Solar 6.6kw or 10kw

Poormansmeal on 03/11/2025 - 17:12

Hi everyone,

I’m currently building a new home and looking into installing solar builder provide 6.6Kw as standard , should I go for 10kw

A few details:

It’s a family home (we’ll be living here for many years).

We plan to run standard appliances, ducted air conditioning, and maybe an EV charger in the future.

The roof has good north-facing space for panels.

I’d love to hear from anyone who’s gone through this recently —

Is it worth spending extra for the 10kW system?

How has your system size worked out in terms of bills, payback period, and performance?

Any regrets or things you wish you’d done differently?

Thanks in advance for your insights!

Electrical & Electronics Solar Energy System

Comments

Search through all the comments in this post.
    • mantra on 04/11/2025 - 01:16
      +11

      during winter cloud cover reduces solar output, more solar panels can compensate

      solar panels last longer than batteries

      solar panels: 25–30 years with a predictable 0.5% per year degrade rate
      batteries: 10-15 years but also influenced by cycle rates, temperatures and discharge depth

      winter output less than half summer output considering you probably won't get enough power to fill the battery during winter sound like a better investment

      especially if your using heating in winter

      • Dr Phil on 05/11/2025 - 16:23

        Makes sense
        Though I doubt the "useful life" of solar panels is much more than 15 years and batteries closer to 10 years.

        But yes, more solar panels would partly compensate for lower solar power generation perios during cloudy days and winter.
        In the end depends whether the extra cost for a 10kW system is worthwhile according to OP.

        Sorry OP but only you can answer that
        Its your house and your money

  • JIMB0 on 03/11/2025 - 17:21
    +74

    Always get the bigger one if you have the space, you'll regret it later on if you don't.

    • supersabroso on 04/11/2025 - 19:26

      Yep, you only get one chance to claim the rebates which are disappearing more and more every year. You can't add more solar panels later on unless you want to pay full price or throw away your new inverter and pay for another one to replace it. Worst case scenario, you might have to throw your entire system in the bin if you want more panels.

      • WhyAmICommenting on 04/11/2025 - 23:53
        +3

        This is not true for STCs associated with panels. For batteries you only get one bite of the cherry though.

      • us3rnam3tak3n on 05/11/2025 - 10:27

        Well the system price has been falling at a rate where the rebates are less necessary but I agree - fill that roof up.

  • Brick50 on 03/11/2025 - 17:25
    +11

    10-13.2KW panels, 10kw inverter, 42kwh fox ess battery all for under 10k

    • 87percent on 03/11/2025 - 19:13
      +2

      From where

    • verio on 04/11/2025 - 14:18
      -2

      First two points yes, but I would stay away from the cheapest battery on the market.

      • UltimateAI on 04/11/2025 - 17:43
        +1

        Good battery though, widely popular in Europe and large volumes here.

        • singingwolf on 04/11/2025 - 20:17
          +2

          Happy with my Fox ESS. Can say definitely get 13.2kwh. And get the 42+kwh battery. Anything less you'll beat yourself up not getting them.

          • UltimateAI on 04/11/2025 - 21:22

            @singingwolf: I went 10k with 42kwh, despite having a tiny solar system, use has been almost zero using the OVO EV plan. Agree a larger inverter may have been better. Although usage has peaked at 15kw, was just a spike and the 10kw inverter just did it's think and usage netted out to zero.

            I force charge from 11pm-2pm FREE period (when I run the pool heater + AC + Charge the EV), then have it set to use-grid between Midnight and 6am, if the battery falls below 30%. That is the EV window.
            Winter will be interesting, we run the AC a lot more now, and have not yet dipped into the EV window.

            • singingwolf on 05/11/2025 - 01:05

              @UltimateAI: At least you have a decent battery. When the free 3 hour period kicks in next year you can fill up free and export more to grid

      • bigbadboogieman on 05/11/2025 - 20:34

        Yeah one of the biggest brands in UK and good presence in Germany and US as well.

