Why It Makes Sense To Fill Your Roof With Solar

roof full of solar

Q. How much solar should you install on your roof?

A. As much as you reasonably can.

In the past, working out just how many solar panels should go on a roof was a difficult job.  But now it’s as easy as 3.141591




…thanks to three developments:

  • The falling cost of rooftop solar.
  • The increasing price of electricity.
  • Higher feed-in tariffs.

It now makes sense for most people in mainland Australia to install as much solar as possible, within the following two limits:

  • The amount your Distributed Network Service Provider (DNSP) allows you to install.
  • The amount that is practical to fit on your roof2

For many homes this will be around 6.5 kilowatts of panels, either because they have single phase power and that’s close to the maximum they are usually allowed to install, or because that’s all they can easily fit on their roofs.  But people who have large rooftops and three phase power or potentially have single phase power and are willing to export limit their systems, can go much larger.

The reason why it now makes sense to go so large is because these systems can now pay for themselves regardless of your energy consumption habits, so feel free to go big and fill your goddamn roof.

Roofer Cogburn

“Fill your roof you son-of-a-bitch!” — John Wayne’s famous line from the classic movie Roofer Cogburn.

Electricity Prices have Risen And Are Heading Higher

Victorians had a hefty 10% increase in electricity prices at the start of the year and almost everyone else in the country suffered a hefty hike up the electric invoice on the first of July.  West Australians, despite a $169 a year increase in every residential electricity bill, are the only ones who haven’t had an increase in the incentive to install solar because of the darstardly way it was done.

Not only have this year’s increases made solar more attractive for most Australians, we are also heading for further price increases over the next two years.  This alone wouldn’t be enough for me to tell everyone to go big on solar but on top of that, solar feed-in tariffs are up as well.

Solar Feed-In Tariffs Are Up

Australians have already had huge increases in grid electricity prices over the past several years, but Feed in Tariffs have stayed low.

This is because the price rises up to 2016 (as shown above) were not driven by the wholesale cost of electricity, they were mostly about the ‘network gold plating’.

But the increase in electricity prices occurring now is due to an increase in generating costs.  Wholesale electricity prices have soared and this has been reflected in feed-in tariffs, which are now much higher for most Australians.

Because households typically export 70-80% of the electricity a 6.5 kilowatt solar system produces, this finally makes going big on solar worthwhile.

Many households with large solar systems will obtain more savings from their feed-in tariff than from their own consumption of solar electricity.

Feed-in-tariffs vary by retailer. If you want to see what kind of feed-in tariffs are available in your area you can use the SolarQuotes electricity plan comparison tool.

Update 6th Oct: If you are going big on solar, one thing to watch out for is limitations on how many kilowatt-hours of feed-in tariffs you can receive.  In South Australia SA Power Networks limits feed-in tariff payments to the first 45 kilowatt-hours of solar exported to the grid per day, averaged over the billing period.  As a result, it is possible people who go over 10 kilowatts of solar panels may miss out on payment for some exported solar electricity when output is high and self consumption is low.  

DNSPs Often Allow 5 kW Of Inverter Capacity Per Phase


Straight from the horse’s mouth.

Distributed Network Service Providers, or DNSPs, are in charge of the local distribution of grid power and make the rules for connecting solar to the grid.  The default position is generally to allow 5 kilowatts of inverter capacity per phase3.

Most homes in Australia have single phase power which means they can install 5 kilowatts of solar inverter capacity.  As panel capacity can be one third greater than inverter capacity, you are allowed up to 6.66 kilowatts of panels.

In practice it will have to be less than this, as the only way to get exactly 6.66 kilowatts is to have your system installed by the Devil4.

An increasing number of homes have 3 phase power which means they can install up to 15 kilowatts of inverter capacity and up to 20 kilowatts of solar panels.  That is, if they can find room for 74 or so average panels it would take.  Unless a home is massive it’s usually difficult to find room for more than 10 kilowatts of panels.

But while allowing 5 kilowatts per phase is fairly standard, there is considerable variation between DNSPs.  In some locations there are no set limits to the amount of solar capacity a property can have, but they can limit the amount of solar power exported to the grid.  Export limiting requires additional expense to set up5.

The Annual Return From A 6.5 Kilowatt Queensland System

Update 22 Sept:  I have improved upon the sloppy comparisons I originally made below by including a comparison of Internal rates of return (IRR). 

To provide a quick example of what kind of return can be obtained by installing the largest solar system that is generally practical for a household with single phase power, I will use a home in Brisbane.  To determine what they pay for grid electricity and the feed in tariff they receive, I will use the SolarQuotes electricity plan comparison tool and use the first plan that appears6 after I enter how much electricity the household exports to the grid.

