In batteries, Blog, Solar for Beginners

Should I get an AC coupled battery or a DC coupled solar battery for my solar system? This post will briefly explain AC and DC and how we change one form of power to another. Next, I will discuss the advantages and disadvantages of both methods of charging a solar battery. It will also point out a unique problem that microinverters such as Enphase microinverters face by locking you into one style of charging. Finally, I’ll explain the “battery ready” scam and the loophole that you’ll want to know if you choose to install either an AC or DC coupled battery.

AC and DC power 101

DC power

DC stands for “direct current”.  Direct current flows in one direction; electrons move from negative to positive. It’s the type of electricity that comes from your solar panels, but it is also the electricity that comes from any kind of battery. From car batteries to watch batteries, AA batteries to Solar Batteries; all batteries are really DC batteries.

AC power

AC stands for alternating current. In Australia, that current alternates or changes direction 50 times a second, or at 50Hz. Alternating current is the type of electricity that comes from the poles and wires on the streets, into your home, and out of your powerpoints.

 

 

Changing from AC to DC

While your solar panels produce DC electricity, that DC is “inverted” (or converted) into AC power by the solar inverter. We then use this AC power for every appliance in our homes.

However, mobile phones, laptops,  modems, tv’s and many other appliances use DC power. So why can we plug these appliances into an AC powerpoint? These devices convert the AC power back to DC power – with an AC to DC rectifier. A rectifier does the reverse of an inverter.

 

AC coupling a solar battery

AC coupled tesla

It’s no different with an AC coupled solar battery like the Tesla Powerwall, the Enphase AC battery, or the SonnenBatterie. AC coupled batteries store DC energy; they are just all charged by being “coupled” or “plugged into” AC power.

You’ll still need either a standard string inverter such as a Fronius, SMA or Sungrow, or microinverters. We connect your AC battery to work alongside – but independently of your inverter.

 

The AC coupled battery has a built-in rectifier/inverter that converts power from the AC power source to DC power in order to charge the battery.

When it is time to discharge the battery, the battery uses the inbuilt inverter to convert the battery’s DC energy back to AC so that it can be used again by the appliances in your home.

So an AC coupled solar battery uses three stages of conversion:

  1. DC (from the solar panels) to AC (at the solar inverter)
  2. AC (from the solar inverter) to DC (to charge the battery)
  3. DC (from the battery) to AC (to be used in your home).

 

DC coupling a solar battery

In contrast, DC coupling a solar battery only requires power conversion once rather than three times. It uses DC power from the solar panels to charge the battery. It then converts the DC power from the battery back to AC power to be used by the home.

Both of these steps normally happen by what is commonly called a “hybrid inverter”. A hybrid inverter is simply a battery charger, and a solar inverter put together in one box.

The battery charger must communicate with the battery to charge and discharge the solar battery at the right rate. What happens when it doesn’t?

 

Disadvantages of DC coupling a solar battery

You might be thinking DC coupling is a better solution. Why would you get an AC Battery that goes: DC-AC-DC-AC, unless you are obsessed with that band from the ’70s. But as it turns out, it has been DC coupling “solutions” that have taken many installers on a Highway to Hell.

Several years ago, it became apparent to the industry that the future of the solar boom would include household solar batteries. It wasn’t long before every inverter manufacturer had their own version of a Hybrid inverter, and every lithium battery manufacturer had their own version of a lithium solar battery.

The lithium battery manufacturers and inverter manufacturers each chose several partners that they wanted to work with. All they had to do was get the communications between the battery and the hybrid inverter right. The lithium battery just needed talk with the inverter to ensure it was charged and discharged at the right rate. How hard can it be?

But it’s a long way to the top if you want to rock and roll. Communications between Brand A and Brand B turned out harder than you might think. As technology rapidly developed, software upgrades were continually done on both the inverter and the charger.  When the installation failed, or lithium battery didn’t perform as guaranteed, the finger-pointing game between the battery and the inverter began. The installer and the customer were both hung out to dry. And we’re not just talking about pop-up brands. These warranty issues are endemic even amongst the well-known brands.

Never ever

Smart installers quickly worked out that if you want to avoid nightmares, never, ever, ever, ever, install a lithium battery and inverter unless they are manufactured by the same company.