  • Naigrabzo on 03/11/2025 - 17:34
    +29

    Biggest possible. Full stop. Cover your roof if you have the cash.

  • Poormansmeal on 03/11/2025 - 17:39
    +2

    Thanks folks

    • Brick50 on 03/11/2025 - 17:48
      +7

      Don't use your builder to install it though, they charge 2x what you should be paying
      If panels only then difference between 6.6 and 10kw should be less than 2k.

      I reckon the builder will back you into a corner and tell you 10kw of panels would be an extra 4k or something, in which case better off using your own installer if you can, or getting bare minimum installed to meet energy ratings and then getting a proper system installed.

      • singingwolf on 04/11/2025 - 20:17

        Agree

      • Excelsior on 05/11/2025 - 11:05

        Get at least 3-4 quotes from different retailers for comparison before making your decision. Caveat - cheapest doesn't always mean the best.

        Case in point - I got stuffed around by a supposed reputable retailer (at least according to Google reviews); long story short, they were the most competitive in terms of pricing - started the job after many follow ups, and not long after gave random excuses as to why they couldn't continue and had to cancel the job.

        I then spoke to my builder and he referred me to another company that he had personally used; I requested a price match with the first quote and they said it was way too low and that there was no way to match it because they'd be basically making next to zero margins. So there you go, all makes sense in the end.

  • Twilight1234 on 03/11/2025 - 17:51
    +4

    Always bigger. Pair it with battery and you're set

  • JDMcarfan on 03/11/2025 - 18:02
    +3

    echo above. dont get from builder. find a trusted installer in the area and you can get a great deal and quality (non-affiliated) product. If you plan to live there long, and have a CBA (or other lender who gives green loans - cheaper than home loan %), use the subsidies and all your roof space and oversize as much as you can. Join Amber and get a smart battery (esp if you are 3-phase) and let the system profit for you.

  • joka on 03/11/2025 - 18:28
    +15

    Get the bigger capacity system and then you can run your own AI computing farm. Just don’t ask us where to start.

  • TEER3X on 03/11/2025 - 18:33
    +1

    How much extra exactly?

    Pointless without numbers.

  • geekcohen on 03/11/2025 - 19:11
    +7

    Go bigger. I got a 6.6kw like 5 years ago and I regret not going bigger. Our usage has gone up and I should've gotten a bigger system at the start.

    • Ocker on 03/11/2025 - 19:19
      +1

      Got my 6kW system about 6 years ago as that was the maximum number of 275w panels I could fit on the available roof area.
      Now, with 475w-500w panels being available I am soon going to upgrade to a 10kw system.
      Not sure that adding a battery is financially viable when I upgrade.

      • TEER3X on 04/11/2025 - 07:58
        +2

        Sound like you're doing it backwards, add the battery first. You likely don't need to upgrade the solar capacity if you get a battery

        • Ocker on 04/11/2025 - 08:08
          +1

          So are you saying that with my peak summer/winter net usage of around 75kW a day , with zero feed-in, a battery is all I need? But what is going to store energy in the battery if I am currently using all of the solar output?

          • TEER3X on 04/11/2025 - 17:12

            @Ocker: Most people aren't running a grow house

            If you're in the right state, you'll be able to charge your battery for free (3hrs free coming soon)

            • Ocker on 04/11/2025 - 18:32
              +1

              @TEER3X: No free 3 hours for Victoria for some time yet.
              Not operating a grow house, just a large house with ducted reverse cycle aircon to 11 outlets, plus pool pumps, etc

              • TEER3X on 04/11/2025 - 20:23

                @Ocker: Give it some thought if you've got all that extra space

  • dreamerman on 04/11/2025 - 07:05

    New house. 13.6kw/10kw solar and 50kw/10kw batteries. This is the way.

    • Poor Ass on 04/11/2025 - 14:24
      +2

      how much will this cost

  • brad1-8tsi on 04/11/2025 - 07:41
    +3

    The market & rooftop requirements are changing rapidly. eg: today's "3 hours free" Fed gov't press release.

    Get a battery or a car that can do vehicle to grid/house. 30-50kWh.

