in.  I’ll be interested to see what happens.


  1. The individual battery cells will be in a box, but some are just naked modules and you are expected to put them in a case or rack you supply yourself.  They need a compatible hybrid (mulitmode) inverter or battery inverter to work. It is also possible to connect batteries with a DC converter such as the Goodwe GW2500-BP,but this approach is not common.
  2. or want to replace or supplement your existing system
  3. Or rather a low power battery.  Its actual size could technically be large.
  4. Because I am providing results in today’s money I need to use a real rate of return for the term deposit.  As the interest rate on term deposits is now nearly 3% and inflation over the past year has been 1.9% I have used a real interest rate of 1% for the term deposit.
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. Tim Efthymiou says

    Hi Ronald and everyone. I’ll simplify everything for you without graphs or complicated calculation.
    I have a 10.4 k.w. solar system attached to three batteries (1 powerwall 1 and two lgs resu (10 k.w. and 6.5 k.w.))
    We have three phase at our place, without getting into too much details of which phases there on, hot water system etc. etc.
    My calculation is simple. I used to pay $1,000 to $1,250 per quarter in electricity before the solar/batteries, and now I get a CREDIT in the vicinity of $200 (winter) and $350 (summer).
    Cost of total system $30,000
    Return time of my investment about 5.5 – 6 years. (not taking into consideration any future price hikes in electricity).

    I could go on and complicate things dramatically, by trying to calculate degradation of batteries, cost per k.w. and a whole lot of other calculations, but, in my opinion, I have made a good decision and it’s paying off for me big time. You and others can draw up graphs and do a million calculations, but mine is quite simple cost of system and return of my money. Can’t get simpler than that.

    If anyone has any question (mine is a very complicated set up, with two individual solar systems and hooked up to the phases that draw the most power and I turn on the hot water system to heat up when the batteries are full (about 10.20 am Sydney, Sept. to April)) I will try to answer any of your questions.

    • Finn Peacock says

      Yep – that is blended payback that makes even the PW1 look good:

      There is nothing wrong with buying batteries to get the last few hundred dollars off your bills. If you’ve got the money for batteries and enjoy getting a credit instead of a bill that’s great!

      But people have a right to know the separate paybacks of the solar and battery components so they can put their money where it will give them the best return. That’s why it is so important not to only use the blended figure.

    • Yes Minister says

      Solar batteries don’t need to be anywhere near as complicated as some choose to make them. Firstly, forget the Tesla / LG hype, they are not cost-effective. Secondly, import batteries direct from the chinese manufacturers. When its all said and done, almost everything is made in China anyway, and purchasing from anyone else simply means you are paying huge markups including the Australia tax. Thirdly, local warranties are rarely worth the paper on which they are written, in fact I’ve had far better experiences with claims on chinese manufacturers than I have from certain well known Australian companies, ESPECIALLY one on the Sunshine Coast. Personally I don’t give a rats what people do. I’m not in the industry so I don’t make a profit from flogging anything. That said, I get very annoyed with the ‘experts’ who for whatever reason spread the word that solar storage is only for rich people.

  2. Mark McClurg says

    “I don’t know why only ‘CEC Approved Retailers’ will be able to install subsidised batteries. This excludes some of the best solar installation companies in Adelaide” If they are truly some of the best solar installation companies then why don’t they apply to join the CEC Approved Retailer code? It’s not that expensive to do so – BUT – you do have to meet the CEC’s strict guidelines on sales behaviour and make sure that your T&C’s are underwritten against Australian Consumer Law and that you honour to warrant the entire system for 5 years irrespective if the battery company is no longer around to warrant their manufactured battery! so I am guessing that this rule was put in place rather wisely to help protect the consumer rather than let ‘pop up’ solar companies profit off the scheme who have no intention of being around to honour their workmanship or product warranty.

    • Finn Peacock says

      I like the CEC Approved Retailer Scheme as a voluntary code of conduct.

      I have an issue when it becomes mandatory. Like in the SA Battery scheme.

      • Patrick Comerford says

        If the Queensland experience is any indicator then installers will do what’s in their own best interests, no need to worry about mandatory pressures. Before the Qld state government introduced their interest free loan for PV installs to qualifying applicants in June this year there were about four or five approved retailers in SEQ. Since then many installers are now approved retailers and able to participate in the scheme. So it’s clear that their is a clear benefit in these “subsidy schemes” in raising the standard across the industry which is often stated as being sorely needed.

  3. Not much use for us here in the West.

  4. Hi Ron,
    One thing to consider for users. How are they going to monitor the batteries for warranty purposes. I spoke to one agent who advised the manufacturer behind their battery products have never paid out on a warranty claim. The manufacturer wants to the minute data log of the batteries to prove that the user has always stayed within the warranty requirements. No user can do that. They then offer them 20% off a new set. The thing I like about Tesla, Tesla monitor their own products and are responsible to the setup.
    I make these claims based on being an engineer, live off grid, have owned a set of Aquion batteries (the engineers behind these batteries should be de registered for falsifying results), now owning 72kWh of Narada RexC and living the issues of living off grid.