The example system has the following characteristics:

  • A 5 kilowatt inverter with 6.5 kilowatts of north facing panels.
  • A cost of $7,000
  • 9,400 kilowatt-hours of solar output per year
  • A solar self consumption rate of 20%
  • A grid electricity price of 22.1 cents per kilowatt-hour
  • A solar feed-in tariff of 11 cents per kilowatt-hour
  • A yearly average of $150 is spent on maintenance and repairs
  • A lifespan of 25 years

This system will reduce household electricity bills by $1,243 a year. After subtracting the yearly average of $150 for maintenance and repairs the household will be ahead by $1,093 a year. By putting this figure into Excel I can determine its Internal Rate of Return or IRR, which is a common performance measure for investments.  The result Excel gives me is 18.2%.

IRR percentages are used to compare investments to see which is best. So the question is, what investment should rooftop solar be compared to?

I think comparing it to the IRR of money invested at 5% is reasonable.  This is because when people have rooftop solar installed they never buy it for their landlord.  They put it on a home they own.  Almost half own their homes outright, while the rest are paying it off.

Home loans interest rates are currently around 4%. So if the IRR of rooftop solar can beat that of money invested at 5% households should be better off using any excess money they scrape together to purchase solar rather than make an extra house payment.

People with homes can also access home equity. My third ex-wife can borrow money at 5% any time she wants using the value locked up in our – I mean her – home.  This means if the IRR of solar beats money earning 5% interest it should make sense to borrow money to invest in rooftop solar.

After using Excel to work out the IRR of investing the cost of a solar system at 5% for 25 years I get 8.6%. This is clearly far below the IRR of 18.2% for the example solar system, so investing in large solar should definitely make sense for a typical household in Brisbane.

Return Over A Shorter Time Is Still Worthwhile

In the example above I assumed the system would have a 25 year lifespan.  As many people won’t stay in one place for that long they may not consider it worthwhile.  But even if you own a solar system for only 10 years and it is worth nothing to you at the end of that time, it can still be a cost effective investment.

Mmmm… liquorice…

If you buy a quality system that will require no significant repairs over the 10 years and only requires an average of $40 a year for maintenance, then provided everything else is the same as in the example for Brisbane above, its IRR will be 14.7%.  That is well above the IRR of 7.1% for instead investing the money at 5% over 10 years.

Even if you only owned your solar system for 5 years before you sold your house, provided it added only $1,500 to the value of your home, then with an IRR of 8.2% it will still best the the IRR of 7.1% for money invested at 5% over 5 years.

No Self Consumption?  No Worries!

In the example I assumed the household would consume 20% of the solar electricity themselves, which is around the average most households with a system of that size can expect to.  But even if the household consumes much less, the return can still be worthwhile.  For the Brisbane example above, in the very unlikely event the household self consumed none of the solar electricity, then over a 25 year period the IRR would still be 13.9%.

Annual Returns For 25 And 10 Year Lifespans In Australian Capitals

As conditions vary across the country, I will use the same process as I did for the Brisbane example to estimate the IRRs for solar systems with a lifespan of 25 years and a lifespan of 10 years in Australian Capitals.  It is pessimistically assumed a system will have no value at the end of its life.

Internal Rates Of Return


As you can see, over a 25 year period the IRR of a 6.5 kilowatt solar system beats that of money invested at 5% everywhere in Australia, with Darwin well ahead of everywhere else thanks to the highest feed-in tariff in the country.

Over a 10 year period, Hobart is the only capital where households are better off using their money to pay off their mortgages faster or where they will save money by not borrowing at 5% to invest in rooftop solar.  But everywhere else in Australia a large rooftop solar system is worthwhile.

Larger Solar Systems Are Cheaper Per Watt

One of the reasons going large makes sense is because, as the size of solar systems goes up, their cost per watt comes down.  This can offset the effect of a decreasing self consumption percentage and make very large systems provide a solid return.

Lower costs per watt for larger systems is especially true for those with conventional string inverters.  These are used in the majority of installations.  A 5 kilowatt string inverter will typically be 20-25% cheaper per watt than a 3 kilowatt inverter of the same brand.

Unfortunately, as one microinverter or DC optimizer is typically used per panel7, their cost per kilowatt of panels remains constant and does not fall as a system increases in size.  So if your goal is to build a large system while keeping costs low, you may need to avoid them.

It Pays To Max Out Your Panel Capacity

Hairy and Furry

The 2010 remake of Roofer Cogburn took things in a different direction.

Solar panels are cheap.  I’m not just saying that because they have fallen a long way in price from what they used to be, solar panels are also effectively cheap for households because the STCs that lower the cost of solar depend on solar panel capacity and not inverter capacity.  As a result, there may be only a small difference in cost between a system with a 5 kilowatt inverter and 5 kilowatts of panels and a system with a 5 kilowatt inverter and 6.5 kilowatts of panels.  The extra STCs mean the panels can pretty much pay for themselves.  For this reason, as long as there is room for them, it always makes sense to install as close to the maximum panel capacity permitted, which is one-third larger than the inverter capacity.

Going Big By Export Limiting

Single phase homes are often limited to 5 kilowatts of inverter capacity8.  But it can be possible to install more inverter capacity by export limiting the system to prevent it from sending more than 5 kilowatts of solar electricity into the grid.  Queensland specifically allows this but Western Australia does not permit it.  In other places it will probably be okay.


