How To Beat The Solar Panel Oversize Rule With Grid-Tied Solar And Batteries

Australian solar oversize rule

Breaking the infamous ‘133% / 75% Solar Oversize Rule’ won’t get you in trouble if you add a battery. Or will it?

It depends on who you ask…

But before we get to that, a brief history of the rule.

The Australian Solar Panel Array Oversize Rule

Why do solar companies everywhere offer 6.6 kW of solar as their standard offer?

It’s because of the rules. Or, to be more specific, the “Clean Energy Council Guidelines For Grid-Connected Solar PV Systems (No Storage)“. This is a set of guidelines that must be followed when designing and installing solar power systems.

If your solar designer does not follow them, you are not eligible for the STCs on the system. In other words, you won’t get the ‘solar rebate‘, and your system will be thousands of dollars more expensive1.

We are talking specifically about “Guideline 4.4 : Array peak power – inverter sizing”:

4.4.1 In order to facilitate the efficient design of PV systems, the inverter nominal AC power output cannot be less than 75 per cent of the array peak power and it shall not be outside the inverter manufacturer’s maximum allowable array size specifications.

The first part of this rule is the one that matters here: “the inverter nominal AC power output cannot be less than 75 per cent of the array peak power”.

Or, to put it the other, more intuitive way…

The solar panel array cannot be more than 133%2 of the inverter’s size in kW.

History Of ‘The Oversize Rule’

Back in ye olden days of solar, shortly after the Renewable Energy Target was conceived, the Small Technology Certificates (STC) scheme was invented as a way to separate out the financial incentives for large (100 kW+) and small solar systems based on their theoretical carbon abatement. Essentially it was a carbon tax introduced by John Howard, but luckily the troglodytes were never bright enough to realise or stupid enough to politicise it.

When you install a solar thermal hot water unit, heat pump hot water or solar PV, there is a value attached to the abatement of greenhouse pollution. You get paid upfront for the coal and gas you don’t burn from now until 2030.

STCs for solar are based in part on how many watts of solar panels are installed. But what if you install more kilowatts of panels than the inverter is nominally rated for? That could result in slightly less kWh of energy per kW of panels installed than a solar array that matched the inverter output.

Bureaucrats hate anomalies like that, so it was decided to limit any oversizing to 133%.  By their judgement, 133% would make good use of the hardware and deliver the best efficiency in terms of money spent for carbon abated.

The thing is that solar inverter manufacturers invariably rated their machines for way more than 133%. You could wire up 150% or even 200% of the inverter nameplate capacity, and it would simply work closer to its maximum throughput for more of its life.

Of course, there are times of good insolation when the inverter wouldn’t be able to handle all of the available energy, it would clip the output, running flat out for an hour or two in the middle of the day. But for most of the day the inverter would be passing through more useful energy than a non-oversized system.

‘The Rule’ Is Way Past Its Sell-By Date

Before the CEC even existed, SEIA were the local industry body. It was a union of like-minded professionals who assembled a standards, training and assessment panel, which in turn wrote a document entitled “Advice on Best Practice”. This was a well-meaning ‘code of conduct’, circa 2007 that was tweaked by 5% and adopted by the CEC as a hard rule without further scrutiny.

The CEC members’ area links to a document explaining the rationale behind the rule. The latest version is dated February 2013.

CEC oversizing explanationJustifying the rule, it says:

“We believe that this limitation will allow the best possible design for the vast majority of cases while still allowing flexibility.

 

At this value, systems installed in the optimum orientation and tilt angle will not suffer greatly from yield loss and also allows for some oversizing to offset losses due to non – optimum installation locations. “

Unfortunately, this rule heralds from a time when:

  • There was 10 x less rooftop solar installed in Australia.
  • Solar cost 2-3 x more than it does now.

And pretty much every drop of sweet solar energy installed back then directly offset fossil fuel based electricity generation.

