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 micro-inverters such as Enphase micro-inverters 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 power points.
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
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 micro-inverters. 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:
- DC (from the solar panels) to AC (at the solar inverter)
- AC (from the solar inverter) to DC (to charge the battery)
- 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 of 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 micro-inverters 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 Micro-inverters, 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.