Liquorice Withdrawal

“Listen, Pilgrim. I ain’t had no liquorice for 30 or 40 minutes, so I’m feeling real mean right now.”

Large rooftop solar systems are now a good investment for most people with unshaded roofs throughout mainland Australia.  But there are some risk factors than can damage its future profitability.

Firstly, electricity prices are likely to fall at some point.  If our politicians get their act together then with a little pen ink they could drop electricity prices by around one-third almost overnight9.  But, as hard as it may be to believe, our politicians may not actually get their act together soon.

Electricity prices are predicted to increase for the next two years, but after that they may fall.  I’ll let you decide for yourselves how likely that is to happen.

As the amount of solar capacity on rooftops and in solar farms increases the wholesale price of electricity during the day will decline.  This will put downward pressure on feed-in tariffs, as in most states they only represent the wholesale price of electricity during the day.  But it is possible payments for environmental and grid benefits provided by rooftop solar will be added to feed-in tariffs, as they have been in Victoria and this may offset the decline in the portion of feed-in tariffs that represents daytime wholesale electricity prices.

But the largest risk to the profitability of installing a large rooftop solar system is electricity retailers being dicks.  If you estimate your large rooftop system will put your bill significantly into credit at times, then you will need to find a retailer who will actually give you that credit and not just take it and steal your feed-in tariff from you by refusing to lower your electricity bill below zero.

You will need to be particularly aware of electricity retailers who claim feed-in tariffs can only reduce the grid electricity usage portion of electricity bills and not the daily supply charge.

I fully intend to look into this and write an article on just who the dickiest retailers are, but until then, don’t let them stop you filling your roof.


  1. First to name the movie in the comments wins a prize
  2. Within reason. Please don’t go this far. You still gotta stick to the installation standards..
  3. This includes battery inverter capacity in AC coupled battery systems such as the Powerwall 2
  4. I would like to unequivocally state that any rumors that the Devil installs for a certain solar company that keeps sending us lawyer’s letters are malicious, unkind, and probably not true.
  5. Additionally, some DNSPs make it difficult to install systems of 5 kilowatts capacity or more and can require additional applications to be filled in and fees to be paid.
  6. Unless the first plan that appears is the one from Origin that requires you to purchase a solar system from them.
  7. In some installations DC optimizers are only used on panels that suffer from shade while unshaded panels in an array do without them, so this is an exception to their usual one per panel use.
  8. SA Power Networks allows homes with single phase power to install up to 10 kilowatts of inverter capacity — or at least they did in the past and they still have documents on their website saying it can be done, so they don’t appear to be preventing it at the moment.
  9. Even a very simple change such as eliminating supply charges and charging a flat rate for the number of kilowatt-hours used could have a major benefit by introducing competition into an area that has been specifically designed to lack competition in the name of increasing competition.
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. 2001 Space Odyssey – what about Tesla PV tiles?

    • Ronald Brakels says

      Ooohh… No, I’m afraid that’s not it. But, tell you what, that was a very good attempt, so please feel free to help yourself to some tiles off your neighbour’s roof.

  2. I agree with the message (we’ve said the same thing – google “ATA bigger solar is better”.

    However if you’re going to compare the annual return against the stock market etc, I don’t think your “one 25th” simple method is rigorous enough. Better to calculate an equivalent interest rate using Internal Rate of Return.

    • Ronald Brakels says

      That is true. Unfortunately, I am incredibly lazy at the moment.

    • Ronald Brakels says

      I have managed to correct the article and add a simple internal rate of return comparison.

      Thank you for helping to motivate me to get it done. I’m afraid what I had previously was very sloppy.

      • Randy WESTER says

        One small correction:

        “with an IRR of 8.2% it will still beast the the IRR of 7.1% for money invested at 5%”

        I tgink you meant ‘will beat the…’ or ‘will best the..’.

        Unless ‘will beast the the’ is an Australian expression, but if so I’d expect it would refer to a much better return on the the. Or that the.

        • Ronald Brakels says

          Oops! Yes, that was a mistake. I tried to think of an appropriate Australian expression to use, but I ended up just changing it to “best”.

        • I have an existing 3kw system but big roof looking at puting in a 6.6kw system growat inv do i need to take down other system or can they be joined or run side by side please .one person qouted to remove it.another company said leave it and just add the other system.

          • Ronald Brakels says

            Often you can leave your old system up and running and put a separate 2nd system on your roof. But it will depend on your location and whether you have three phase or single phase power. If you let me know your general location I can give you a better idea.

            I do recommend considering if you would be better off having your old system removed and getting a large new system that’s over 6.6 kilowatts. This way your solar will be covered by a brand new set of warranties and you won’t need to worry about the old system’s inverter failing outside of its warranty.