Fast forward 10 years, and maximising the yield of your system in the middle of a perfect solar day is not so important. There’s often so much solar electricity generated at those times that the wholesale electricity price goes negative. That’s the grid’s way of telling you there’s too much energy in the system.

When the grid really needs your solar to pump out more energy is over winter, on overcast days and early and late in the day – exactly when well-oversized systems will deliver.

But due to the outdated 133% oversizing rule, solar owners are limited to a solar array 133% more than their DNSP’s (local electricity network’s) mandated maximum inverter size. Often that’s a measly 5 kW inverter per phase.

Which goes to the point I’ve made for 15 years now: I’ve never met a customer who’s complained they installed too much solar.

What Does Oversizing Solar Have To Do With Batteries?

Well, to quote Tom from the Clean Energy Council recently,

“That’s an interesting one.”

If you have a home battery connected to your solar installation, you can now exceed the 133% oversize rule and go to the full inverter manufacturer’s recommended capacity. That means you can claim STC incentive payment for 150% or 200% of the inverter’s nameplate capacity.

Hence the CEC Installation Guidelines with the ‘133% rule’ are “For Grid-Connected Solar PV Systems (No Storage)“. The suffix is recent.

CEC guidelines for grid-connected systems without batteries

So, a 5 kW solar inverter with a battery is no longer limited to 6.666 kW of connected solar panels. You could have 7.5 kW or 10 kW of solar connected.

If you are lucky enough to have a DNSP that allows a 10 kW inverter with a 5 kW export limit, with a battery you could connect 15 kW or even 20 kW on a single phase.

That’s a boon for solar energy output in the morning, evening, winter and in shitty weather.

Sounds like a winner, doesn’t it? Yeah, but this is solar, and the rules are sclerotic.

The CEC is telling us that the guidelines they write and oversee technically mention the inverter being not more than 75% of the array capacity (the inverse of 133%), and that ratio is what matters to them. They specifically point to the front page of the document stating batteries are not considered where solar PV capacity is concerned.

So, the CEC won’t provide any assurance that a battery system oversized by more than 133% will be eligible for STCs (AKA the solar rebate).

I Swear, This: ¯\_(ツ)_/¯  ..Should Be The New CEC Logo.

It’s the Clean Energy Regulator who actually have the legislative clout. They are the ones who make the determinations, and it’s at their discretion that systems claiming over 133% are approved. When I sought clarification from Peta at the CER, she referred me to Tom at the CEC as being the body they farm out the technicalities to. I was told the CER don’t have enough technical expertise.

Wot?

It’s a question others have asked in the solar industry, and I know of at least one installer who has doggedly pursued some clarification until he got a “definitive” answer about a single point. He satisfied himself that the CER would allow extra STCs to be created for hybrid battery inverters, but NOT for systems using AC-coupled batteries such as the Powerwall3.

Many installers will pick their combination of technology and submit the paperwork to the STC dealer/aggregator and take the money when it comes through. But they are taking a risk that the system is deemed non-compliant and the entire STC payment has to be returned4, or they have to argue the point.

It’s this sort of ambiguity that drives people out of the solar industry.

If you think the 133% oversize rule is way past its usefulness, then please send an email to:

[email protected]

Comments are open until tomorrow (December 6th) but may be accepted afterwards, at the CER’s discretion.

Personally, I would just like to see them pay out STCs to full inverter spec, or at least allow you to claim 133% and then install the extra capacity you want without losing the entire STC payment.

Oversized Solar With Batteries Help Deliver More Solar When It’s Needed Most

We need to install more solar panels well over the inverter capacity to deliver:

  1. more power in the mornings and evenings, while output will be clipped or throttled at midday.
  2. more power up to the DNSP export limit for more hours of the day.
  3. more power that is more valuable to you, the customer.
  4. more energy, over and above the DNSP export limit, to charge the battery (depending on your design).

So let’s hope the CEC and CER can lose the 133% oversize shackles of the past. A few hundred emails from you lot may just help.