    • Finn Peacock says

      My personal (non-lawyer opinion) is that that the battery manufacturer is breaking Australian Consumer Law by being unreasonable.

  5. Bret Busby in Western Australia says

    Bob P –

    “Not much use for us here in the West.”

    Assuming that, by “the West”, you mean Western Australia, rather than Western NSW;

    1. Lobby your members of the state parliament, to get the same for here – you have a member of the lower house, and, several members of the upper house. Email each of them, and tell them that wer should also have at least the same assistance – if it is good enough for South Australia to have it, so also should Western Australia have it.

    2. We need for a lobby group, for increasing solar energy usage, like Solar Citizens, here in Western Australia. Amongst other things, a general feral election is coming up soon, maybe in a month, maybe in six months. We need a local (state, as in WA) lobby group, to promote the use (and, increased assistance for the use) of solar energy, including battery storage (I have received two notifications, from Western Power, showing that the week after next, we are to have two days of no grid electricity – two days of day-long “scheduled outages, in the same week), so, battery storage with backup provision, would be useful.

  6. So where have you heard that it is to be released October 18th? Can’t seem to find any updates on the dates it is to kick in.


  7. Folks,
    Leaving the specifics of SA aside, I take issue with the first sentence in this article “For a long time now Australia has been drawing closer to the day when home batteries will pay for themselves”.

    Home batteries will pay for themselves only while governments subsidise them to the point at which they look financially attractive.

    Don’t think the price of lithium batteries is going to fall in a big way. Remember, all over the world there are sites like this, talking up solar/battery power and promoting the use of lithium-based batteries. Lithium isn’t like coal, in the sense that coal is readily available in virtually unlimited quantities. Lithium is more like oil – we’ve passed the point of “peak oil” and I suspect we’re close to “peak Lithium” – from now on the price may actually start rising.

    The sensible approach would be to use sodium ion batteries. They are a great deal heavier (so they are no good for cars or camcorders) but are fine for houses and businesses, or any application where you don’t actually have to move them about. You don’t see many of them around, but wait until the lithium price starts to rise, then you’ll see R&D going that way. But, it will all take time, so – as I said – don’t expect to see the price of batteries falling to economic levels any time soon.

  8. Yes Minister says

    Home batteries can already pay for themselves if one rgnores the experts pushing sexy, grosdly overpriced products. Almost everything is manufactured in China, and local resellers gladly pad the price substantially. Anyone who imports their batteries direct from the manufacturer can save heaps. In my case it was the difference between $6,000 and $21,000 for the exact same batteries.

    • Tim Efthymiou says

      Totally agree, I have first hand experience, read above comment by me.
      What most comparisons/calculations fail to address is the PEAK rate cost in N.S.W. for people with smart meters (all who have solar systems installed). peak rate is set at .62c per kwh between 2pm and 8pm. now I have not ever used electricity from the street during these times, due to my 10.4 k.w. solar system and 3 batteries, totalling 23 k.w. of storage. Like I said in my comments above, my system will pay for itself in 5.5-6 years time. I DON’T CARE WHAT COMPARISONS, CALCULATIONS, CHARTS ETC. ETC. ETC. that so called experts are doing, I’m saving, (big time) and it’s as simple as that.

  9. Yes Minister says

    Lithium batteries are unlikely to drop in price while the demand greatly exceeds the supply. Other than the sexiness created by folk with a vested interest, there is little to recommend Tesla / LG batteries. Size and weight are immaterial for solar storage.

  10. How do i get you people to stop sending me updates on this blog, no more please.

    • Ronald Brakels says

      Hi Bob

      Click on the “unsubscribe” link in the email. If there is a problem we can unsubscribe you from this end.

      (Note from Finn: I’ve found you in our system and unsubscribed you!)

  11. I have a 3.6 kw monocrystaline system that averages 1000 units a quarter of which I use 100.
    So 900 units a quarter go into the grid.
    With gas for cooking, watching TV for 8 hrs and a small refrigerator my consumption averages 300 units a quarter.
    So that is 900 units a quarter feed in tariff of 20 cents with AGL in NSW.
    So a REDFLOW 10 kWh battery would be my battery of choice starting with a fully charged REDFLOW battery supplying up to 7 units a day.
    With OFF PEAK top up occassionaly when required.
    Having paid $16000 for the 3.6 PV system that now costs about $2000 it may pay me to wait for lower battery prices.
    Does that add up please

    • Ronald Brakels says

      Hi John

      Because its round trip efficiency isn’t high a Redflow battery needs to be charged with around 1.5 kilowatt-hours to provide 1 kilowatt-hour of stored energy. This means for each kilowatt-hour of stored electricity you use you will forgo 30 cents of solar feed-in tariff. If you have a standard flat tariff you might be paying around 31 cents per kilowatt-hour of grid electricity used. This means the battery will only save you around 10 cents a day or $37 a year. So batteries will have to become considerably cheaper before they can pay for themselves.