  3. I agree totally. In my case, for my 2 home units for persons with a disability, I fitted 13Kw of panels (3Kw on the East, & 10 on the West: its a big roof! I can still fit another 10-12Kw if I wanted to. This 13Kw feeds into a 10Kw Fronius inverter. I am limited to 5Kw export at the moment, even though I have a 3 phase connection.
    I have exported 14Mw of power over the last 12 months. I feed into Enova, so now get 16c/KwH feed-in tariff. Because this system cost me $9500 installed, the payback period will be really short: I estimate 5 years when the flats are tenanted. (The Tenants will get power included in the rent, up to a reasonable level, then they re-imburse me, as landlord for .excess.)

    • Aaron Murphy says

      You’re a good man Doug for putting panels on a building you rent out. I’m trying to convince people to do this also!!

  4. Classic Movie … John Lithgow great actor! The movie is 2010 the year we make contact.

    Re Stephen’s question on Tesla PV tiles, have you done any comparisons or analysis as yet as I would be interested to see whether these stack up versus decent panels. Also on the filling your roof with solar theme, solar PV tiles make this easy to do and less unsightly.

    We are building next year and no builder is offering Telsa tiles / they arent available in Australia yet but hopefully the cost of them to replace concrete roof tiles isnt going to be exhorbitant as Tesla’s marketing is all centred around cost of solar panels + cost of roof tiles = saving over time from what I can see, and not ditch all your roof tiles and retrofit?

    • Ronald Brakels says

      We have a winner!

      2010: The Year We Don’t Explain What Happened In The First Movie.

      Email your address to [email protected] and I’ll send you your prize!

      Would you like to know what it is or would you rather be kept in suspenders?

      • Thanks Ronald will do. Lets go with the suspense option, but from memory this movie did answer why HAL malfunctioned in 2001 Space Odyssey, at least that was something.

      • Hey. Any relation to Peter and Danny Brakels. Went to primary school Toowoomba East State with them. Oh yeah? Are cells with Arise any good.

        • Ronald Brakels says

          Any Brakels you meet is a relative of mine. We have a name no one else wants! (You wouldn’t believe the number of marriage proposals I’ve made that have been knocked back…)

          If you mean Arise the solar installation company, based on feedback we’ve received, that’s not a company we recommend using.

    • Ronald Brakels says

      When Tesla Tiles were first announced I wrote this article:


      And later, when more information became available, but some important details were missing such as how they are wired up, I wrote this article:


      The bottom line appears to be they look good, but are far from cheap.

    • I know I am late (very Late) to the party and I am glad I didn’t submit my guess:
      The 1984 Cult Classic: Bukaroo Banzai
      With a Star studded cast including John Lithgow, Peter Weller, Jeff Goldblum
      All the Aliens names are John, with Christopher Walker famously as John BigBootee.
      I thought you were making a reference to the Alien nature of a Solar system.

  5. Michael Kent says

    let’s be obtuse in our guess. How about the life if Pi. Mmmmmmmm Pie

  6. love your electricity comparison website – I think it will save me around 20% from my current supplier!!
    I’m hoping to go from an annual bill of $4,500 (ducted aircon) to $2,000< with a newly installed 10kW system and the new supplier from your website! (domestic+super economy)
    Am I dreaming or is this feasible? (Qld household)

    • Ronald Brakels says

      Hello StephenG

      With a 10 kilowatt solar system, you can expect to generate an average of around 40 kilowatt-hours a day which comes to 14,600 kilowatt-hours a year. If you have a feed-in tariff of 10 cents a kilowatt-hour then that alone is enough to reduce your electricity bills by $1,460 a year. If you self consume 25% of the solar electricity generated then that will reduce your electricity by by roughly $2,100 using the average cost of residential electricity in Queensland.

      If you can increase your self consumption of solar electricity then you could get your electricity bill down to $2,000 but it will depend on your electricity consumption habits and if you are willing to make some minor changes such as setting a timer on your air conditioner and other appliances.

    • Steve Charles says

      My household is all electric and my power bills had hit $3400 per annum with just 2 people in the house. I now have 10kW of PV on the roof and I am on track to reduce my power bill to zero and may even make a bit of money. My payback period is 2.5 years. I am in SA.

      • How are you offsetting your supply charge Steve? Are you feeding in enough power to both power your home and pay for this section of your bill?

        • Steve Charles says

          Yes, I am including the supply charge in my figures. The two separate inverters are both grid connected. I have no battery at this point in time, but that will be the next step.