    Summer outputs more than double the kWh daily than winter. 10kW will be great for winter but you may have to curtail in Summer depending on how you choose to export.

    From October-Mar FITs are negative from around 06.00-16.00. The retailers can't ignore that forever

    • kiitos on 04/11/2025 - 07:54
      +1

      you may have to curtail in Summer depending on how you choose to export.

      What does this mean?

      • gyrex on 04/11/2025 - 09:43
        +2

        Curtailing is the process of limiting solar exports; usually to limit exposure to negative feed in tarrifs imposed by your electricity retailer.

    • tellhimhesdreaming on 04/11/2025 - 13:03

      I reckon you're on the money. Solar landscape is changing a little, with today's announcement an EV and battery tech becomes a lot more appealing than a larger array. Who knows when it'll actually flow through the retailers though

      • airzone on 04/11/2025 - 16:15
        +4

        They'll just jack up the daily connection fee.

  • Rob-4x4 on 04/11/2025 - 07:42
    +1

    As big as possible and ensure you get a reputable installer who uses quality products like inverters etc.

    Other ozbarginers may be able to recommend manufacturers of solar components to look for or avoid.

    The solar/battery installation industry is full of dodgy scammers at the moment.

  • giftcardinspector on 04/11/2025 - 08:03
    +4

    I put 3kw on my first house, wish I went bigger.
    I put 6.6kw on my second house, wish I went bigger.
    I put 20kw on my third house, wish I had a bigger north face to maximise it.
    If you're building like I did in my third house, don't get the builder to do it as they will slap a margin on it.
    Engage a Solar contractor yourself, get them involved early, that way they can rough in the electrical before the walls go up.
    They will come back at the end and install the panels.
    Get a battery, as big as you can afford.
    Pay back figures - System turned on in Mid Feb 2025
    Solar Cost 12599.6 paid back - 38.79% (exports @12c FIT, self consumption)
    Battery Cost 11259.49 paid back - 12.44% (self consumption)
    Total 23859.09 paid back - 26.35%
    Sungrow SH15T inverter
    Sungrow SBR192 battery (19.2kw)
    20kw Jinko panels
    Pretty happy with 26% paid off overall in less than 9 months with a full summer to come (SEQ)

    • djsweet on 04/11/2025 - 12:10

      That's pretty wild. You will break even in only 2 years. I have a similar set up and am looking at probably around 5 years to break even - given elec bills of around 5k per year. My payback could be reduced by rising electricity prices I suppose.

  • gidxg03 on 04/11/2025 - 08:58
    +2

    We had a similar system (8.36kW) solar plus a 24kWh battery installed about 2 months ago in Melbourne.

    For us the battery has been the best investment. In Melbourne you’ll struggle in winter or very overcast days to fill a 24kWh battery but we’ve signed up to the glowbird zero hero plan with 3 hours free electricity each day (which fills our battery regardless of the weather).

    The battery is seeing us through a full 24 hours without any power from the grid (outside of 3 free hours). We’re a family of 4 using about 30Kw per day. Zero hero gives $1 rebate per day if you don’t draw from the grid during peak hours (6-9pm) which is offsetting our daily charge. Our first bill last month was $1 and I’m confident this will be norm year round.

    System cost just under $19k but our bills were circa $2500 per year so hoping payback on current usage will be about 8 years.

    The most important thing i found was finding a good reputable installer. A massive cheap system installed by dodgy installers is a much worse investment than an expensive system installed by a good installer. Ideally a middle ground but we spent a lot of time researching and getting quotes. Check ABN history to make sure they have been in business for a good number of years. Check a wide range of reviews and don’t buy a system off anyone who does not come onsite to quote. We used BJ White in Hastings Victoria and I could not recommend them highly enough.

    • The Hawk on 04/11/2025 - 11:35
      +1

      We upgraded from 6.6kW solar on 5kW Inverter to 13.2kW panels, 10kW Inverter + 32kWh battery and have a similar pay back period and are on the same plan. On really grey days like the last couple in Melbourne we might not get the battery to full capacity if we are doing things (like washing/drying clothes and dishwasher) but we have barely touched the grid at all outside those free hours.

