There’s a strange certainty creeping into battery talk lately. The idea that DC coupling is the grown-up option, and AC coupling is something we did back when we didn’t know any better. I keep seeing installers and Facebook pundits claiming the only proper way to attach a battery to a house is with a DC-coupled system. Anything else is framed as inefficient, messy, a historical mistake or just deeply uncool.
I don’t get it.
Stylish Simplicity
AC coupling is simple. You have one inverter for your solar panels and a separate inverter for your battery. The battery charges using solar that has passed through the solar inverter and become AC. The solar is coupled to the battery via an intermediate AC conversion in the solar inverter.
DC coupling removes the solar inverter. The solar panels share the battery’s inverter. This allows the raw DC solar electricity to go directly from the solar panels to the battery. The solar is coupled to the battery via DC electricity.
For years, AC coupling was common, mostly because the Powerwall 2 was AC coupled. The Powerwall 2 had an integrated battery inverter but relied on a third-party solar inverter, making it easy-peasy to retrofit. It sold in big numbers. Then Tesla released the Powerwall 3 with an integrated battery and solar inverter. It was DC-coupled, and the mood was already shifting.
Integrated, DC-coupled battery systems were becoming fashionable. Sigenergy arrived on the scene with their stacked tower of white plastic that integrated everything. Separate inverters began to look old-fashioned, even if they worked just as well.
I’ve got both types of coupling running. Two DC-coupled systems, Sungrow on the Airbnb and iStore on the shed, with one AC-coupled Powerwall 2 plus Enphase solar on my home. If I’m honest, the AC-coupled setup is the one I trust and enjoy the most, and not for sentimental reasons.
The biggest reason is no power bottlenecks. A DC-coupled system is constrained by the rating of its hybrid inverter. That inverter becomes a choke point for everything. Imagine a house with 12 kW of solar panels, a 10 kW hybrid inverter, and a battery that can also push out 10 kW. Now picture an EV charging at 11 kW while the air conditioning is pulling another 5 kW. The house wants 16 kW. The system can only deliver 10 kW. The remaining 6 kW comes from the grid, even though the sun is shining and the battery is sitting there ready.
With AC coupling, those limits don’t stack in the same way. The 12 kW of solar can feed the house through its own inverter. The battery can add another 10 kW through its inverter if needed. You get full use of the solar being generated, and the battery can cover the extra. No single inverter throttles the whole system. You get access to the capacity you paid for.
Single Point Of Failure
With DC coupling, if your hybrid inverter fails, both solar and battery stop working.
There’s also the boring but important matter of redundancy. Hybrid inverters eventually fail like any other electronic device. When one fails in a DC-coupled system, everything stops until it’s fixed. Solar, battery, all of it. You flip the bypass switch, and you’re back on grid-only. With AC coupling, solar and battery live separate lives. One can keep working while the other waits for a service visit. That redundancy becomes more valuable as systems age.
Upgrades tell a similar story. AC coupling is forgiving. You can add solar later without redesigning the battery system. You can add a battery later without touching the solar. DC-coupled systems are more rigid. Often perfectly good equipment gets replaced early just to make room for something else.
Many objections to AC coupling are leftovers from another era. Whole-home backup used to be awkward or impossible with AC coupling. That changed years ago when inverter standards allowed batteries to control solar output using frequency shifts. If someone says AC coupling can’t do whole home backup, they’re repeating something that stopped being true over 10 years ago.
With AC coupling, one inverter failure doesn’t take out everything at once.
An Argument That Never Goes Out Of Fashion
The efficiency argument is another one that refuses to die. Yes, AC coupling involves an extra conversion step. The losses are real. They’re also small. I’ve already run the numbers. It’s not something a normal household should lose sleep over, and it certainly isn’t a reason to rule out an entire system design.
The real issue in Australia is that we’ve drifted so far from AC coupling that we barely have any batteries designed for it anymore. Right now, the only decent-sized, decent-looking battery specifically designed for AC coupling I can think of is the Franklin.
So when you hear folks insisting there’s only one correct way to install a battery, take them with a grain of salt. AC coupling isn’t a mistake. It isn’t obsolete. In some homes, it’s the better option, even if it means a second inverter on the wall.
Phase Shift is a weekly opinion column by SolarQuotes founder Finn Peacock. Subscribe to SolarQuotes’ free newsletter to get it emailed to your inbox each week along with our other home electrification coverage.
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I was under the impression that the powers that be allow you a maximum of 10kw of inverter in any configuration / configuration here in Qld for single phase.