  7. As always, thanks for the article, which is not only informative but this time, more entertaining than usual. I installed 12 panels on the west-facing roof (that’s all that would fit) of our Melbourne home, and a 3 KW inverter, in April 2014 for $2,995 (after rebates) and began saving, by my calculation, about $200 a quarter, or $800 a year. Although I work from home 3-4 days/week so use much of the power I generate, at this rate the system should have paid for itself in just under 4 years, although my calculation at the time did not take into account the increased savings due to rising electricity prices over time, or the recent increase in the Victorian feed-in tariff, so rather than early 2018, ithe payback point may already have been reached, or is about to be, a few months earlier than I calculated. From this point, I’m in front. A recent question to solarquotes.com.au confirmed that at this point in time, adding a battery isn’t economically justifiable (it may be, in 3 or 4 years, depending on what happens to battery prices), and we are also limited to 5 KW generation, as per your article. I understand that for 5 KW, your article indicates I could instal 6.5 KW of panels, which seems to be an additional 14 panels, if I’m right. Due to nature of our roofing, this would be 10 facing north on the garage roof (all that will fit, it’s been measured) on angled racks, and perhaps 2 on the east-facing roof. But I would also be up for a sub-station in the garage (I’ve been advised this would be necessary), burying a conduit for the wiring from there to the house, and a new 5 KW inverter. I’m happy to do all this if the payback period is say, ten years or less: another $200/quarter + current $200 = $1,600/year assumed savings seems possible compared to where we’d be with no solar system; even at $1,200 it’s $6,000 saved over five years. Panels and inverters may now be a bit cheaper than they were in April 2014. What do you think? Is it likely to be viable? I’m happy to take the risk that electricity prices may drop in 2-3 years time and I really, really, hate paying electricity bills…

  8. I guess the big unknown is how the electricity suppliers will adjust their pricing structure in the future as they react to less daytime consumption and/or greater feed in tariff payouts.
    I suspect the “poles and wires” component will keep being increased to meet the cost of keeping the CEO’s wives in 7 Series BMWs.

    • Steve Charles says

      It will be interesting to see what happens. As power prices continue to increase, more households will install solar to offset these costs. This also means revenue to power companies will fall due to increasing solar installations and this will drive further power price increases.

  9. Steve Charles says

    I totally agree with going big on solar PV. I originally put 5kW on my roof which worked so well I added another 5kW system giving me the maximum of 10kW per phase. I am on single phase. Both systems are monitored using CarbonTrack so I can see how much power I am using and how much is being generated and exported.

  10. Jackson Wallace. says

    Come along now!….If you ” have your system installed by the Devil” y’wouldn’t need ANY solar panels…..Y’d just run a steam-pipe up and not give a stuff about the weather.

  11. Alex Bertini says

    Good morning,
    I would like to know if there is an office/company to check if Solar Panel installation works properly?

    • Ronald Brakels says

      Hi Alex.

      Well send you an email and recommend an installer in your location who can inspect your system for you.

      Two things you can do yourself are:

      1. Check your inverter to see if it is producing power. On a clear day at around noon you can expect it to be producing about 80% of its panel capacity. So if the system has 5 kilowatts of panels it should be producing around 4 kilowatts of power.

      2. Check your electricity bill to see if you are receiving the expected amount of feed-in tariff.

      • Steve Charles says

        Many solar inverters already have monitoring built into them via wifi. You should be able to find out how to set this up in the inverter handbook. If you have wifi at home, you can log into your inverter and give it your network SSID and password. Then you go to your inverter manufacturers website portal and set up a monitoring account. It is all free and data is typically logged every 15 minutes. It will also log your historical data.

    • Steve Charles says

      You can always monitor the performance of your solar system through the company website. Most inverters these days are wifi capable. If you link the inverter into your home wifi then open an account with the inverter company, you will get monitoring for free. I have a Goodwe inverter and a Growatt inverter. Each one is connected to wifi (you would have been given instructions as to how to do this. Inverterts typically upload data every 15 minutes or so, and this will provide you with daily, weekly and monthly power data.

  12. A quick question relating to the 5 KW inverter / 6.66KW PV array limitation which applies to single inverter installations.

    How does this rule apply to micro inverters? Will we be limited to a 5 KW array?

    • Ronald Brakels says

      Hello Niels.

      Microinverter capacity is treated the same as any other inverter capacity. So if you are limited to 5 kilowatts and your microinverters are 260 watts each, the maximum you can install is 19 for a total of 4.94 kilowatts.

      Your total panel capacity will be determined by the wattage of the panels you put your microinverters on. The largest panel you can put a 260 watt microinverter on and still receive STCs that reduce the cost of solar is 345 watts. That will be just under one-third larger than the microinverter capacity. So if you had 19 microinverters on 19 345 watt panels your total PV capacity would come to 6.555 kilowatts.

      Note that a panel that is 345 watts is probably a very high efficiency panel with a igh cost per watt.

      • niels faerch says

        Thanks for that, Ronald!
        I was wondering about the size of panel one can put a micro inverter on.
        Now I’m starting to get some clarity as to which option will better suit.

  13. Yep, totally agree. I loaded my home and barn with solar. 6 separate solar strings, 73 Enphase microinverters (M215, M250), 18kW AC. No real utility bill. Last year, I got a check from my utility for $192, imagine that, them paying me! Where I live, they used to pay retail at $0.12/kW, but now they’re down to $0.05/kW, but still worth it to install solar. I am grandfathered in since I installed my system in 2014 timeframe. I haven’t had a single microinverter fail yet either.

  14. I have 5kw of panels and a 5kw inverter. I have unencumbered Nth facing roof that I want to use for an extra 1.5kw of panels. Here in Adelaide, I can’t get anyone to provide a quote. They just don’t get back to me. One even called to inspect – that was 5 weeks ago & still no quote. Even the original installer won’t answer my calls.