      • Tim Efthymiou says

        Hi John, battery prices might not fall in price anytime soon. As you may have heard that Tesla has actually increased their price for the pw2 as more government grants come into effect. (which in turn, companies will want to rip off consumers as much as possible). We just need more players in the battery producing industry. When this will happen is any ones guess. Prices could fall dramatically (as with solar systems) or they could remain stable for years to come. My guess is that they will remain stable for a while. I have spent $30,000 on a 10.4 k.w. solar system with 3 batteries (24kw battery capacity) about 21/2 years ago, and the prices have remained the same as when I installed my system.

  12. Hello, everyone, first of all, excuse my impertinence

    I’ve been thinking about it for a couple of days, I’d like to know what you guys think.

    In this 2016 post “Is The Powerwall 2 A Good Investment?”

    After calculating the gross savings in 13.5 years of useful life Ronald discounts the initial cost of the battery because the battery “dies” after 13.5 years and we assume it is worthless.

    “…Using these assumptions, after 13.5 years I can save around $23,790 on electricity bills in Sydney and $22,370 in Perth with a Powerwall 2.

    But because my initial $10,150 investment in the Powerwall 2 is now worthless I would actually only be about $13,640 ahead in Sydney and $12,220 ahead in Perth.”

    After which we obtain the net savings,

    “But, to make a fair comparison, I should also put all the savings from electricity bills into a term deposit. When this is done the total savings of a Powerwall 2 owner in Sydney would be $15,950 and in Perth they would be $14,600 ahead, which puts the Powerwall 2 clearly ahead of a term deposit in both cities.”

    In this post that we are dealing with, I do not see that the initial cost of the batteries is subtracted, despite reaching the end of the useful life of the battery, and we put the gross savings in the term deposit to “work”

    Which of the two calculation scenarios would be correct?

    • Tim Efthymiou says

      Hi Manuel, I can answer you very easily. NO CALCULATION (S) ARE CORRECT. It is extremely difficult to get a clear answer as there is way too many variables at play. For e.g. here in Sydney, time of use rates go up to 61cents pkw/h for peak rate 2pm to 8pm which is the bulk of any household electricity use. But in return you get a great off peak rate. So I have sat down and worked out the best option for me. I installed 3 batteries to my solar system, and have never paid off peak electricity rate for the PAST 5 YEARS. How many of the experts or over opiniated people actually have batteries installed at their homes ??? 90% of the time my batteries and hot water system are charged by the sun. Now in Sydney it has been raining for the past 7 days straight, so I force charge my batteries from off peak rate at night and have plenty of juice in them for the afternoon when peak rates apply. I HAVE BEEN DOING THIS SUCCESFULLY FOR THE PAST 5 YEARS. Also a word of advise, do NOT go for powerwall. I have, one powerwall 1 and two lg chem resu batteries. The L.G. batteries are way, way cheaper and have been performing better than the powerwall. They sit outside, rain/hail/shine and have never, ever let me down. So, if your interested in having a solar / battery system, get the biggest solar system you can fit on your roof and then calculate how many k.w. you use on a daily basis and purchase battery or batteries capacity to make sure you have enough battery power so you draw from the street minimal amount.
      My system consists of a 10.4 k.w. solar system split into two phases / two inverters. One is hooked upto the two l.g. batteries totalling 16.9 k.w. storage and the other is hooked upto the powerwall 1 (6.9k.w. storage). BUT i also got an electrician in, and moved most of my high demand wires (in the fuse box) to the phases that have the batteries connected to them. The third phase that doesn’t have a battery connected to it, only runs the led lights. I also have a switch on my hot water system, so when my batteries are full from the sun, I flick the switch on and the h.w.s. heats up from excess solar power, whalllla “solar hot water system”. if the weather is cloudy or rainy, I flip the switch to off peak and the h.w.s. heats up from control load 1 rate (very cheap rate). So you see my friend there is no easy way to calculate and use different scenarios to get a clear picture. You need to sit down and start from the beginning, first step is look at your electricity rates and then work from there of how you can set up your home the run as cheap as possible without running around screaming at the kids to stop using appliances. At the moment I get about $450-$480 credit every quarter and have been for the past 3 years as I have fine tuned my system. (first couple of years I didn’t look closely at how to fine tune my system and in turn I wasn’t paying electricity but I also wasn’t receiving a big credit) I also supply electricity to a tenant and he gets charged about $300 a quarter from a private sub meter which is fed from my system. Before installing my solar / battery system, we were paying about $800-$1,100 per 1/4. it’s been a great investment for me and has been working great for the past 5 years.

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