Footnotes

  1. A note on Australian Consumer Law – if your solar retailer is at fault here – they lose the dollars – not you.
  2. 133% is the reciprocal of 75%
  3. However, there weren’t any rules forthcoming for a grid connected battery system using a multimode battery inverter charger with an AC coupled solar inverter. And off-grid systems weren’t covered in the series of emails either, because of course you can have AC coupled batteries in off grid systems too.
  4. you lose all the STCs – not just the ones over 133%
About Anthony Bennett

Anthony joined the SolarQuotes team in 2022. He’s a licensed electrician, builder, roofer and solar installer who for 14 years did jobs all over SA - residential, commercial, on-grid and off-grid. A true enthusiast with a skillset the typical solar installer might not have, his blogs are typically deep dives that draw on his decades of experience in the industry to educate and entertain. Read Anthony's full bio.

Comments

  1. You dont necessarily lose ALL the STCs – your installer can just remove the serial numbers of the necessary number of panels to ensure the system remains at 133% of the inverters capacity, allowing the customer to get the majority of the STCs.

    • energy companies will use things like google maps to see how many panels you have on the roof so this is not a good idea. better to go the storage battery path mentioned in comments

  2. Peter Brett says

    I haven’t gone back to the “books” but one of Selectronics configurations lets you do what you like with a battery/inverter system. The input/output from/to the grid is limited but I seem to remember battery and solar panel capacities are open slather. Maybe someone could comment on this?

    • Erik Christiansen says

      As off-grid will have a (big) battery, the battery related certificates rule is great news for my impending rural install.

      Diverging a little bit now: I’m hoping to use a 7.5 kW Selectronic 482 to have a better chance of starting the 1.5 kW induction motors on lathe, mill-drill, and compressor, or run the arc welder, Its heavy transformer is said to add high surge capacity, I’m wondering if there are other good inverter options here, 10 or 12 kWp for several seconds? An additional AC coupled PV inverter will handle another array on a second roof, but the sun might not be shining when the compressor kicks in.

      I’ll probably have to go to whirlpool to ask if anyone has a better way to monitor an SP-Pro on Linux, other than to run the MS-Windows software in a virtual machine? (Windows would be new for me, and excess novelty is a nuisance at 68, when there’s so much else to do.)

      Some other mastodon of an inverter, able to start even the compressor, would neatly sidestep the ugly SP-Pro monitoring restriction.

      I have to know what will be tolerable in service, beyond simple energy delivery, before talking to installers, I figure.

      • Anthony Bennett says

        Hi Erik,

        I’m not sure if you’re across the Selectronic monitoring solution that is SpLive? It’s a fairly customer friendly graphic rich display with numbers to show energy flow and detailed history. For the non technical customer, the state of charge indicator on the output graph is the thin blue line to live by.

        It also gives you or your installer to remote access to the full suite of functions available via a conventional SpLink session with your laptop.

        I think there are some people who still persist with SMA SUnny Island or Schneider inverters. Victron make some great packages with a mind boggling number of options and great online visibility… but you need to rate them for their battery charging capacity. ie a nominal 5kVa Victron is only a 3.6kVa battery charger in continuous service.

        The SpPro is a 7.5kW machine in both directions, continuously at 25degC, with surge to 18kW and overload ratings for various thresholds in between. There’s simply nothing better, especially when you have a 10 year warranty and brilliant onshore support here in Australia. If the RFDS are going to save your life at Innaminka Clinic, they’re relying on an SpPro.

        Another interesting tidbit for you is that off grid solar PV can be designed and wired for a full 1000VDC under AS5033, the grid connect AS4777 doesn’t impose the silly 600v rule for inverters that aren’t grid connected.

