Adding Solar Panels To Existing System? Here’s What You Need To Know.

solar panels with space for adding more

Here are your options and gotchas when adding solar panels to an existing system.

So you’ve got solar panels on your roof and you want more so you can get your bills even lower?

(Note: If you are on a premium Feed In Tariff then you need to understand how solar upgrades can affect your eligibility. 

If you are on the NSW solar bonus scheme, then your FiT is ending on 31 December 2016 – and you should time your upgrade to happen as close to this date as possible)

You are probably weighing up 3 alternative approaches to your solar upgrade:

1) Add more panels to your existing system – using your original inverter.

2) Get a new, bigger inverter and add more panels too.

3) Get a whole new separate solar system installed next to the original.

Let’s go through each option in detail so you can weigh up the best approach for you.

Option #1 Adding more panels to existing system using your original inverter.

The first thing to be aware of (and not a lot of people know this) is that you are allowed to have a panel array with a peak power up to 133% of your inverter’s rated peak power.

For example:

If you have a 1.5kW inverter, you can have up to 2kW of panels attached.

If you have a 3kW inverter you can have up to 4kW of panels attached.

If you have a 5kW inverter you can have up to 6.65kW of panels attached.

And if you don’t go over the 133%, you can claim the solar rebate (STCs) on those extra panels which will cover a big chunk of their of their cost (excluding installation costs and installer margin).

So if your inverter is big enough, adding panels is a cost-effective option to get more energy. You may be wondering how on earth this can be the case if the inverter limits the power output. Let’s use a 3kW inverter as an example.

3kW of panels will generally only produce 80% (2.4kW) of their rated peak power due to losses.

4kW of panels, after losses will produce a peak power of 3.2kW. A 3kW inverter will safely clip this down to 3kW. So you are only losing 0.2kW of power. And that is only for a couple of hours each side of midday on a summer’s day. In the mornings, evenings and winter the peak power will be way less than the rated 3kW of thew inverter.

So oversizing your inverter by 133% is a good way to squeeze more energy from it.

Two big caveats here are that:

1)Unless your original solar installer does the upgrade you will void your existing system warranty.

2)It depends on your installer finding the same or very similar panels to your existing ones.

If you want more panels than your inverter can handle (using the 133% rule), or you can’t find compatible panels, then your next option is:

Option #2 Replace your small inverter with a bigger inverter and add panels.

It seems like a waste of money and resources to remove a perfectly good inverter.  However the sad truth is that over the last 7 years a lot of the installed inverters that have been cheap and nasty and are probably on their last legs. If your inverter is a Sunny Roo, KLNE, JFY, JSI, Aerosharp, Sharp, or other inverter with a penchant for going bang after a few years, then this is a great opportunity to replace the thing with a genuine premium inverter, such as SMA.

Another reason to bite the bullet and swap your old inverter for a new bigger one is that one inverter is much easier to add batteries and backup to than two separate ones. So if a battery ready system with backup is a priority, having a single, large inverter is the way to go. (This post explains what a battery ready system with backup is and how that differs from a battery ready system without backup)

If you do decide that up-sizing your inverter is the way to go then watch out for these gotchas:

a) With panel technology changing so quickly, you may struggle to find extra panels that match your existing array. In this case get an inverter with 2 or more inputs so you can install new, different panels without affecting the old array.

b) If possible get the original company to do the work, then they keep the responsibility for the system warranty.

c) I recommend getting an inverter with SUNSPEC/MODBUS. This is a communications protocol that will make it easier to add batteries in the future. All the SMA models on sale now in Australia have this baked in. I would say that SMA are the safest bet here.

Note: ABB inverters can also talk MODBUS with a $500 interface box. And although the Fronius data sheets seem to claim all their inverters talk MODBUS, the word on the street is that in practice it is only usable in their expensive ‘hybrid’ models. Perhaps some one from Fronius would like to comment?

d) You can’t claim any rebate on the inverter, but you can claim the solar rebate on the panels. So put as many panels as you can on that inverter (133% of inverter rated power) because the rebate (STCs) covers a large chunk of their cost.

If you don’t want to replace your existing inverter, then your third option is:

Option #3 Get a whole new separate solar system installed next to the original.