    • Ronald Brakels says

      That’s odd. Many installers won’t want to work on a system installed by someone else, but they should be upfront about that.

      Perhaps what you want isn’t feasible, but again they should tell you if that’s the problem.

      You can try getting someone through us if you haven’t already. Just go to our home page and enter your postcode in the top right corner of the screen and answer the questions as best you can:


      If all your panels are currently facing one direction, then it may be possible to add extra panels on your north roof, but if your panels are split between two directions then it won’t be possible to add panels, at least not in a normal way, as normal inverters can only accept panels facing in two different directions. Using DC optimizers or panels with panel string optimization may allow you to get around this.

      I am afraid that adding panels to an existing system is generally expensive, so you may not find it economically worthwhile. It’s possible other energy efficiency measures around the home may be more cost effective.

      One possibility is to wait until your current inverter fails and replace it with a 4 kilowatt inverter. Then you could install a small 1.3 kilowatt system on the north roof, but I am afraid that is not likely to be cheap either, as small systems tend to be quite expensive per watt.

      • Thanks for the comprehensive reply. My 5 kw system comprises 3250 facing north and 1750 facing west. The inverter is an SMA Sunny Boy.

        Should it be possible to add Nth facing panels to the existing Nth facing string?
        The West facing panels are on their own string.

        Cost isn’t a big issue. It just gives me mental satisfaction knowing that my roof is achieving maximum power production.

        My system is currently producing 26 kWhr on a good day and this grows to 35 during December.

        I also have an 11.6 kw battery purchased under the South Oz VPP scheme.


        • Ronald Brakels says

          Yes, the Sunny Boy inverter should be able to accept an additional 1.5 kilowatts of panels.

          SMA has an online design tool you can use if you want to double check, although it can be a bit confusing if you haven’t used it before:


          I overlooked the fact you are in Adelaide, so, since your goal is to maximize production, the best approach may be to install a second inverter and install however many panels fit on the north roof and then if there is suitable space on your east roof you could put panels there.

          SA Power Networks currently still has information on their site saying 10 kilowatts of inverter capacity for a home with single phase power is permitted, so going over 5 kilowatts of inverter capacity shouldn’t be a problem and it is likely to be the lowest cost option per watt for increasing your solar generation.

      • Depressingly, its not odd.
        To date, my experience with 3 installers has been they are not interested in doing installs with anything but the panels and inverters that they provide.
        Nor are they interested in unusual roof mounts. In fact unless they can provide a cookie cutter solution they won’t even quote – this strategy delivers the maximum profit to work ratio.
        For example, I am pretty much set on the idea of micro inverters – the three sales guys that turned up to measure and quote did their best to talk me out of them with their spin about unreliability, cost, and that no one is using them. etc etc.. When they failed to change my mind, I never heard from any of them again. Get rich quick spivs and charlatans best describes some of the installers.
        So now, I am contacting installers listed on the Enphase web site for quotes.

  15. dave cadwell says

    String Inverters are like everything else,they fail. If you loose your inverter you are toast. Microinverters are as single units replaceable.
    That said are we back to the which came first” the chicken or the egg” conversation?

  16. Life of Pi

  17. While I was happy with my 3 kW system, I have now come to regret that I have only installed that much (that was what I could afford then). Trying to bump it up to the maximum now (I have single phase) limit me to very limited options. Either I get rid of my current inverter and put in additional panels and a new 5 kW inverter or convince some installer to top it up somehow. Most will not do the latter.

  18. Steve Charles says

    Best not to try to upgrade the existing system as it will be expensive. If it is working well, leave it be and look at adding another system. On single phase, you are allowed a maximum of 10kW, in SA anyway. I don’t know about other states. I originally installed 5kW on the house which is working well, but then came along even more power price rises, so I installed another 5kW on a shed. The total performance is impressive and I hope to drive my annual bill to zero.

  19. I am now thinking to install a second solar system as it is very difficult to get anyone to add panels to my existing system. Would both systems provide charge to my AGL VPP battery? Would I retain my 16.3c FIT in South Australia.

    Would a second system simply supply additional power and be used in the same manner as my existing 5kW system.

    • Ronald Brakels says

      Hello Dave.

      If you add a second solar system it won’t affect your 16.3 cent feed-in tariff. Your battery will be able to charge off the power it produces. And it would act the same as your existing system.

  20. Hi, love your posts Ronald… I have a 10.9 kW system on my roof in part because I agree that you should go as large as you can. One issue that I only found out about after the installation is that in South Australia, SAPN sets a maximum daily export of 45 kWh per day, averaged out over the billing period. My summer production is about 70 kWh per day so I have to make sure I self-consume a significant amount of that to avoid giving it away for free. Thankfully I have a power-hungry air con that I use on a timer to keep the house cool and stop it getting too hot before I get home in the evenings.

  21. Hi Ronald,

    Just a couple of quick questions, if you don’t mind.