        Cheers

        • Erik Christiansen says

          Many thanks, Anthony, for your info. (That 1kV array on off-grid is very interesting.) I now like the Selectronic SPMC1201 better than the 482 for off-grid, as more power can be pushed into the battery at a limited current at 120 volts. (I’m retired, so the $30k that my brother spent on Zenaji batteries is a bit much, despite their high allowed charging current.)

          The Zenajis might be necessary, though, as I’m struggling to integrate an EVSE with the SP Pro. Victron have an EVSE which will only use excess PV generation for EV charging, and that’s vital off-grid. With their little data cables, it may be their Cerbo GX control gizmo which tells it what to do. I’ve asked them, as I’d like to have a full system capability concept nailed down before the installer nails any gear to the wall. (An install which works OOTB is a good install.)

          As you indicate, that runs into a charging limit of 2 x 3.6 = 7.2 kW from two 5 kW Victrons. With global warming observably increasing cloud cover in East Gippsland, I could always throw another inverter at it.

          The Cerbo GX apparently has a Generator Start relay, which could be used for signalling power diversion to the HWS on selected battery SOC, with inverted logic and swapped thresholds. That’s if Victron have no other way to control a third priority power diversion.

          I’m ditching V2H, as EV batteries have limited cycles. (And as Finn has shown us, a bidirectional EVSE is just far too expensive to be worth a retiree considering. The difference can pay for more or better batteries.)

          My brother’s install has two 5 kW Victron battery inverters, and they start his compressor, and run his welder, with the grid breaker thrown, so I can be persuaded to relinquish my attachment to the beefy 7.5 kW 50Hz transformer of the SP Pro, if that’s what gives a system integrated EVSE.

          I’ll need to gather more information, as system integration is not a strong point in this developing market, I find.

  3. Richard Courtenay says

    How long would it take to pay back the cost of a Tesla 10 kWh battery on feed in tarrif alone? Would it be more economical to use the additional panels (beyond %133) to charge the battery and disconnect from the grid at night? In other words, do you earn more feeding in at night with the battery than what you would pay for night time grid power ?

    • Finn Peacock says

      As a rule of thumb: You’ll save about $500-$800 per year if you use a 10 kWh battery to power your home at night instead of the grid – if you are on a flat tariff and if you use most of the capacity most nights.

      The sum is: annual savings = 10 kWh x (flat rate – FiT) x 365 days
      e.g. in SA: 10 x (30-8) x 365 = $803,
      e.g. in VIC: 10 x (22-7) x 365 = $547

      If you are on a Time-of-use and use the battery to ride through the peak period, and employ the battery most days you’ll save a bit more:

      SA: 10 x (35 – 8) x 365 = $985
      NSW: 10 x (30 -7) x 365 = $839

      These savings require enough solar to charge the battery from solar even in winter – which is where extra panels rally shine.

      • At $500 to $800 savings, you are getting back less than the interest on your mortgage on a $17k battery, and therefore you never recoup the capital cost.

        • David Thrum says

          The point is not just saving money. It is also about climate change and the planet.

          • The point is it won’t be popular if it makes no financial sense. Nor does financially silly small systems make sense if larger scale systems are more viable.

        • Rudolf Ruyter says

          For our investment in our 10kw battery, payback over time was not the deciding factor…..We live in the Dandenong Ranges and experience storm power loss regularly , and sometimes for days…. The battery investment is to make us operational and electrically self sufficient during such outages…..
          The battery is backed up by a 20kw solar array….
          It’s a new installation (2 months old) and so far, the battery has been a brilliant buffer for sometimes greatly variable solar input…. and easily powers the house all night.
          The only downer is the inverter clipping our available power down to 10kw, which is something I did not expect….as our intention is/was to go totally electric… which will require well over 10kw (prob about 15kw) for about the summer 1/2 of the year.

  4. Presumably making it easier to have more solar panel capacity for any given inverter capacity will particularly benefit those who can install a combination of east and west facing panels, as they have separate peak generation times, if the inverter can handle that?

