Solar Accreditation Australia (SAA) has just dropped version 1.3 of their solar installation guidelines, and the good news is that they’ve tweaked the rules in Section 4, making it easier to oversize your solar setup without having to tell porky pies.
The new SAA guidelines modify the longstanding “0.75 rule” that your solar inverter’s capacity (in kW) must be at least 75% of your solar array size. Or, to put it more intuitively, the rule that your solar array size can’t be more than 133%1 of your inverter capacity. That’s why you see so many 5 kW inverters with 6.6 kW of solar panels attached. 5kW x 1.33 = 6.65 kW.
Previously, the rules said you could exceed the “0.75 rule” if you had a DC-coupled solar battery. The idea is that even if your potential output was curtailed by the AC capacity of the inverter, extra solar power could bypass the solar inverter and charge a battery directly.
However, the rules implied solar with an AC-coupled battery could not break the 0.75 rule. That’s a bummer if you have, for example, a Tesla Powerwall 2 and want a nice big solar array to charge it all year round.
But thanks to the latest rules change, you can now exceed the 75% rule with AC or DC-coupled solar battery systems as long as the inverter manufacturer allows it.
Manufacturers commonly allow 150% or 200% oversizing of solar, so it shouldn’t be unusual for a 5kW inverter to have a 7.5kW or 10kW array. If you’re really mad then I think SolarEdge will even handle 300%
Pragmatists Have Been Doing It Anyway
While an AC-coupled battery didn’t qualify, some electricians sought clarity from SAA’s predecessors, the Clean Energy Council and STC fraudbusters, the Clean Energy Regulator, neither of whom were interested. I know of at least one installer who browbeat the authorities into providing documented proof that oversizing didn’t apply to systems using the Tesla Powerwall 2.
However, others have asked about different brands using a similar architecture and received different advice.
The upshot has been that solar-rebate (STC) forms are filled out with the battery box ticked off, a serial number added, rebates paid, and no questions asked.
I have the emails to prove this assertion
A Little History
In 2002, the Business Council for Sustainable Energy published a good practice book with rules of thumb for inverter sizing. This was before standards existed for solar installation, so because Solar PV was incredibly expensive, it was thought you shouldn’t load too many panels onto an inverter or regulator.
The 0.75 ratio was a guideline to ensure that this really expensive glass wasn’t wasted and that the systems would perform properly.
“For some stupid reason this became a RULE!!!”
– Glen Morris
BCSE Standards, Training & Accreditation committee member.
Solar Is Now CHEAP
People used to worry about the efficient use of glass on the roof, because solar PV was $5 per watt. Now that wholesalers are ringing me to offer respected brand name panels for 19 cents per watt, it really doesn’t matter where you point them, just so long as they’re not totally shaded.
Where The Devil Did 6.66/5kW Combo Come From?
As mentioned, the 0.75 rule led to 6.6 kW arrays with 5kW inverters becoming the most popular size for many years. But where did the 5kW inverter limit come from?
Network companies have insisted on export limits of 5kW per phase for many years, so at any given instant, you couldn’t drive more than 5kW (or about 22amps) back into the grid. This was achieved by consumption monitoring, applying a throttle to large systems, or using a 5kW inverter.
By moving to flexible/dynamic control, many networks are starting to lift this hard limit to 10 kW of inverter power, allowing 20 kW plus of solar panels on a single-phase inverter if you have a battery. Now we’re talking.
Now AC & DC Batteries Both Officially Comply
Whether you’ve got an AC coupled or DC coupled battery, you can now oversize your system with solar to your inverter’s limits. This means you can push your yield further and claim STC credits on all the panels legally, no matter what kind of battery you’re using.
The new rules are live as of June 21, 2024 so you can get straight into installing big solar battery systems without breaking any archaic network rules.
Footnotes
- 133% = 1.33 = 1 divided by 0.75 ↩
About Anthony Bennett
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Not only was the rule stupid, it was applied to all solar systems. Meaning those that had east-west oriented solar panels couldn’t bump up their solar panels to make up the loss of output when the panels were facing east/west as opposed to facing true north.
The rule blindly assumed that all solar panels were facing north for maximum power output. Not all houses in Australia have large roofs facing north. Which is another dumb thing that councils do. They don’t lay out subdivisions so houses are built for maximum solar position. That is the widest part of the house should be facing north and the narrow part of the house facing east/west to avoid the summer sun since it’s hard to shade whereas maximum frontage should be north since the summer sun is almost overhead, the eaves shade the window during the middle of the day.
Everything you’ve said is true, I’ve been saying the same for years.
Exactly! And just add planning controls over the height of vegetation to gutter height along with planning restrictions on roof design to complete a solar friendly environment.
Hi,
We have 13.2 KW of solar panels and a 10 KW battery.
What we did was take out loans to buy it all!
In the Summer, it works, but the Winter sees us with monthly bills of around $500 per month! I’ve added it all up together and even with the worst-case scenario, we are still as much as $300 per month better off than before the install. So, for those who say they can’t afford to pay for solar, I just ask how do they pay their electric bill. If you can pay that, you can afford solar and save money.
Whilst I agree somewhat, the assumption is that people can either obtain a loan and or have the cash sitting in the bank to be able to make the purchase.
I think that many people for whatever reason, believe that by installing solar and batteries, they won’t get an electricity bill and this is many times inferred by dodgy sales practices.
Hi Eric,
I’m just wondering what appliances you’re using in your house, to still have a $500/month electricity bill in the Winter, even with 13.2kw of solar panels, and a 10kwh battery?
I have an archaic system with only 2kw of panels and no battery, and my bills in Winter rarely exceed $300 per quarter.