      My last months bill was $9.

      Part of me wishes I got a little more battery, even if I haven't needed it yet. All the math says this is the right sizing for me and extra would be totally unused most of the time (especially in winter where I likely couldn't even get that much charge into it)… but I think the incremental cost (with the current rebates) for another 6 - 12 kWh of battery would also provide for a longer life as the total capacity eventually degrades.

      If you're building, as has been said, get a solar guy to do it, rough it in before walls and install later. If you're not already, I'd also make sure you provision things like a dedicated circuit for future EV charging (possibly two!) now since it's going to be so much easier at build. Same story for your switchboard, bigger is barely any extra cost but saves an upgrade down the path should you need it.

    • Poor Ass on 04/11/2025 - 14:26
      -5

      We’re a family of 4 using about 30Kw per day

      ridiculous you are using too much

  • Summoner on 04/11/2025 - 09:38
    +1

    10 kW for sure — especially if you’ve got ducted aircon. Those systems alone can pull 4 kW+ at peak, even more if it’s a bigger unit, so a 6 kW system will struggle to keep up.

    • UltimateAI on 04/11/2025 - 21:26

      4 kw, that's a baby…:) or a modern ducted systems a lot more efficient than old school systems.

      • Summoner on 04/11/2025 - 22:01
        +1

        Yeah, I upgraded our ducted system last year after the old one failed — the repair cost wasn’t worth it.

        Our Mitsubishi Electric 14 kW inverter unit normally runs at a low, steady draw, but on hot days it peaks around 5.5 kW several times.

        I assumed lower peak consumption as it could be a smaller unit if OP's home is single story.

  • Sito85 on 04/11/2025 - 09:41
    +2

    If you’re going with a big builder (domaine, masterton, edenbrae, etc.), they will overcharge you for any changes. I got the 6.6kw panels for free as well and when I asked for the 10kw or for a 6.6kw hybrid inverter (battery ready), they wanted to charge me full price.

    My advice, get your freebie and upgrade after handover.

    • Ulysses31 on 05/11/2025 - 21:31

      (domaine, masterton, edenbrae, etc.)

      Who?

  • Mr BoMBAStiG on 04/11/2025 - 10:59

    I am def in the "fill your roof" camp but you need to ensure you have the ROI.
    Do you have appliances and needs to consume the majority of the solar you generate. Feed in tariffs will be $0 in a year or two. You need to be able to use most of the solar you generate or store the energy in a battery or hot water system.

    Feed in your install cost, approx usage (number of people in your household, appliances, pool pumps, HWS specs etc), current energy prices and AI will be able to calc your ROI fairly well.

    • UltimateAI on 04/11/2025 - 23:35

      Not wrong, but now having a battery\solar, the mindset change is worth the $$$. No more skimping on heating\cooling, just live our lives and go nuts.
      I call it free, wife calls it "pre-paid". But so much more relaxing, not worrying about energy costs. A single hot day was costing us around $40.

  • djsweet on 04/11/2025 - 11:41
    +1

    The panels aren't that expensive so go as big as you can. The battery is the expensive bit. But it's good to be able to achieve 24hr coverage. We use most of our electricity in early morning and evening where solar isn't producing, so without a battery it's almost worthless. Get a system big enough that it will be able to fill a battery in winter which will take you through the evening and early morning. In winter you will produce about double your system size in kwh. So you will get about 40kwh out of a 20kw system. If you use 40kwh in a day and around 30 of that is when the sun isn't shining, then you want a 20kw system with a 30kw battery.

  • BiotBargin on 04/11/2025 - 11:50

    10kw but get some of your panels facing west (roof space provided) or adjust some of the north panels to tilt west if all on one roof face.