This is not to say that you won’t have issues if you keep your brand consistent. It does mean that hopefully, the communications between the inverter and the battery will be better. And when you do have a problem, there can be no finger-pointing between two manufacturers. I should also note that lead batteries are a lot simpler than lithium batteries and don’t have the same type off communication issues.

… never, ever, ever, ever, install a DC coupled lithium battery and inverter unless they are manufactured by the same company.

AC coupled batteries, however, don’t have the same issue because the inverter/charger is built into the battery. However, AC coupling does have another significant issue in many homes around Australia. It’s to do with regulations.

 

Disadvantages of AC coupling a solar battery

In Qld, ACT, parts of NSW and parts of Victoria, we are limited to a maximum of 10kW of inverters on each phase. The problem is, the 10 kW limit includes the sum of all the inverters and the AC coupled inverter/battery charger. On single-phase, if you want to have a large 8.2kW Fronius solar inverter with 10kW of panels, you are left with a piddly 1.8kW AC coupled battery inverter. In reality, if you need a larger solar system with a larger than 5kW solar inverter, you’ll need to use a DC-coupled battery …. if you can.

If however, you chose an Enphase microinverter system, you don’t have the option to DC Couple a battery, because the DC power is inverted to AC behind each solar panel. So if you choose Enphase, you can only ever install an AC coupled battery. Consider the following examples:

 

AC coupled battery example 1. Install a healthy 10kW solar system with 8kW total capacity of microinverters on your roof.
The problem? You will only be allowed to install a (future) 4kWh Enphase battery with a 2kW inverter charger. A 4kWh battery is a bit of a token effort.

AC Coupled battery example 2. Install 6.6kW of solar panels on 5kW Fronius inverter. Later, install a 13.5 kWh Tesla battery with a 5kWh inverter charger.

The problem? It’s likely you won’t have enough excess solar to charge your battery.

 

 

Advantages of DC coupling a solar battery

If you choose to install anything other than Microinverters, you leave your options open to either an AC coupled or DC coupled battery in the future. Under current Qld regulations, DC coupling allows you to install a 10kW inverter, 13kW of panels, and any size battery that you like. (Read the blog: “can in install 10kW solar on single-phase”). This is because a DC-coupled system does not need an additional inverter/charger just to charge the battery. We just use a DC battery charger (the “hybrid” part of a hybrid inverter).

 

DC-coupled battery example 1. Install an 8.2kW Fronius Primo now with 10kW of solar panels. In 5 years, replace your 8.2kW Primo with new 2024 model 10kW single-phase SMA Hybrid inverter (not invented yet). Install 15kWh of batteries.

DC-coupled battery example 2. Install 2x 5kW Fronius inverters with 13kW of panels.  In the future, purchase a 2024 model SolarWatt battery charger. Install it before the Fronius inverters. Install 15kW of SolarWatt batteries.

If regulations happen to be changed and the 10kW restrictions are lifted, you could install an 8.2kW Fronius now and AC couple a battery later.

 

Advantages of AC coupling a solar battery

In essence, an AC coupled solar battery is a battery with an inverter charger built-in. One obvious benefit of this is there can be no finger-pointing. If there are issues with either the Powerwall battery or the Powerwall inverter/charger  – it’s a Tesla issue.

The option of AC coupling also allows you to install an affordable string inverter now, and keep it for the ten year warranty period. If you choose to install a battery five years down the track, just wack in whatever battery comes on the market without being concerned about compatibility (assuming you are keeping to the local network’s regulations).

The other clear advantage of the battery and the inverter not being interdependent is if either the inverter or battery fails five years down the track, you can replace that product with an updated product without being concerned about compatibility.

 

Other Solar Battery considerations

The ol’ Battery ready inverter scam

Another common mistake made by unsuspecting customers is purchasing a battery ready inverter, so they can simply plug a battery in when they become more affordable.

  • First, some companies call any old inverter “battery ready” and yes, I guess theoretically you can, because every inverter is “ready” to have an AC coupled Tesla connected to it.  If a salesman is telling you that scam, block his number. The term “battery-ready-inverter” is correctly used to describe a hybrid inverter that is used for DC-coupling.
  • If you do happen to purchase a true battery ready inverter, just be aware that solar technology moves fast. By the time you are ready to purchase a battery, your “battery ready inverter” may be old technology, and not compatible with the batteries on the market. I’d suggest the only purchase a new model battery ready inverter – if you are planning to purchase a battery in the next 12 to 24 months. As I type it is November 2019, I can’t see batteries being affordable in the next two years. Batteries are still a discretionary spend.