I’ve been considering upgrading to a larger system with a battery, but your figures are giving me second thoughts.
That’s a great size system and battery that should cope well for most households.
I’m perplexed though, your usage must be massive !
A large family that uses power a lot, or a lot of power hungry work equipment ?
Our household is empty nest now, just me and the wife.
I love working though, and regularly use a compressor, heat lights, all sorts of power tools, also a lot of IT stuff running 24/365.
We are an electric home (no gas) and regular HWS, slow combustion fire that heats whole home, well insulated, no air cond needed summer until 5 + days of over 35c.
With 11.23kw of panels facing east, north, and west, no battery, we killed it in first summer just gone with no bill, shoulder $250, and now looking at $500 first winter.
We now struggle with 2 massive lemon scented gums that shade the whole winter roof through the day, but they are going early August.
Basing power on pricing, we are in SA, and paying the highest tariffs in OZ, with a 50c peak rate.
Before solar, our bills were $500 – $900 / close to $1000 a quarter sometimes.
Bottom line, if we had a battery we’d have no summer or either shoulder bills, and probably a very small winter bill, but then I’d then charge a battery on EV rate 8c through early hours 0000-0600 to cover all day use.
I’m struggling to see how you are not getting a lower power cost in winter, maybe you do use that much, have big shade issues, and / or live Tassie or such.
Just takes a big ducted AC system, HW, charge car, no wood fire anymore, to easily go from a 12 kWhr average to 30kWhr per day house.
You really should go through your home and see what’s on standby and learn to turn the appliances you don’t need on standby off. if you have old appliances maybe consider updating to more energy efficient ones.
My off grid inverter converts the 140 V dc from solar panels to a controlled 50v dc for my backup batteries it uses an mppt charging system is this a dc coupled sysrem
This is very welcome news. Thank you for sharing it.
“Now that wholesalers are ringing me to offer respected brand name panels for 19 cents per watt” = wow, that is unbelievably cheap.
Surely someone out there must be thinking about selling a 50 or 80kWh house battery soon? At the start of this year the wholesale cost of a 50kW EV battery was $9,500. So just take an EV battery, put it in a case with some software, and sell it for $20-25K? Then everyone can stop dicking around with Tesla powerwalls and modular 5kW batteries, and you’ll be set for the foreseeable future. If cars get bi-directional charging this will also be a good thing, but still could be many years off actually happening, so in the meantime there’s a huge opportunity for a battery company to storm the market. I don’t get why it isn’t happening?
Where are you finding an 50kw EV for $9,500?
That doesn’t sound like an unreasonable “assumption” for a car battery. But even if you could get an actual replacement battery at that cost , it still wouldn’t take into account all of the extra bits of gear that come with the home battery setup.
As an example, I’ve been gradually converting all of my heavy duty work tools to battery over the last few years to avoid the joys of carbon monoxide inhalation when using them in confined areas. Just one of the many benefits ;).
The expensive ones have far superior features, mostly safety related. I’d hazard a guess that’s similar to the car battery situation.
Apples to oranges, or a-hloles to ear holes.
There’s no way the BYD cars I’ve looked at have 3x the 20Kwh tank that runs my house strapped to the bottom of them.
Hi Anthony, Will this rule change apply to Victoria as well
Yes – it’s national guidelines.
Do something about the ‘got any questions’ thing – makes reading your excellent articles near impossible on an old fart mobile!
Cheers
Bob Tumath
I see what you mean! Will fix today.
do you have any info on when SA will let everyone use export limiting, instead of just one (wealthy) suburb at a time? It was idiotic of them to slam the doors shut on that tech given that it did exactly what they said they wanted.
Here is SAPN’s Flexible Exports rollout schedule:
https://www.sapowernetworks.com.au/your-power/smarter-energy/flexible-exports/flexible-exports-eligibility/current-flexible-exports-areas-in-south-australia/
Interesting. I’m considering a 2nd 6.6 kW system. Could be worth considering the cheapest small battery available in order to be able to install more than 6.6kWs of panels. Especially since i don’t have room for 6.6 on the remaining north roof, so some will need to be on the south.
We are export limited to 5kW, were at times the voltage rise is blamed at other times the size of our transformer. We are the sole user of the transformer, which by the way is brand new. I’ve asked for an explanation, but are still to receive one and the installer has questions on that ruling too. It is not that I am after a bigger income stream from the FIT, but the waste is what puzzles me.
So does this mean I can add more panels to my 6.6kw system if I get a battery? How do you know what the true limit of your inverter? Does it say that somewhere on it?
If you add a battery to your existing solar system, and choose to add additional panels to oversize the inverter at the same time as installing the battery, are the additional panels eligible for STC credits?
I have been told this does not apply to Queensland – can you please confirm if the ‘national guidelines’ are not really so?
They are national guidelines. What changes is the local network rules. i.e. Ergon and Energex dictate the maximum inverter size that can be connected to the grid.: Single phase: 10 kW solar inverter limit + 10 kW battery inverter limit, 5 kW fixed export limit. (SWER limit = 15 kW solar inverter + 15 kW battery inverter)
Three phase: 30 kW inverter limit, 15 kW fixed export limit. (SWER limit = 15 kW inverter per phase)
Optional, dynamic export limit allows 1.5-10kW export per phase.
What a great update to the rules. Many thanks for the article.
How much can the Fronius Gen24 10kw 3p unit be oversized by? I threw the 19.3kwh DC coupled BYD HVM on after a windfall just prior to prices going crazy.
I’ve got a great East facing roof that would pretty much bullet proof my place no matter the weather or time of year. Roughly 2 years into use and something like this rule change could really fill in the gaps we get on multiple rainy days.
Kind regards.