  • knasty on 04/11/2025 - 12:43

    Depends how much the extra is (chances are it won't be good value from a builder). I've got 15kW of panels across east, north and west aspects and a 28kWh battery and that works for us as a reasonably high usage household. Also consider which retailer you will go with eg. I'm with Globird on their ZeroHero plan so I get free grid power from 11-2pm every day - all the heavy use appliances are scheduled for this window. With a big enough inverter this allows for full charging of the battery every day during this window, irrespective of season/clouds etc. So saving on additional panels now may allow you to get a bigger battery (again likely better value getting it from a 3rd party vs your builder).

  • drprox on 04/11/2025 - 13:12
    +1

    You do mention an EV and that is a big factor.
    In which case absolutely yes 10kw.

  • Vash1984 on 04/11/2025 - 14:39

    Don't forget the opportunity cost of forking out more to get a bigger system. That additional $3000 could be invested elsewhere. So when you calculate break even, you need to take into account potential investment earnings, especially if you are taking out a loan. Also im with agl and on a good day they buy back $1.60 and there is a cap. So on good days I really have no use for solar. I'm not going to switch to battery any time soon because roi is probably 7 years and the battery will be cooked by then.
    Best thing I've done so far is buy additional large freeze so I can buy meat and ice cream whenever it's cheap. Also no longer hang out my clothes, just use a dryer (don't get heat pump) saves me heaps of time.

    • UltimateAI on 04/11/2025 - 23:38
      +1

      Agree about not getting the heat, standard dryer on delicate\normal doesn't use that much power. Even with my small solar system, on a cloudy day can run it fine. So much quicker than a heat pump.
      If it's really cold, we just hang on a rack in-side and durn on the central air.
      Ironically having solar\battery, now far less concerned with energy consumption.

  • drewbytes on 04/11/2025 - 16:12

    I went from 5 > 10 > 18. The more the merrier (to a point).

  • fedex11 on 04/11/2025 - 19:54

    With a 10kW panels, would a 5kW inverter be ok? Or a higher capacity inverter be recommended for a better system?

    • noz on 05/11/2025 - 08:01
      +1

      Unless you are getting a battery, you can only oversize the panels up to 33% of the inverter size. Hence a 5kW inverter can only have 6.6kW of panels connected to it.

  • Kegsta on 05/11/2025 - 08:24

    My advice would be just to wait until it's built, your settled in, then get quotes，target the instead date to be late 2026 before the next STC price drop.

    Batteries are only getting cheaper and bigger each year.

    Im in Tassie with a 13kw system and want to get a battery so I can DC couple and add another 7kw of panels, but it's not financially viable at the moment.

  • Poormansmeal on 05/11/2025 - 12:55

    Thanks very much indeed for your thougts

  • bigbadboogieman on 05/11/2025 - 20:42

    Depends on your usage to be honest. We used ChatGPT and 2 years historical data from our retailer to identifying our sizing. It turned out we are fairly low usage household so that 6.6kW was perfect for us. The sizing we got for our battery was 8kW. We said double that to future proof so 16kW. Went to market and found that FoxEss was selling 42kW for the price of 20kW from other brands. Did our research and found FoxEss has quite a massive presence in the UK. There is a 15K community for FoxEss. The lowest we have managed to get in the battery overnight is 82%. Even with 6.6kW system, our battery is topped up to 100% by 11am everyday. During the last week when it was pretty much fully overcast, raining or heavily raining, the battery was compensating for lack of powerful sun and still managed to get to 100% by 2pm.

  • serpserpserp on 05/11/2025 - 21:12
    +1

    If you are going to get a battery and an electric car eventually I'd get a 10kw system.

    I have 6.6kw system currently and it is fine for my needs.

    Just make sure you get above a 5kw inverter with your 10kw system

  • soan papdi on 05/11/2025 - 21:19

    Visit https://aldisolar.com.au/ and look for the Estimate my payback period form. It's a very simple calculator but enough to understand if more kwh is going to help you or not. For me, even a 6kwh unit with 10kwh battery takes 10+ years to pay back, I've decided to not install solar for now.

  • misu p on 05/11/2025 - 22:29
    +1

    Go for 10kw. Always, always max the solar panels.

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