    I currently have a 3.12kW system, with 12 x REC 260w solar panels and a 4.0kW Fronius Primo Inverter. And I live in East Gippsland in Victoria.

    Based on your article, I can add another 8 x REC 260w solar panels (totalling 2.08kW) to the existing 3.12kW to bring the solar panels up to 5.20kW – as the inverter can handle up to 5.32kW (4.0kw x 1.33) – is this correct?

    Does the increase in solar panel capacity above the inverter capacity have any detrimental effect of the life of the inverter?

    And based on the above addition of panels, if I use the average daily production figures for Melbourne, where a 1kW system produces 3.6kWh, then the average daily production figure for my system with the additional panels would be 18.72kWh (ie 5.20 x 3.6) – is this correct?

    Appreciate your advice

    Cheers Peter

    • Ronald Brakels says

      Hello Peter

      A 4 kilowatt inverter can have a maximum of 5.33 kilowatts of panels, so you could add an extra 8 panels for another 2.08 kilowatts of panels. I doubt adding the panels will have any appreciable effect on the lifespan of the inverter. Nothing worth worrying about, anyway.

      Unfortunately, adding panels to an existing system can be expensive and most installers prefer not to work on a system they haven’t installed themselves. A cheaper option may be to install a new system instead. Your Distributed Network Service Provider may limit you to one additional kilowatt of inverter capacity, but depending on your location you may be permitted to install a larger additional system, even if you have single phase power.

      Unfortunately, in Melbourne you are only likely to get average 3.5 kilowatt-hours a day per kilowatt of north facing panel capacity, rather than 3.6, so you could expect an average of around 18 kilowatt-hours a day from a system with 5.2 kilowatts of panels.

      • Hi Ronald,

        thanks for your quick and informative reply.

        You’re right that adding panels to an existing system can be expensive – have just received the quote!!

        However, it is from the same crowd that installed the original system and they are using the same REC panels so that the roof will look neat and tidy.

        And they are the resellers for the Solar Analytics Monitoring system that Finn recommended on another blog entry (www.solarquotes.com.au/blog/got-33-power-bill-solar-even-crappy-8c-buy-back-rate/#comment-100453) and that Choice magazine recently reviewed.

        So, as they were going to install the monitoring system, I thought that after reading your article, I’d get them to quote on the additional panels which they have done.

        cheers Peter

  22. Hi Ronald,

    You mention that the total wattage of the panels can be one third higher than the rating of the inverter. What is the basis for this? Is it a rough rule of thumb based on technical calculations, or a legislative limit, or something else again?

    I would have thought that the panel string(s) and inverters would have to be compared on a case-by-case basis to ensure voltage limits, current limits etc. for the inverter are not exceeded.

    • Ronald Brakels says

      It is a limit that must be adhered to in order to receive STCs as part of Australia’s Renewable Energy Target that lower the cost of rooftop solar. Normally inverters can handle higher panel capacity than this. Because having panel capacity 50% greater will result in only a small loss of PV output while making grid operators happier by smoothing out PV production, I am of the personal opinion the limit should be increased. (Provided it is within the manufacturer’s specifications for the inverter.)

  23. Two things to keep in mind

    1) Many retailers in NSW who provide high feed in tariffs limit the size of the system that can take advantage of them. Many at 10kw, one even at 5kw so you need to do you homework before installing

    2) Converting to 3 phase power to allow a larger export can be expensive. I was told $1,800 – $2,000 but it got complicated and ended up costing closer to $5,000 – which hammers to rate-of-return on the extra panels.

    So 5kW inverter (and up to 6.6kw of panels) – no brainer, but actually MAX sizing the system and going beyond that? There can be a lot of things that trip you up. Just something to be aware of

  24. Hi, regarding the South Australia limit

    “In South Australia SA Power Networks limits feed-in tariff payments to the first 45 kilowatt-hours of solar exported to the grid per day, averaged over the billing period. As a result, it is possible people who go over 10 kilowatts of solar panels may miss out on payment for some exported solar electricity when output is high and self consumption is low.”

    Does that same limit apply to both single phase and three phase households? It just seems a little strange that a three phase house could have 30 kW of solar, but be limited to export only 45kWh per day. I’m currently buying a three phase house with existing solar, and trying to work out the economics of expanding what’s already installed.

    • Ronald Brakels says

      I have never seen any indication that it does not apply to three phase households, but it is something you may want to check with SA Power Networks. (If you do get in touch with them, let me know what they say, thanks.)

      • Will do! As soon as I get a reply, I’ll let you know.

      • Ok, emailed them, but I’ve tracked down where the limit actually comes from. It’s from “Electricity (Miscellaneous) Amendment Act 2011”, and it’s phrasing answers one question and raises another.

        Section 36AE(2)

        “the holder of the licence authorising the operation of a distribution network is only required to credit a qualifying customer under subsection (1)(b) for the first 45kWh of electricity fed into the network each day”

        So it’s the the customer that has the limit, not the phase. But it also says that they are only required, not limited. There is no legal block to being authorised to export more than 45kWh…

    • In SA, does the 45kw limit apply to all systems irrespective of when they were installed?