I also thought the issue that remained with AC coupling was black starting?
My main comment is you’ve forgotten to mention oversizing and DC coupled batteries.
Here and i assume most regions don’t always let you just add more inverter capacity.
For Ergon last i checked they take as standard contracts 1-10KW and 30-100KW system capacities.
The 11-29KW range requires a negotiated connection which i could find literally no references on how that goes last i went searching.
Though I haven’t checked if they still count battery inverters to the inverter capacity limit.
Having recently gone through this I don’t disagree with your points but I think you neglect a couple. Firstly installer simplicity- the all in one seems to have won the battle here. Secondly price. I had a really good quality but a couple years old pv which I originally planned to ac couple. However by the time you buy another inverter and the gateway hardware etc it was thousands cheaper and neater to get a high quality dc coupled battery. Anyone wanting a barely used fronius inverter please get in touch 🙂
The power bottleneck argument doesn’t stack up on Energex, as they limit total combined inverter capacity (solar + battery) to 10kVA per phase.
So if I have two 5kW AC coupled inverters, then I am power bottlenecked to 5kW of battery charging in the day and 5kW of usage at night.
With my 10kW hybrid inverter I can supply the house with 10kW of solar AND simultaneously charge the battery.
The rest of the arguments stand, but DC coupling is a helpful strategy to get the most out of your investment without breaking the rules.
Good piece Finn. I guess the main draw back for many residential installations is that adding two inverters for a.c. coupling (solar one and battery one) means you may be limited by DNSP rules on maximum IES capacity (total inverter power) at your site. Typically 5-10kW for single phase homes.
Would have liked to keep my old inverter, but the local rule for only a max of 9.99kW inverter and hybrid battery combined meant it wasn’t possible.
However now I will be able to charge an EV at 12.5kW and provide another 9.99kW to rest of the house without drawing anything from the grid with a DC only coupled setup.
Hi Finn, thanks for the reminder on the differences between AC and DC coupled systems.
I’m AC coupled 6kw Trini panels, with 20kWH Enphase batteries.
So I have Enphase micro inverters on the roof, and my batteries have micro inverters incorporated in them.
Regards
Jenko
Would you say the inverter capacity limitations with certain networks play a part to some degree in certain cases? Especially if someone wants to be able to expand upon the system at a later date. In Ausnet for example, if someone had an 8.2kW Primo and a Tesla Powerwall 2 they are well and truly stuck having already utilised a manual assessment with the network to exceed 10kW with an AC coupled battery. Throw in Emergency Backstop requirements into the mix and it makes things even more complicated.
Hi Alex,
You’re really onto something here.
DNSP limits are a pain in many instances & backstop requirements are causing a lot of good functional equipment to go in the bin.
I’m going exactly through this right now. I have an existing fronius (6kw) PV system and have an Sig battery (with 10kw inverter) about to go in.
Different installers recommend different setups.
My only thought on this is that I regularly see my fronius maxing out at 6kw from my 8+kw panels on the roof. Am I missing out on a potential of 2kw generation each day by having the fronius sit in the middle?
Hi Dreg,
Fronius have superior current capacity so depending on how the strings are arranged, your Sig may not be able to handle all the DC solar without rewiring the array.
The number of hours your inverter spends under full insulation is quite small, so the “extra 2kW” really doesn’t yield much.
My advice is to add more solar, doesn’t matter which direction, correct orientation is facing UP.
On my DC coupled system i have 12kw of AC inverter attached to 24kw of battery and 19kw of pv.i also have 12.5kw of dc EV charging. I can be running at full speed on the EV charger and/or putting 12kw into the house battery and still have everything the AC inverter is capable of producing available if i need. Of course I actually can’t, the PV capacity is the limit but if it produces 19kw i can use it all… if everything ran through the inverter then only 12kw would be available in sum… complex modern systems have both DC and AC loads so use what ever backbone coupling works for you but make sure the bottleneck that exists, and there will be one, doesn’t restrict what you want to do. in other words AC or DC coupling is used as marketing to differentiate, above all make sure your total system does what you need…once your boxes are ticked why would you care what the backbone is?
Not sure I understand the point you were making about the Franklin. My sig is clearly designed to support a number of strings on the DC side, but in my case there are none and it is more than happy to accept my enphase output via AC coupling (even when islanded). All the pretty charts show solar production and where it went between battery load and grid.
So what’s the advantage of a battery that must be AC coupled (vs one that can be either)?