Oversizing on a solar battery

Clean Energy Council installer “guidelines” (read “solar bible”) only allow us to install a maximum panel capacity 33% more solar than the size of the solar inverter. Yep, that’s why you see a 6.6kW solar systems on a 5kW battery advertised everywhere you go. However, many in the industry are unaware that, if you have a solar battery connected to your solar system, the 33% rule doesn’t apply.

Direct from the relevant standard:

Clause 9.4 Array peak power – inverter sizing

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

 

The key-words are on the cover page of the guidelines: “no battery storage”. The 33% oversize rule applies only to systems without battery storage. If you install a battery, the standard no longer applies. While there is nowhere that says you can oversize panels by more than 33 per cent if you have a battery, there is no standard or guideline that says you can’t.

  • This loophole works particularly well with DC-coupled batteries. If you have a 5kW inverter and you are producing 8kW of power, 5kW of solar power can go to the inverter and 3kW can go to the battery.
  • Oversizing doesn’t work as well with an AC coupled battery, because the 5kW solar inverter is limited to 5kW output, and the battery charging happens after the solar inverter.

The catch is, because batteries are not yet a viable option, you’ll need to wait until you have a battery installed if you want to oversize your array.

 

Conclusion

AC coupled and DC coupled batteries have distinct advantages and disadvantages. The take-home for DC batteries is: be wary of purchasing a battery and inverter of different brands. When it comes to warranty time, you may be hung out to dry. AC coupling solves the communication issues, but it may limit your options to install a larger solar system. You’ll need to be sure that your local network regulations will allow you to install a solar inverter and an AC battery big enough to handle your needs. If you want a reasonably large solar system and a large battery, you may need to look at DC coupling.

When you are looking into Solar, be aware of the “battery ready inverter” scam. If money isn’t a problem and you really want a solar battery, you should know what many salespeople are still unaware of: the 33 per cent oversize rule does not apply.[/vc_column_text][/vc_column][/vc_row]

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13 Comments on The difference between an AC & DC coupled Solar battery

Mark C said :administrator Report 3 days ago

Hi Ian, A few interesting points, Yes, it's only the stored energy that gets converted 3 times. But Hybrid inverters will also only convert excess energy to the battery charging voltage. DC to dc inversion happens in all inverters, including Enphase. DC to DC conversion is a lot more efficient than inversion, but I'm not really concerned about the efficiency losses. I think that is a minor point compared to all the other considerations. I'm all for AC coupling as a simple and reliable method, as long as regulations don't limit the size of the solar system and battery you need.

    TT said :Guest Report 4 days ago

    Hi Mark, excellent blog. Thanks for taking the time to educate us. I have a question that may be of interest to your blogees. It’s to do with central protection for systems over 30kW. AS 4777.1 talks about methods of disconnection. A lot of installers use a contactor controlled by a MainsPro or similar to isolate the IES from the supply. According to clauses 3.4.4.2&3, you can also shut the inverters down by asserting DRM 0 to the inverter from the MainsPro relay. My question is - With multiple inverters can you connect them in a master/slave configuration where you would assert DRM 0 to the master and use the comms function so the master shuts the others down. (I’m thinking of Fronius inverters). Just wondering if this would satisfy the requirements of AS 4777.

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    Mark Cavanagh said :administrator Report a week ago

    Hi Robart, My advice is don't waste your money on a battery unless you have blackouts often. But if you really want batteries and have three-phase, I would personally stick with Tesla. You won't be able to run a 3 phase appliance (eg ducted air-con) off it, but it should go the distance and have a decent warranty.