    • Leap of faith says

      My place is north east and south west facing. Recently put a 10kw solaredge system (it’s alright, does the job. For the data it has, the app it woeful. But I digress)…

      Had an old 5kw solax system that I incorporated half of the panels on top of the new 10kw system. Worked out to be about 13kw.

      The 10kw went south west facing and a couple spare panels supplemented the north east original panels. Turns out my afternoons are unrestricted by shadows unlike the north east in mornings. A shallow pitch roof also help.

      Longish story short. Even though I’m speed limited to 5kw export to the grid. If I have something to dump the energy into, it’ll do 7-10kw from 7am to 4pm.

      With the Powerwall and the Tesla arriving a few weeks ago. My commuting and home electric is essentially off-grid.

      Round figures, the car, Powerwall, heatpump hot water and the solar 10kw system costs about $1100 per month. But I’m saving $650 in fuel, $200 in home electric, $100 in servicung.

      All of the above is roughly costing about $200 per month.

      For the $110k leap of faith it was. Couldn’t be happier. Fuel and electricity prices can do what they want from here on in.

      • In sincerely doubt that with only the equivalent of 3kW of panels facing NE (with shadows) you are generating anywhere near 7-10kW at 7am. I expect the SW facing panels would be generating near zero at 7am.

  5. David Morgan says

    I looked at this for my real world data. This is a system facig three directions with some shading and micro-inverters. I downloaded the half hour energy outputs for my system over the last two years, and looked at the effect of power limiting on the total energy delivered. I found that a imit of 40% of nominal power capacity reduced the total energy output to 89% of the energy with no limit. Limiting to 30% power would give 77% energy output. So panels up to 3 times the export limit as well worth having.

  6. Ian George

    I find i am going backwards with my Origin Plan. !0c Solar Feed, 32.7c Gen Use, 17.4c Control Load, 179.3 /Day & 17.3c/day Supply. 6.6 Kw of Solar. Going Backwards by $100 per qtr and expect more Rises.
    Looking at 5kw Modular Batteries and Charger and going off the grid, plus EV Charging, Plus using Solar to replace fossil Fuel Machines.
    Regards Ian George

    • Best of luck paying your battery off at $100/quarter.

    • Suggest you look at another provider, I’ve recently switched from Origin for the same reason. Origin then attempted to offer me a capped FiT. (They have lost the plot)
      My new provider is Power$%#@! , apparently, its a dirty word around here
      The new provider actually discounts the connection fee. I assume you are an Essential Energy customer, as the rates are identical

      • John Rogers says

        Another possible energy retailer Simply Energy. Their EV plan pays me 11 cents /kWh feed-in tariff and has a very low Super Off Peak rate of about 12 cents /kWh.

  7. Peter Brett says

    I can’t offer much help but where are you to be hit with 179.3 c/Day?

  8. I just add on myself after the intial install..

    • Mike Stringer says

      Yes! this was my thought. What would happen if I added extra panels to an existing system that was already at the 133%? Clearly, I would not be claiming the STC rebate for the new panels, but would I then have to pay back the rebate I have already received for the original ones. I have a Powerwall 2 so that does not help as it’s AC coupled and therefore excluded.

      • Anthony Bennett says

        That’s a very good question Mike, one which would I think turn on the possibility somebody from the CER is ever going to audit your property. You’re not claiming any more STCs so beyond that it’s really just electrical work on your roof.

        Cheers

  9. S Campbell says

    Thanks Anthony for the practical advice to email CER. I have done so, but could only speak from my own experience and what it would mean to me, which could be politely titled ‘novice’. Let’s hope a few more send emails, amplify the problem and make the road a little wider for solar set up options.

    • Anthony Bennett says

      Thanks Stephanie,

      I can only assume that the more responses there are, the more weight we have, so every bit helps.