This option can make a lot of sense – if batteries with backup are not on your radar. You get a new system, with a separate warranty. And because installers are so efficient at installing whole systems, and don’t have to fiddle around integrating into existing solar wiring, it can often work out cheaper than upgrading your existing system. And yes – you can claim the solar rebate (STCs) again on your second system – it will already be included in any quoted prices.

So to summarise:

Be aware that you can legally oversize most inverters by 133%. This may give you all the energy you need with your existing inverter.

If 133% of your inverter rating is not enough, then you can replace your inverter to allow you to add more panels. Just be aware that this can be surprisingly expensive as it is not as straightforward as a fresh installation.  If you are getting a new inverter ask for one with MODBUS/SUNSPEC communications as it will be easier to add batteries to in the future. And if the original company does the work then your warranty should stay intact. You do waste your old inverter though.

It is often cheaper to add a whole new solar system using either a conventional string inverter or micro inverters . In grid connect mode these 2 systems side by side will work great, but it can make adding batteries+backup more complicated down the track.

And I’d be remiss if I didn’t mention that you can get quotes for a solar system upgrade using my free service here.

About Finn Peacock

I'm a Chartered Electrical Engineer, Solar and Energy Efficiency nut, dad, and founder of My last "real job" was working for the CSIRO in their renewable energy division.


  1. Hi, I bought a 1.5KW solar system from True Value Solar six years ago. Originally it had a Chinese made Aerosharp inverter. It failed about six months later. After much pressure to True Value Solar, they replaced it with the Growatt brand. So far so good, although it does interfere with my AM radio stations sometimes.

  2. Hi Finn , there are a lot of people upgrading their solar systems by just adding AC Solar modules. They don’t have to add a whole string and allows for easy future upgrades. You can also install on various roof faces to match the energy consumption and you do not have to touch the original system, potentially voiding installation warranty. Plus no high voltage DC wiring and associated issues. If they use Enphase Envoy S Metered they can see the consumption monitoring of the entire solar system (old and new) and can simply add the optimum number of Enphase AC batteries once they know exactly how much energy they are exporting.

  3. Ahh.I will correct you here.The Solar rebate does not pay most of the cost.
    To put an extra 4X260watts on your roof will still cost you around $800.00 to $900.00 .I shopped around but I used my original installer and that is what it cost.As for claiming the rebate you can’t claim the rebate.The rebate if any is claimed by the installer and I can assure you the panels are not for free.

    • A 260W Tier 1 panel wholesales for about $200 + GST. STCs are approx $800 per kW installed. So the wholesale cost of the panels is pretty much covered by the STCs.

      Of course the installer must add a margin and installation cost and racking etc (as I point out in the post). But the majority of the cost of the panel hardware is covered by the rebate.

      And you are perfectly entitled to claim the rebate yourself. It is just a lot easier to get the installer to do it for you.

      • Of course but it is on the fitted price of the panels and then the STC,s are applied so it works out roughly at $200.00 per panel but the starting price is in my case on 20 panels was 10,000 plus GST less $4,000 (107 stc’s @38.00) equals $6,000 plus GST cost $6,600.
        The fitting of just 4 panels still requires a road trip of 2 hours and 4 hours fitting for one person plus the profit on the panels and hardware so in my case having 4 panels the starting price would have been $1600.00 (allowing for no inverter) in total less the rebate, so apply that to 20 panels it equals $8,000 plus the cost of the inverter which looks about right give or take $80-90.
        So the retail fitted price is around $400.00 to $500.00 per panel and of course many operators charge much more than that so the cost of 4 panels is still $800.00 and upwards.

  4. G’day Finn,

    I recently heard from a solar installer that, in NSW, the size of the system (for the purposes of the 60c FIT) is gauged by the size of the inverter. Their advice to people faced with the FIT running out was to add more panels, i.e. oversize the array up to 133% as you suggest. According to them, this will not affect the FIT because the inverter size hasn’t changed. They also suggest that the 60c FIT will pay for most of the extra panel cost before it runs out.

    Your thoughts?

    • Hi Andy,

      My understanding is that you cannot add panels to an existing inverter if you are on the 60c scheme and keep getting 60c for all the energy.

      Although the offical guidelines are really badly written:

      What is clear from the FAQ above is that:

      “You must notify your distributor of any change to your Scheme generator that would affect your receipt of Scheme payments. Fines and penalties of up to $110,000 may apply for failure to notify.”