      Mark Cavanagh said :administrator Report a week ago

      Hi TJ, the blog is https://mcelectrical.com.au/blog/microinverters/

        Mark Cavanagh said :administrator Report a week ago

        Hi John, Yes fair point. "Solar batteries" are designed to be used to store excess solar energy, and in many places, they cannot be charged from the grid due to regulations. It's like calling a "car battery" just that. But I used the term for more pragmatic reasons: heaps of people search for the term "solar battery ...." and I want them to read the blog :)

          Jon Edwards said :Guest Report a week ago

          Once again, another great blog post Mark. Clearly explained and sequentially presented. My question is about terminology and I am happy to be accused of nitpicking! I don't think we should be referring to batteries as "solar batteries". A battery is an electrical energy storage device. A PV Panel is a solar energy collection device. What exactly do you mean by "Solar Battery"? Are there batteries out there that are exclusively for use with solar systems, implying that they cannot (or should not?) be charged from the mains? I believe that we should strive to be clear in our terminology as people can easily jump to conclusions and assume things that we are not saying. Your thoughts?

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          TJ Roberts said :Guest Report a week ago

          Mark, I was unable to find the blog. Do you have a link? I've written a lot of promos on Enphase's TechTalks and on SeekingAlpha.com.

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          Mark C said :administrator Report 2 weeks ago

          Agree with most of what you are saying TJ. I wrote a blog on Enphase. Google it. I'm also a fan of Enphase but realistic about its application and limitations. I'd be keen to hear your feedback on my negative comments.

            Mark C said :administrator Report 2 weeks ago

            TJ Roberts, ah, correct, should be "a standard string inverter OR microinverters"

              Graham Chadwick said :Guest Report 2 weeks ago

              Excellent Blog, well pitched for all to understand. For those who want more its a good spring board into the solar questions and answers. Enjoyed the read.

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              Robert said :Guest Report 3 weeks ago

              What is the advice for a home with three phase power considering a 15KW solar system possibly with suitable batteries?

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              TJ Roberts said :Guest Report 3 weeks ago

              Great article. That's why I like Enphase "AC" batteries; of course, the original ACB was too small and "load shaving" was just not worth it. But, Enphase's newest Encharge ACB's promises a much better solution with backup capability. One thing people need to realize, also, is that Enphase's concept of "micro" batteries like their "micro" inverters has 2 advantages — 1) size and weight means single person installs instead of needing a team to move the giant battery like a Powerwall, RESU, eco, etc, and 2) fault-tolerance aka reliability in multiples. In 2020, Enphase's Ensemble software which is what they call their latest software solution which includes microgrid-forming capability, will allow for a solar PV system to form is own microgrid w/o the use of storage. All other microgrid storage solutions require storage to operate. This is a uniquely independent solution and one which may require the average home to just need a smaller storage form factor to operator, like the low-end Encharge 3.3kWh product. Tesla, Sonnen, LG, Pika, et cetera, all want you to buy huge storage products, but Enphase may have a more budget-conscious solution. IQ8PV is due in 2020; Ensemble Storage with IQ8 is being released now. In addition, customers should consider fault-tolerance when making a purchasing decision, that is, what happens in an active cooling storage solution if the fan fails? Will the entire storage product be inoperable? Enphase's solution is passive cooling so no fan. Then, what happens if the hybrid inverter fails? Will all the storage be down? With Enphase's duplicitous, decentralized power topology, each Encharge 3 has four IQ8 microinverters integrated such that even if 3 of the 4 fail, the storage product will still operate. The Encharge 10 has twelve IQ8 microinverters. Low-voltage DC-to-AC gives customers the optimal longevity for electronics. I honestly believe that with hybrid inverters running more than their solar siblings, the longevity for these will be even shorter. Of course, I am Enphase-biased with a 20,000-Watt AC solar system on my home and barn since 2013-14 that has never failed albeit the replacement of 2 microinverters, and even then, only a single circuit breaker needed to be switched!!!

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              ian holland said :Guest Report 3 weeks ago

              i agree with you blog on from a battery charging standpoint, but i feel you missed a major part of why ac coupling can be an advantage. AC coupling means that the solar inverter converts energy and feed houseloads directly. only excess energy is then converted to charge the battery. which means one conversion dc to ac to consume the solar power during the day and only having the extra conversion on power not directly consumed. so when you say the ac coupled is three conversions this is true for only part of the energh not all of it. making it a lot more efficient than you state here. dc coupling means 2 conversions to use the power normally dc-dc conversion to battery, dc- ac conversion to loads. ( this is not the case with all dc coupled arrangements)

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