      Cheers

  10. allan hamblin says

    “The CEC is telling us that the guidelines they write and oversee technically mention the inverter being not more than 75% of the array capacity ….”
    Did they really say that, or did you make an error? Shouldnt it be “.. not LESS than 75% ….”?
    Types of inverters, connection types, etc. confuse my old brain. Can I install say 10kva of panels with a hybrid inverter, tempoarily some lead acid bats, but add an ev car when a suitable one is available, or is such an inverter (for V2H) not yet economically available?
    The various rules, options of system etc, are very difficult comprehend.

    • Anthony Bennett says

      Hi Allan,

      Thanks for the correction. As for the solutions available for powering your house and car, I understand your confusion. There are so many options it’s mind boggling but nothing I would say is a perfectly good one. You can have a hybrid Sungrow with 48v lead acid batteries, or a Selectronic inverter charger, but the only 48v vehicles I know of are forklifts and luggage carts.

      As for V2G or V2H, again there a lot of options which have varying capacities and functions. We’ll shortly have an article out for V2G that is approved in Australia but it’s currently under embargo. Please stay tuned.

      Cheers

  11. Peter Robinson says

    I have two phases to the house. Can I get the solar output linked to both phases. This would mean more of thr generated power would be used rather than going to the grid at 20% of what we pay to get it back overnight.

    • I have mine linked to 3 phases.

    • Anthony Bennett says

      Yes Peter,

      Most if not all DNSP approvals are around 10kW conected inverter capacity per phase, (with a 5kW export limit) so you can have a much larger system at your place using two single phase inverters.

      Cheers

  12. I think the over sizing rule should only apply where you have AC coupled systems. If you are DC coupled and charging before the inverter go for the max!

    The CER will issue approvals to bypass the rule (recently seen a 200% overrun on a Solar Edge system approved) with out looking at the overall job.

    All this was was an attempt (successfully) to by-pass the need to have any secondary protection. 29kW of inverter with 60kW of panels. Clipping for 40% of the day. Battery was 2 powerwalls.

    Oh to have the money….

    • David Thrum says

      That is my understanding. I had a single phase 10kW system (SolarEdge) with 13kW panels (max allowed) with 5kW export limit (Energex rules – QLD). Upgraded with SolarEdge hybrid inverter and SE Energy Hub inverter and SE AC-couples batteries. Added another 3 kW panels to total 16 kW (no issue with STCs – installer said I could have gone up to 20 kW panels).

      Yes there’s curtailment on sunny days, but on overcast or rainy days still mostly get enough charge to last through the night (< 1 kWh imported). In winter curtailment will be less, and also I charge my EV mostly from generation that would be otherwise be lost due to Energex 5kW export limit. EV is 100% charged from my solar using the "ChargeHQ" app (chargehq.net) to control the car's charge rate.

      Was it worth the money? Probably not in financial terms (at least not for a few years yet) but shows what can be done. Smart EV Charging (and battery charging in general) can (and must) play a significant role in addressing the infamous solar duck curve issue.

      • Anthony Bennett says

        That sounds like a great solution David.

        Cheers

      • Sounds like a good system.
        Doing my research into getting my own system.
        The max 3 Phase Hybrid Inverter is 10kw
        Is that 10kw per phase or 10kw in total (roughly 3.3 kw per phase)?
        Was thinking as I have 3 phase power available
        60kw of panels – 20kw per phase
        10kw 3 phase hybrid inverter
        Appropriate sized battery
        dc coupled

        Is this even feasible?

  13. Jeff Ellis says

    We can talk all we want about adding more panels to a roof but if those panel are inappropriately positioned it won’t help anyone increase their solar generation or save on their power costs

  14. Tim Falkiner says

    I can understand governments limiting grants to a specific solar system size but I do not understand how it can be argued that the owner cannot install as much solar as the owner wants subject to limiting exports to the grid. The only reason I can think of is that it forces the householder to buy more energy from the grid and this is environmental vandalism. These rules are too important to be made by statutory bodies and should be passed by parliaments in legislation.

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