      • My reading of the the “Electricity (Miscellaneous) Amendment Act 2011” is that it only applies to systems installed or modified after 1st October 2011.

        The 2008 act applies for systems installed between 2008 and 2011.

      • gday

        I have noticed that here in NSW, Origin limits its “Solar Boost” contract with the terms:
        “To be eligible for this energy plan:
        your solar PV system must not be larger than 10kW ”
        So, for their 15c FiT, you cannot go ‘as big as your roof allows’.
        This may stop me going to 15kW

        • Des Scahill says


          Your local council may impose a further restriction as well (they do in QLD) that sets a maximum of 5 kw inverter and 6.66 kw of panels on your roof.
          That’s mainly for electrical safety standard reasons I believe.

  25. hello,
    I am planning to install solar system in my house in Adelaide SA 5095. Any advises would be great help.


    • Ronald Brakels says

      Hello Chandra

      I recommend installing as much solar as will reasonably fit on your roof and is within your budget. This is because it’s usually the best investment a household can make.

      Our Solar 101 Guide is a good introduction to solar:


      If you’d like some quotes from installers who have been vetted by us and we know do good quality work, you can go to our homepage:


      And enter your postcode in the space at the top right. Answer the questions that come up as best you can.

      If you have any questions, let me know.

  26. Hello Roland,

    Thank you so much for this wonderful introduction for solar power. It was indeed a big wisdom for me. I will get back to you again when I am ready to get the solar power.


  27. John Witney says

    We have an existing solar system installed by Mark Group & commissioned 25/11/2011,Using 8-Eoplly 125/72-185 panels facing due North on a 20 degree pitch tile roof with a Samil Power-Solar River Inverter Model SR1K5TLA1 Rated at 1700 watts. This nominal 1.5 KW system is working wonderfully well for these
    8 years,although its O/A efficiency each year decreases (42% in the 8 yrs).
    Should we investigate extending or modifying the system to get more energy & save on our power bills with Origin
    Upon reading your article I discover what I think is perhaps feasible for our situation (we are in our 80 s) & only wont to look forward to any solar advantages for the next 3-5 years.
    With solar panels capacity up to a third greater than the Inverter capacity we could extend our panels to a max of 12 & perhaps utilize all the existing system.What would be the cost to add 4 extra panels or is it better to replace all the existing with the potential max 2.267 KW inverter capacity It would be most interesting to here from you,Regards John.

    • Ronald Brakels says

      Hello John

      Because your system was installed by Mark Group I’m guessing you are in NSW and so don’t have a high solar feed-in tariff locked in. My normal advice is that it’s not worth expanding an old solar system like yours. You will almost certainly be better off replacing it with a new solar system and getting a new set of warranties. In NSW a new solar system should be able to pay for itself in around 5 years or less. So it is within your time frame.

      This article gives an estimate of the payback time for solar in state and territory capitals:


      It assumes a price of $6,500 but it is possible to now get a quality system installed by an installer who does good work for under $5,000 which should allow it to pay for itself in under 4 years. But as you will probably want to have your old system removed, that will add to the cost.

      If you’d like to get some quotes through us you can go to our homepage here:


      Enter you postcode in the space at the upper right and then answer the questions that come up as best as you can.

  28. Feel free not to publish this if there’s already an answer (but I’d love you to point me in the right direction!).
    I live in WA in a home connected to 3-phase power, but I understand the Renewable Energy Buyback Scheme (REBS) is only provided to installations with a maximum capacity of 5kW. Is that true

    • Ronald Brakels says

      Yes, I’m sorry to say that solar systems with inverters larger than 5 kilowatts won’t receive a feed-in tariff (REBS) in WA. Also, it’s not permitted to get around this by export limiting a larger inverter to 5 kilowatts. This means 6.66 kilowatts of panels is the maximum that can normally be installed and still get a feed-in tariff. Someone with batteries and/or an electric car might find it worthwhile to get a larger system and go without the tariff, but they’d have to use a lot of electricity. Of course, it could be done for environmental reasons.

  29. One of the companies quoting us recommends as many panels as possible (28), to be battery ready, even though I can’t see us being able to afford a battery in the short-medium term. We are dual phase power. Is there any point in this many panels, or is it just a sell? We are in Southern Highlands NSW. Thank you.

    • Ronald Brakels says

      Hi Katie

      I normally recommend installing as many panels as will reasonably fit on your roof. This is because the extra capacity will usually pay for itself even if you use hardly any of the extra generation yourself and most of it goes into the grid for a feed-in tariff. If you do get a battery in the future it will be useful and will also help if you get an electric car. (You may not think you’ll want an electric car in a rural area, but give it 5 years and you may find it’s a good choice.)

      That said, if you decide to go with a smaller system it will still be useful in reducing your electricity bills. The reduction just won’t be quite as large.

      Because you have dual phase power that will create some issues with the installation that I hope your installer has discussed with you.

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