      So my advice is not to just do it and hope the rules are in your favour. I have heard reports of distributors using nearmaps to look for extra panels on roofs and then penalising owners.

      I would call up your distributor and and them if you are OK adding extra panels and not changing your inverter (as the official guidelines are ambiguous). If they say it is OK and you won’t lose your 60c – get it in writing before proceeding.

      Hope That Helps,


      • The official guidelines you have linked clearly indicate that the ‘size’ of the system [vis-a-vis changes to it] is not gauged by the inverter size since you can upsize the inverter [up to 10kW] without affecting the tariff but that the 60c tariff will not be paid on power generated by new panels [vide ‘separately metered’, etc.].

        • Finn Admin says:

          How do you separately meter extra panels if they are on the same inverter as the old panels?

          • That’s your problem, not the government’s. The rules say what you may or may not do (in isolation) not necessarily what you can or cannot do together. The fact that the rule on separate metering doesn’t mention separate inverters doesn’t mean you can somehow avoid it – you can’t make assumptions from what the rule doesn’t say.

  5. Hi

    I am going to use one of the three solar installers that you have put forward to me. Thank You.
    I Just want to confirm a few things to make sure that i have read your helpful articles and hints correctly.
    1. We have just been told that we have 3 phase power.( we didn’t realise when we bought our home last September)
    From what you say, It is preferable and more straightforward to just apply solar to the one phase?
    2. We wanted a big system because our electricity bills are around the $1,000 mark with no heating or aircons in the home. The Plumber we used to install a Sanden heat pump for cheaper hot water told us.
    Reading your info regarding oversizing with extra panels, I’m thinking that a 5 kw system inverter (which is the maximum we can install using the single phase) May suffice and instead of the 24 x 260 panels = 6 kw, we have been offered, Maybe we could ask for 25 x 260 panels and this would add up to about 6.5 kw? The maximum to we are allowed to still get and receive our stc’s ? Does that sound right?
    3. We have a very large roof with areas East, West, North and South.
    Would it be wise to put most panels on the North side and some on the West for the late afternoon sun and some on the East side for the early morning sun?
    4. We are on the Sunshine Coast QLD and the Tariffs we have in this house are 33, 31 and peak.
    What is standard tariff ? And should we be on this tariff to use as much solar power before it goes to the grid?
    Apologies if i havn’t understood the info you have supplied correctly.

    Gillian Armstrong

    • Hi Gillian,

      1) Yes – if you are happy with a 5kW maximum inverter, then it is fine to go into a single phase with the solar. A 3 phase inverter would allow you to go bigger.

      2) Yes 6.5kW of panels into a 5kW inverter is a good move.

      3) I would recommend 2kW East, 2kW West and 2.5kW North. A 2 input inverter with the E and W on one input and the N on the other input can work well. Micro inverters would work well too. The E/W and N split will give you a more even spread of power from sunrise to sunset, increasing self consumption of the solar, saving more money.

      4) Tariff 31 is very cheap and can be used to boost your HW overnight very cheaply – so I’d keep it. Depending on your consumption – it may be worth putting everything else that can be run in daytime on the standard tariff so the solar can offset it. But your installer will be able to advise you on this based on what appliances you have.

      Hope That Helps,


    • Tom Roberts says:

      Go with Enphase microinverters which give you a truly decentralized, fault-tolerant solar PV system. Central string inverters are an Achilles Heal!!

  6. Hi Guys,

    You cannot claim STCs for addtionnal panels if your inverter is not on CEC list. Most of the old inverters are not on the list anymore because of the change of regulation. !

  7. Hi All, we are looking at putting a second 5kw system on our shed roof. We have a current 5kw system our house which was installed about 4 years ago. We live in Brisbane and receive the feed in tarif rate of I think 45c or there about per kw which was before the axed the higher FIT’s. My question is if we runs a separate system alongside the existing and feed it back into the grid will we still receive the same FIT with double the feed in. Does this present any issues. Thanks.

    • Ronald Brakels says:

      Hello Wayne. I’m afraid that any increase in the capacity of your solar system will result in you losing your high feed-in tariff. While you can repair or replace defective components of your old system it’s not permitted to make any change that increases the system’s original capacity. Unfortunately, this includes adding an additional system.

  8. Hi Ronald, thanks for the reply. Do we loose the feed in tariff if we up the existing system to to 133% panels?

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